# Is Time Travel (going backwards) Possible?



## Bullgrit (Oct 16, 2012)

In your opinion or educated knowledge, is it possible to travel through time (backward and forward again)? [I don't mean possible with current science, but possible to do at all, even in the far, advanced scientific future.]

I'm not talking about dual timelines, or splitting timelines, or anything other than true travel up and down our "real" timeline.

Bullgrit


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## Scott DeWar (Oct 16, 2012)

I don't think so, just because if you travel forward, you would be able to change it, changing it possibly creating a paradox.


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## Hand of Evil (Oct 16, 2012)

Guess it depends on your view of time, is it linear or a ball of string that is always looping back on itself?  

If linear, I don't think so.  You become part of the timeline or are unable to because time would be a barrier.    

If a ball of string, you create alterations (loops and alternates) and overall the time line is not impacted, just the period you are there.


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## Morrus (Oct 16, 2012)

No, because Hitler existed. If time travel were ever to become possible, his bedroom would have been be filled every night with thousands of would-be asssssins from the future.

I guess it's possible that time travellers from the future are very circumspect when they go into the past, but that doesn't seem likely to me. Humanity doesn't exactly have the "circumspect" trait down as as a species. And it would only take one or two idiots out of potentially millions upon millions of future time travellers.

So, in my opinion, if time travel is ever going to become possible in the future, we'd know about it now.


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## Stumblewyk (Oct 16, 2012)

Morrus said:


> So, in my opinion, if time travel is ever going to become possible in the future, we'd know about it now.



 This.  I honestly believe if it were possible to travel backwards in time, someone, somewhere would turn it into a vacation of sorts.  You'd see time travelers visiting key points in history to "be there" and witness it in person.

Hell, _I'd_ pony up the dough to do it.  Wouldn't you like to have been there when the Declaration of Independence was signed?  Or the Magna Carta, or the Treaty at Versailles?  Witness a pyramid be built, see ancient Greece in all it's glory?  Be at a safe distance when Mt. Vesuvius erupted?

And with all these vacation destinations available to future society, don't you think one of the morons they send back in time would screw up, would reveal that though they're dressed in a appropriate garb and trained to speak in a language or accent appropriate for the time period and place (or given a "babel fish" of some sort), that _one_ of them would give up the goat and pull out a future cell phone to take a picture for that future Facebook page?

And if _one_ idiot would do it, it stands to reason that thousands, maybe millions would as well.

No, time travel back in time is not possible.  If it was, we'd know all about it because some idiot from the future would have let the secret slip out.


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## Kaodi (Oct 17, 2012)

Morrus said:


> No, because Hitler existed. If time travel were ever to become possible, his bedroom would have been be filled every night with thousands of would-be asssssins from the future.




In my view this is actually completely incorrect. Going back in time more than a couple months has potential to be FAR more _evil_ than even a combination of Hitler, Stalin, Pol Pot, and the like. 

When you go back in time and change things, you completely alter the outcome. Going back in time to kill Hitler (or do anything else for that matter) would also have the effect of murdering almost every single person who was born after that; billions of people.

Edit: Also, I cannot think that "space" travel has does not have to be taken into account during time travel. And that could just lead to wacky things, like it being impossible: either physically or in terms of surviving it.


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## RSKennan (Oct 17, 2012)

I agree that same-timeline backwards time travel is almost certainly impossible, but forward time travel isn't. Just approaching the speed of light should do it. 

And despite the OP's prohibition, there's less of a case against time travel that *does* split the timelines. Sure you can kill Hitler, but there's an infinite number of other Hitlers left. Assuming the Many-Worlds theory is true, which I like to believe it is, if only because it's cool.


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## Hand of Evil (Oct 17, 2012)

RSKennan said:


> I agree that same-timeline backwards time travel is almost certainly impossible, but forward time travel isn't. Just approaching the speed of light should do it.




But you are not travelling in time, you are just viewing it.


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## RSKennan (Oct 17, 2012)

Hand of Evil said:


> But you are not travelling in time, you are just viewing it.



How do you figure? Travel near c time slows down for your frame of reference. Stop travelling near c, and bam, you're in the future. Sure, you pass through the intervening time at an accellerated rate, but you still get to the future in a shorter period of time than it would take if you had stayed "still" and waited it out.


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## Morrus (Oct 17, 2012)

RSKennan said:


> How do you figure? Travel near c time slows down for your frame of reference. Stop travelling near c, and bam, you're in the future. Sure, you pass through the intervening time at an accellerated rate, but you still get to the future in a shorter period of time than it would take if you had stayed "still" and waited it out.




Yup. You're time travelling into the future right now. Bam - now you,re in the future - you travelled  forward a second! It's easy! Accelerating lets you do it faster than other people, of course, and that's the only context in which it makes sense here.

Time travel without space travel? You're safe in a TARDIS; not in a DeLorean (unless Doc Brown was keeping the teleportation power secret). The Earth is millions and millions of miles away from where it was in 1942 (heck, not just its orbit - how far does the sun move in the time?). You'd find yourself floating in space.


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## Quickleaf (Oct 17, 2012)

What, you mean like John Titor? 

John Titor - Wikipedia, the free encyclopedia

I'd like to believe in time travel, but it seems pretty dang unlikely.


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## Dire Bare (Oct 17, 2012)

RSKennan said:


> I agree that same-timeline backwards time travel is almost certainly impossible, but forward time travel isn't.




Isn't traveling forward through time just traveling in someone elses' past?


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## sabrinathecat (Oct 17, 2012)

Well, one problem would be similar to the Trek Transporter: you are creating or altering a few billion molecules in order to make yourself that new body. Or you are altering their position and form.

As for killing Hitler, there was a great story about that. Short version: hit-squad goes back in time and gets caught. During the interrogation Hitler walks in, expressing his concern. One of the hit-squad breaks free, and the squad and Hitler all get killed. The top generals look at each other. "Now What?"
"Well, there is always the clone..."
"I never liked that thing--it always struck me as unstable."

And another story where the entire crowd at the crucifixion is made up of time travelers and 4 or 5 actual locals.

Yeah, the Earth spins. The Earth orbits around the sun. The Sun orbits within the Milky Way. The Milky way moves within the Galaxy. The Galaxy drifts out as the universe expands. Thus, it is impossible to come to a full and complete stop. Try that one next time you are pulled over for not stopping at a stop sign. Include the math for the officer.


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## Stormonu (Oct 17, 2012)

Yes, but the energy involved would be ridiculous.

What is going back in time?  Putting every relevant molecule except your own back where they were X seconds, minutes, hours, days or whatever.

If you could find "the center of the universe" and the equivalent of some sort of super magnet that put everything back in its place (and we're talking food that was eaten, pulling corpses out of the ground and hauling back dessicated atoms of flesh and the like...), you'd have gone back in time.

But we're talking here enough energy to reverse "the Big Bang".  Unless the universe is going to collapse back in upon itself at some point*, I don't see it happening any time soon.

Perhaps someone could find a way to do a localized version - say, with a few properly placed black holes?  In that case though, the fun would be in stopping the reversal of time...

* In which case, the Battlestar Galactica mantra "this has all happened before and will happen again" takes on new meaning...


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## Fast Learner (Oct 17, 2012)

Morrus said:


> No, because Hitler existed. If time travel were ever to become possible, his bedroom would have been be filled every night with thousands of would-be asssssins from the future.




Unless, say, Hitler's and the Nazis' actions serve as an essential historical lesson, preventing countless future genocides, maybe even the destruction of entire species. 

Time and history are incredibly complex.


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## Morrus (Oct 17, 2012)

Fast Learner said:


> Unless, say, Hitler's and the Nazis' actions serve as an essential historical lesson, preventing countless future genocides, maybe even the destruction of entire species.




What, and every person from the invention of time travel until the end of time abides by that?


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## Someone (Oct 17, 2012)

Morrus said:


> No, because Hitler existed. If time travel were ever to become possible, his bedroom would have been be filled every night with thousands of would-be asssssins from the future.




I think you got your sci-fi wrong. Any alteration to the past, even stepping on a butterfly, results on nazins winning WWII. The nazis didn't win WWII, therefore nobody has travelled to the past.

Also, the current set of events aren't one where anyone will ever travel to the past, or else the nazis would have won WWII. So, if time travel is possible, no time machine will ever be built.


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## Hand of Evil (Oct 17, 2012)

RSKennan said:


> How do you figure? Travel near c time slows down for your frame of reference. Stop travelling near c, and bam, you're in the future. Sure, you pass through the intervening time at an accellerated rate, but you still get to the future in a shorter period of time than it would take if you had stayed "still" and waited it out.




FTL does not take you out of "now".   No matter where you go...there you are.


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## RSKennan (Oct 17, 2012)

Hand of Evil said:


> FTL does not take you out of "now".   No matter where you go...there you are.




Irrelevant to the topic at hand. The point is, it's time travel to the future, because all such future time travel would take some interval of subjective time that is less than the interval of subjective time that would have passed without the time travel.


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## Hand of Evil (Oct 17, 2012)

Time is a concept created in hell, you always want more, you never have enough, you try and save it, it flys and it crawls.  It is always out, is money and there is a limit to how much you get.


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## Bullgrit (Oct 17, 2012)

> No, because Hitler existed. If time travel were ever to become possible, his bedroom would have been be filled every night with thousands of would-be asssssins from the future.



But what if Hitler's existence is the lesser of evil options? What if our history was orchestrated by time travelers? Things have happened the way they have because even the very terrible things were better than the other options? Or what if the time traveler is himself evil, and chose to have Hitler rise and other stuff?

What if our history is constantly changing due to a time travel war changing everything in the past. We wouldn't know a difference between one day and the next. Yesterday the history was that the Nazis won WWII. But a time traveler changed that and now we have history as we know it. Tomorrow our history may be different, but we wouldn't know it changed.

For the record, I don't think time travel to the past is possible. But I just don't think the argument that history is as we know it is a logical argument to disprove reverse time travel.

Bullgrit


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## Umbran (Oct 17, 2012)

Morrus said:


> Time travel without space travel?




Here's an important point.  Time travel is equivalent to space travel.  Einstein showed us that time and space are not separate entities - they are wrapped together in a single "spacetime".



Hand of Evil said:


> FTL does not take you out of "now".   No matter where you go...there you are.




Well, he's talkign about travel *near* c, not faster than C.

FTL does not take you out of "now", but "now" is *not* what we intuitively think of.  There's a set of events you think of as "now".  But, someone else will see those same events happening at different times.  This is "relativity" - the fact that perception of the order of events is relative to the observer, not absolute.

It thus can be set up that, if you can move faster than light, you can see an event, and then move in such a way as to arrive at the location of the event *before the event happens*.  

From your perspective then, that one event can happen at two different times!  If you can travel FTL, then you can remember a clown at your 6th birthday party, and then on your 30th birthday you can be the clown performing for your 6-year-old self.  Same event, two times, two "nows".


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## Raunalyn (Oct 17, 2012)

The problem with same timeline time travel if that it did occur, and something in the past was changed, we would not be aware of that change because all future events from the point that history would change would occur in that set direction (if that made sense).

So, someone may have traveled into our past and changed something...we just don't know what it was. Another possibility that has been approached in many sci-fi and fantasy novels is that travel into the past is quite possible, but nothing can be done to change the past. Events happen that force the timeline to occur as it is supposed to.

All hypothetical of course. Einstein even accepted the possibility of time travel...it was a matter of stepping out of the stream and walking upstream a little way and getting back in. 

I'm of the firm belief of multiple timelines. A multiverse where time branches off in different directions.


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## RSKennan (Oct 17, 2012)

Dire Bare said:


> Isn't traveling forward through time just traveling in someone elses' past?




This is interesting, and I've often thought of the possibility of time as a surface; one timeline defines its length, multiple timelines define its width. In such a universe, every possibility could be predestined, existing somewhere on the surface. In other words, time is not dynamic, and the present is a matter of perspective, and yet there would be free will- you could freely travel laterally.

The part that relates to your post is the part about the present being relative.


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## dogoftheunderworld (Oct 17, 2012)

Morrus said:


> No, because Hitler existed. [...] So, in my opinion, if time travel is ever going to become possible in the future, we'd know about it now.





Only if you assume we are living in the past   If we are at the front edge of the timeline (i.e. the future hasn't happened yet), then the past can't be changed until time travel has been invented, which it hasn't... yet 

On the flip side I time travel into the past/future every Spring and Fall, thanks to the government being able to control time


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## Fast Learner (Oct 17, 2012)

Morrus said:


> What, and every person from the invention of time travel until the end of time abides by that?




Access to time travel doesn't require massive amounts of power or some other limiter that keeps it in the hands of very few?


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## Fast Learner (Oct 17, 2012)

There's also an easily probable combination of time travel being possible but humankind being wiped out before discovering/inventing it.


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## Nellisir (Oct 20, 2012)

Umbran said:


> It thus can be set up that, if you can move faster than light, you can see an event, and then move in such a way as to arrive at the location of the event *before the event happens*.




This is what keeps breaking my brain.  I'm sure I've understood it before, but I haven't been able to recently.  And the recent articles about exceeding the speed of light have brought it up again.

The light from Alpha Centauri takes about 4 years to get here.  If you exceeded the speed of light and teleported there, right now, you still wouldn't journey into the past - you'd arrive right now, four years after the events that you just witnessed.  You could use your ftl phone to call home and confirm your arrival, but you couldn't use it to call yourself and remind you to bring your fuzzy slippers.  And if you teleported back, you'd arrive back here a few minutes after you left, not a few minutes before...although the pictures of your vacation wouldn't arrive for another four years.

Or is that all wrong?  It seems like something's wrong, but I'm not sure what.

So, set it up for me.


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## Morrus (Oct 20, 2012)

Fast Learner said:


> Access to time travel doesn't require massive amounts of power or some other limiter that keeps it in the hands of very few?




Even one person per thousand years until the end of the universe is still billions and billions and billions and billions of people.

How rare are you suggesting something can be kept _forever_?


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## Iosue (Oct 20, 2012)

Morrus said:


> No, because Hitler existed. If time travel were ever to become possible, his bedroom would have been be filled every night with thousands of would-be asssssins from the future.



Here are two of my favorite takes on the killing Hitler issue.

The thing about using the non-death of Hitler as proof of no time travel is we have no idea how our current timeline is supposed to be different.  Maybe Hitler is relatively small potatoes, and we don't know about anyone far, far worse because time travelers have already taken them out.  Maybe John F. Kennedy led us to worldwide nuclear holocaust, with a death count in the billions, so...


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## Morrus (Oct 20, 2012)

Iosue said:


> Here are two of my favorite takes on the killing Hitler issue.
> 
> The thing about using the non-death of Hitler as proof of no time travel is we have no idea how our current timeline is supposed to be different. Maybe Hitler is relatively small potatoes, and we don't know about anyone far, far worse because time travelers have already taken them out. Maybe John F. Kennedy led us to worldwide nuclear holocaust, with a death count in the billions, so...




Sure, we can invent fanciful stuff to explain anything we want.  We're talking about time travel, after all - if any of us actually know, I'm sure we'd find better things to do than post on a D&D forum.  So we post what we think sounds reasonable to us, and the more invention you have to introduce, the less reasonable it sounds to me.  But I agree - anything it _possible_.


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## Nellisir (Oct 20, 2012)

Bullgrit said:


> In your opinion or educated knowledge, is it possible to travel through time (backward and forward again)? [I don't mean possible with current science, but possible to do at all, even in the far, advanced scientific future.]




No.

(I typed some qualifications here, but really, just no.)


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## Fast Learner (Oct 20, 2012)

Morrus said:


> Even one person per thousand years until the end of the universe is still billions and billions and billions and billions of people.
> 
> How rare are you suggesting something can be kept _forever_?




Again, that suggests that mankind somehow survives forever, or even a million more years. Far, far from guaranteed.


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## Fast Learner (Oct 20, 2012)

Morrus said:


> Even one person per thousand years until the end of the universe is still billions and billions and billions and billions of people.
> 
> How rare are you suggesting something can be kept _forever_?




Big assumption that mankind makes it to "forever", much less even another million years.


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## Morrus (Oct 20, 2012)

Fast Learner said:


> Big assumption that mankind makes it to "forever", much less even another million years.




Unless you have some special information about the future that the rest of us don't, I'm afraid you're going to have to settle for this thread containing nothing but guesses and assumptions.  You're gonna be horribly disappointed if you're hoping that a thread about the invention of time machines in the future is anything else.


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## Fast Learner (Oct 20, 2012)

Morrus said:


> Unless you have some special information about the future that the rest of us don't, I'm afraid you're going to have to settle for this thread containing nothing but guesses and assumptions.  You're gonna be horribly disappointed if you're hoping that a thread about the invention of time machines in the future is anything else.




I'm with you on assumptions and guesses. Precisely why I think the "no one has killed Hitler" line of reasoning is full of holes, my assumptions and guesses, and yours.


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## Scott DeWar (Oct 21, 2012)

*my assuptions and guesses*

Maybe we are just so insignificant on the history line that we are not visited by time travelers.


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## Derren (Oct 21, 2012)

Steven Hawking made an experiment about it.
He threw a party for time travellers and no one showed up.


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## Starbuck_II (Oct 21, 2012)

Morrus said:


> What, and every person from the invention of time travel until the end of time abides by that?




But Nazi sympasizers would also go back in time to prevent that. They would be the time roaches ofdthe universe.
Who says Neo-Nazi's wouldn't be the one to create the machine? 
Hitletr did many great things without him, we would have less tecnologfy related to medicine.

In fact, if you are saved by a life threatening operation, Hitler probably saved your life. What he did in the Holocaust was horrible, but humanity benefited by it far beyond anything we have done since.

It helps when you can use trial and error on subjects closer to human  than animals (humans are closest), so yeah.


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## Janx (Oct 22, 2012)

Fast Learner said:


> I'm with you on assumptions and guesses. Precisely why I think the "no one has killed Hitler" line of reasoning is full of holes, my assumptions and guesses, and yours.




I think Morrus's Hitler ain't dead concept is an independent variant of a view I espoused on some other stupid thread of mine.

Which is, if time travel existed in the basic form that Bullgrit espoused (just plain old poof, you're in the past, not a forked quantum reality, etc), then eventually, the future would consist of enough time travellers that ONE of them would travel back in time and screw up in a way that spilled the beans.

Some traveller would get hit by a car, have his arrival witnessed, people sieze his gear and figure out its a time machine.

timeline violators are likely to be ahead of the curve compared to time cops.

As such, BECAUSE there hasn't been an obvious detection of accidental time travel/manipulation, it likely isn't possible.

Not very scientific, but couched in the overall confidence in the human race to invent something cool, and have somebody screw up with it.


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## Nytmare (Oct 22, 2012)

I personally subscribe to the Many Worlds theory regarding time travel.  Beyond that, my layman's view of things would make me suspect that true sci-fi-esque time travel isn't possible for anything other than elementary particles.

Why we think there’s a Multiverse, not just our Universe – Starts With A Bang


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## Janx (Oct 22, 2012)

Nytmare said:


> I personally subscribe to the Many Worlds theory regarding time travel.  Beyond that, my layman's view of things would make me suspect that true sci-fi-esque time travel isn't possible for anything other than elementary particles.
> 
> Why we think there’s a Multiverse, not just our Universe – Starts With A Bang




I'm too lazy to read that, but it's probably the same as my "forked quantum reality" concept.  basically, there's infinite distinct realities that represent each outcome of a random chance (does an electron go left or right = poof! 2 new quantum realities).

Michael Crichton's novel Timeline used this approach (then ignored it later).

The premise being, if you COULD travel back in time, you are in an alternate quantum reality from the original that you experieced/came from.  So the original reality where you were not present from the future exists and YOU experienced that (or descended from it).  

Now you travel back in time, and that creates a NEW quantum reality.  the current YOU and the Chrono-Native inhabitants of this NEW quantum reality experience whatever it is you and they experience (like the fun of trying to kill/protect Hitler).  Let's say you're successful.

To the Chrono-Natives, they experience a world where Hitler died at your hands.  To you, assuming you stay there, you watch the world experience a different sequence of reactionary events to Hitler's death.  But that doesn't erase your memory of your past experience (or time spent in history class) of your original Quantum reality.  If you hang out long enough, you'll get to see if this Quantum Reality's version of you gets concieved and watch you grow up (or not, if it turns out your mom marries somebody who wouldn't have existed if the Holocaust happened).

This is how Quantum physics guys explain away the paradox of killing your grandpa before your dad was born.

The time traveller is the observer.  He witnessed or descended from a timeline where his grandpa concieved his dad in his Quantum Reality of Origin.

Going back in time, isn't going back in the QR of Origin. It's shifting to a new QR set in the past.  You whack Gramps in THAT QR.  If you then hop to the future, you are moving back to your Origin QR or the future position of this new QR.  You don't cease to be because YOU were born in your Origin QR, not this new one where an equivalent you doesn't get made.

By travelling back in time, you BECOME the initiating choice factor for a new Quantum Reality.  because there's the original reality where you were NOT present in 1940 Berlin, and the new reality where you just appeared from nowhere in 1940 Berlin.

there's probably more sciency details, but that's my basic understanding of Quantum physics and multiple realities.  A few sci-fi authors have played with this idea, like Crighton, and Neil Stephenson if I recall.


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## delericho (Oct 22, 2012)

Janx said:


> I'm too lazy to read that, but it's probably the same as my "forked quantum reality" concept.  basically, there's infinite distinct realities that represent each outcome of a random chance (does an electron go left or right = poof! 2 new quantum realities).




The problem with that is that every single event (on a quantum level, no less) creates an entirely new universe - duplicating everything that exists. Where does all the energy come from?

I suppose it's possible that there could be a finite (and fixed) number of parallel timelines, constantly splitting from and converging with one another, in some sort of ball of wibbly-wobbly timey-wimey _stuff_. But that does seem rather unlikely.


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## delericho (Oct 22, 2012)

I'm inclined to think that time travel is probably _theoretically_ possible, but may not be technologically possible - it may require an impossible large power source, or a computer that is too powerful to build, or whatever.

In any event, Morrus is right - if we were ever to develop time travel, someone would almost certainly have gone back and killed Hitler. That suggests that either time travel is impossible, or at least that we'll never get there.


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## Jhaelen (Oct 22, 2012)

delericho said:


> In any event, Morrus is right - if we were ever to develop time travel, someone would almost certainly have gone back and killed Hitler. That suggests that either time travel is impossible, or at least that we'll never get there.



But how about being able to observe the past (i.e. without being able to change anything)?

Is there any argument against that being possible?

No matter if time-travel is possible or not, I certainly enjoy reading stories about it, most recently Connie Willis' 'Blackout/All-Clear' and Neal Stephenson's 'Anathem', two very different takes on the idea.


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## Janx (Oct 22, 2012)

delericho said:


> The problem with that is that every single event (on a quantum level, no less) creates an entirely new universe - duplicating everything that exists. Where does all the energy come from?




Beats me.  Ask a physicist.  Like  [MENTION=177]Umbran[/MENTION].

It's also possible that we all confuse "create" with actual creation.

What if it would be more accurate that all matter exists in all states and all positions in the entire universe.  Much like the idea that an atom from Julius Caesar is now a part of you.  the different Quantum Realities are the defined states.  So it's less creating, and more just shifting to a QR that already exists, which to a layman such as myself, might be described as "created".

That all gets pretty complicated.  I think it's simpler to acknowledge that we haven't seen any evidence of massive time vandalism that Hitler would probably attract.  Sure, it's possible one guy could pop back and cap him in a bunker and set it on fire, and nobody'd know the difference.  But as the future expanded more people would get access and be jumping back to mess with Hitler further.  And some point, there'd be a soup of time-jackers hopping in around Hitler to save him or kill him, and we all would notice that something unnatural was going on around this one dude as 2 parties with laser guns were battling it out.


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## delericho (Oct 22, 2012)

Jhaelen said:


> But how about being able to observe the past (i.e. without being able to change anything)?
> 
> Is there any argument against that being possible?




Well, you'd probably need to insert some sort of sensor to pick up the light/sound/whatever that you wanted to observe. And if you can send a sensor back in time, why not anything else?

But we're rapidly getting beyond the point I'm comfortable talking about (while sober, at least), so I'll have to leave off with "I don't know".


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## Thotas (Oct 22, 2012)

I have this hypothesis, that I'd love to talk to a qualified physicist/cosmologist about someday, regarding a possible explanation of the increasing rate of the expansion of the universe.  If that idea of mine is even close to right, no travel back in time at all, no how, no way.


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## Bullgrit (Oct 22, 2012)

> Some traveller would get hit by a car, have his arrival witnessed, people sieze his gear and figure out its a time machine.
> 
> timeline violators are likely to be ahead of the curve compared to time cops.



If some time traveler screwed up and revealed time travel, another could just go back and fix it. We in the "now" would never know. Just like with the "Hitler wasn't killed" idea, it isn't proof against time travel.

You can't really say that time travel doesn't exist because things are as they are. Things could be as they are *because* of time travel.

Anyone see the Family Guy episode where Stewie and Brian go back in time (again), Brian screws up the timeline, and they keep having to go back further to stop themselves from stopping themselves from stopping themselves? Eventually there were a couple dozen Stewies and Brians in the yard stopping their previous selves.

Bullgrit


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## Scott DeWar (Oct 22, 2012)

but then you create a paradox, which really screws things up worse.


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## GMforPowergamers (Oct 22, 2012)

What about the idea that you turn on your time machine on, and then can only go back that far...
Or the Sam becket only within your life time
Or 7 days


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## Nytmare (Oct 23, 2012)

Scott DeWar said:


> but then you create a paradox, which really screws things up worse.




Paradoxes don't exist in a Many Worlds setup.


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## Dannyalcatraz (Oct 23, 2012)

Time a travel is SOOooo yesterday.


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## Corathon (Oct 23, 2012)

Theoretically, general relativity allows one to create a time machine. (that doesn't mean that it will ever be possible in practice). Such a  time machine allows one to travel back in time, but no further back than the creation of the machine.

So, until someone builds a time machine there's no traveling back in time.


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## Morrus (Oct 23, 2012)

Corathon said:


> Such a time machine allows one to travel back in time, but no further back than the creation of the machine.




I don't know what version of General Relativity you've been reading, but that's not what it says about time travel at all.


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## Flatus Maximus (Oct 24, 2012)

Starbuck_II said:


> But Nazi sympasizers would also go back in time to prevent that. They would be the time roaches ofdthe universe.
> Who says Neo-Nazi's wouldn't be the one to create the machine?
> Hitletr did many great things without him, we would have less tecnologfy related to medicine.
> 
> ...




How the hell did this post not get struck by lightening immediately?


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## Dannyalcatraz (Oct 24, 2012)

I'll just say this about that: most of the "experiments" done by Hitler's "scientists" on humans lacked proper controls & methodology, and as a result, almost none of it was of any scientific value.


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## Nytmare (Oct 24, 2012)

Corathon said:


> Theoretically, general relativity allows one to create a time machine. (that doesn't mean that it will ever be possible in practice). Such a  time machine allows one to travel back in time, but no further back than the creation of the machine.
> 
> So, until someone builds a time machine there's no traveling back in time.




Which movie did you pick up this version of general relativity from?


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## Corathon (Oct 24, 2012)

Nytmare said:


> Which movie did you pick up this version of general relativity from?




I can't cite a movie, but how about a paper published in a well-known physics journal?

Tipler, Frank J., Rotating Cylinders and the Possibility of Global Causality Violation, Physical Review D, 9, 2203, 1974.


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## Corathon (Oct 24, 2012)

Morrus said:


> I don't know what version of General Relativity you've been reading, but that's not what it says about time travel at all.




See (coincidentally), Morris et al, 1988 (ref below) in which the authors speculate that a wormhole could be turned into a time machine. Such ideas may never be physically realizable, but they do exits in theory. General relativity is most definitely not my field, and maybe ideas have changed since I last paid much attention to it - but ideas for time travel have been discussed in physics journals.


Morris, Michael S., Kip S. Thorne, and Ulvi Yurtsever, Wormholes, Time Machines, and the Weak Energy Condition, Physical Review Letters, 61, 1446, 1988.


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## Morrus (Oct 24, 2012)

Corathon said:


> See (coincidentally), Morris et al, 1988 (ref below) in which the authors speculate that a wormhole could be turned into a time machine. Such ideas may never be physically realizable, but they do exits in theory. General relativity is most definitely not my field, and maybe ideas have changed since I last paid much attention to it - but ideas for time travel have been discussed in physics journals.
> 
> 
> Morris, Michael S., Kip S. Thorne, and Ulvi Yurtsever, Wormholes, Time Machines, and the Weak Energy Condition, Physical Review Letters, 61, 1446, 1988.




Nobody disputes that physicists have been using the theory to discuss theoretical time travel for decades; it's your added "can only go as far back as when the time machine was built" part that we're disputing.


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## Corathon (Oct 24, 2012)

Morrus said:


> Nobody disputes that physicists have been using the theory to discuss theoretical time travel for decades; it's your added "can only go as far back as when the time machine was built" part that we're disputing.




I think that, using the mechanism described in the Morris paper that I cited that the statement is true; one couldn't use the wormhole time machine to go back to a time before the time machine existed. Can you give a counter example?


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## Morrus (Oct 24, 2012)

Corathon said:


> I think that, using the mechanism described in the Morris paper that I cited that the statement is true; one couldn't use the wormhole time machine to go back to a time before the time machine existed. Can you give a counter example?




Of a_ time machine_? No, of course not. 

Perhaps it would help facilitate discussion if you were to quote the section of that paper you are referencing. I'm assuming (guessing), though, that the mechanism is wormhole-specific and requires each end to be at a location in spacetime; so yes, that particular method would be limited to the lifetime of the wormhole itself (not that a wormhole has ever been observed). Those theories also - as I understand them - require quantum effects to be taken into account, and relativity does not play well with those: thus the prevalent theory is that wormholes, were one ever to be found, could not be used in that manner and would destroy themselves. Kip Thorne (one of the co-authors of that paper) wrote a book about it, I believe. That wormhole theory is only one tiny part of the discussion on time travel, though. You can't take that and make global statements about time travel.


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## Janx (Oct 25, 2012)

Morrus said:


> Of a_ time machine_? No, of course not.
> 
> Perhaps it would help facilitate discussion if you were to quote the section of that paper you are referencing. I'm assuming (guessing), though, that the mechanism is wormhole-specific and requires each end to be at a location in spacetime; so yes, that particular method would be limited to the lifetime of the wormhole itself (not that a wormhole has ever been observed). Those theories also - as I understand them - require quantum effects to be taken into account, and relativity does not play well with those: thus the prevalent theory is that wormholes, were one ever to be found, could not be used in that manner and would destroy themselves. Kip Thorne (one of the co-authors of that paper) wrote a book about it, I believe. That wormhole theory is only one tiny part of the discussion on time travel, though. You can't take that and make global statements about time travel.




yeah, assuming a limit like that is like declaring that you need to consume gasoline in order to move from one place to another.  Just because one machine requires that, doesn't mean another does.

It's certainly reasonable that a machine built to talk to itself or to a partner component cannot do so prior to the existance of both ends of the pipe.

But that's not the model most fictional time machines work on.  Instead, they transfer themselves or their subject from now to then directly (without a reciever/landing platform).

Did we ever get a Physicist to answer the basic question of whether time travel was possible for BG?

Not from the sense of do we have a machine, or can we generate enough energy to do it, but in the same fashion of can we go faster than light (which is No.)


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## Corathon (Oct 25, 2012)

Morrus said:


> Of a_ time machine_? No, of course not.




I'm not asking for an example of an actual time machine; I thought that was obvious. I'm asking for a *theoretical* time machine (much like the one in the paper that I cited) that allows time travel back to a time before the machine existed.



Morrus said:


> Perhaps it would help facilitate discussion if you were to quote the section of that paper you are referencing.




I can't just swipe the the text, but the paper is available online. The relevant paragraph is just below Figure 2, on page 1447. The time machine is created by accelerating one end of the wormhole to relativistic speeds and then reversing the process. Essentially, it is the twin paradox, with the two mouths of the wormhole taking the place of the twins. Since time travel in this method would involve travel through the wormhole, one can't go back to a time before the wormhole was created.

I do understand that such a time machine may never be physically realizable; maybe no time machine is physically realizable, but that's not relevant to the point I'm trying to make. My point is that I am unaware of any theoretical time machine (using general relativity) that allows travel to a time before the machine was created. Since you seem sure that my statement to that effect was false, can you provide a (theoretical, not actual) example of a time machine based on general relativity that would allow one to travel back to a time before the machine was created? I could very well be wrong about this, but I'm not willing to accept that I am just based on an assertion that I am.


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## hopeless (Oct 25, 2012)

Bullgrit said:


> In your opinion or educated knowledge, is it possible to travel through time (backward and forward again)? [I don't mean possible with current science, but possible to do at all, even in the far, advanced scientific future.]
> I'm not talking about dual timelines, or splitting timelines, or anything other than true travel up and down our "real" timeline.
> Bullgrit




Only if it has already happened.

I'm not a fan of paradoxes because from my viewpoint time travel should only be possible if its revealed the world is as it is because they _had to go back to the past_ for their world to be what it is.

For example if someone from now hadn't been stuck in the past say around the second world war whose to say where we would be now, what if because of their presence and memories of what was to come it allowed the world to develop far more peacefully than had he or she not been present?

What if the microchip or the massive build up of American industry wouldn't have happened if someone hadn't gone back and recommended certain things be done to prepare his homeland for what was going to happen, he could have even tried to warn them about 9/11 but the information may have been passed to the wrong person and misused to bring about Homeland Security.

The idea about going back to kill your own Grandfather has always struck me as ridiculous after all your target of choice is based on what you know not what actually happened and its more likely you end up bringing about what caused you to make the trip in the first place after all Primeval might be a good sci fi tv show but when they got around to explaining the origin of those anomalies thats when they fell apart.


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## Janx (Oct 25, 2012)

Corathon said:


> I'm not asking for an example of an actual time machine; I thought that was obvious. I'm asking for a *theoretical* time machine (much like the one in the paper that I cited) that allows time travel back to a time before the machine existed.
> 
> 
> 
> ...




I suspect you have 1-upped the rest of us.  We all entered this discussion assuming a lack of information on a time machine, whereas you have arrived with specific information on a time machine proposal.

Since the rest of us are unhindered by our lack of knowledge, we have no such preclusions that a time machine has to be limited to its own existance timeframe.

My own time machine concept shifts the subject from our current quantum reality to one that is running "behind".  As such, I do not assume I need to build a landing pad (or expect one) to be on the other side.  to me, it would be like taking all the contents in the chamber and changing it's frequency to the new QR.  And poof.  time travel.  without paradoxes, per my other explanation on quantum realities.

I would expect it is a one way trip, unless I send the parts for a machine in advance (or send a guy capable of making a new one).

good times.


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## Dannyalcatraz (Oct 25, 2012)

The retromotive time machine was invented a long time ago:


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## Morrus (Oct 26, 2012)

Corathon said:


> I do understand that such a time machine may never be physically realizable; maybe no time machine is physically realizable, but that's not relevant to the point I'm trying to make. My point is that I am unaware of any theoretical time machine (using general relativity) that allows travel to a time before the machine was created. Since you seem sure that my statement to that effect was false, can you provide a (theoretical, not actual) example of a time machine based on general relativity that would allow one to travel back to a time before the machine was created? I could very well be wrong about this, but I'm not willing to accept that I am just based on an assertion that I am.




You've got your burden of proof backwards. You made the assertion - the burden upon you is to prove it, not in me to disprove it.  I cannot prove a negative. I cannot prove what relativity *doesn't* assert.  Failure to provide a real fireball spell does not prove that Sesame Street lyrics are a fireball spell; similarly, failure to provide a theoretical example of a time machine does not prove that your claim is true.

The logical flaw in your cited evidence is that it does not directly address the question. The question is what General Relativity stipulates about time travel; your cite does not address the overall question of time travel, merely one suggestion for a time machine. It at no point claims that this would be only method of time travel; thus it can not validate your assertion.


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## Richards (Oct 26, 2012)

I posted in this thread the mathematical formula that proves conclusively that time travel is possible, in either direction.

However, after people started building their own time machines based upon my posted formula, and getting into all sorts of trouble as a result, I went back in time and prevented myself from posting the formula here, thereby preventing all of that ruckus and mischief.

So never mind - please carry on with your discussion.

Johnathan


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## jonesy (Oct 26, 2012)

In the many worlds interpretation, assuming time travel to the past is possible, you have already gone back and killed your grandfather. You have also, at the same time, not have gone back at all, and have gone back and not killed him. Many worlds accounts for all possibilities. But nothing would change for your grandfather in your original starting point, because that starting point exists in a timeline where you didn't go back to kill him. The timeline where you did go back to kill him isn't your original starting point, and the 'changes' there were technically already there and lead through a different crossroads.

It's like a choice between being Doctor Manhattan or Paul Atreides. Doc can't change anything because he experiences every point of change at the same time and he is already having been making every one of his choices at the same time in every single point. Paul on the other hand sees every possible and impossible choice at the same time, even the ones he has already passed (like recognizing the other potential Kwisatz Haderach who were not to be).


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## Umbran (Oct 26, 2012)

This resident physicist will be back at his keyboard on Sunday or Monday, and will be able to address some of th questions then.


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## Corathon (Oct 26, 2012)

Morrus said:


> You've got your burden of proof backwards. You made the assertion - the burden upon you is to prove it, not in me to disprove it.  I cannot prove a negative. I cannot prove what relativity *doesn't* assert.  Failure to provide a real fireball spell does not prove that Sesame Street lyrics are a fireball spell; similarly, failure to provide a theoretical example of a time machine does not prove that your claim is true.
> 
> The logical flaw in your cited evidence is that it does not directly address the question. The question is what General Relativity stipulates about time travel; your cite does not address the overall question of time travel, merely one suggestion for a time machine. It at no point claims that this would be only method of time travel; thus it can not validate your assertion.




I've made a statement  - let's call it a hypothesis, "statement" is too definite - of the form "x does not exist". Citing 1 example of an x would surely prove me wrong.  Not logically impossible at all.


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## Morrus (Oct 26, 2012)

Corathon said:


> I've made a statement - let's call it a hypothesis, "statement" is too definite - of the form "x does not exist". Citing 1 example of an x would surely prove me wrong. Not logically impossible at all.




I can't think of how to present the logical flaw here more clearly: I recognise it, but my literary style is not up to the task. But there's a fundamental logical flaw in those two sentences, located in the logical fallacy of demanding proof of the negative (impossible). All I can say is - demanding that I disprove you is not proof of your assertion. Don't say "prove me wrong"; prove you're right.

There's an alien from Neptune behind my couch. Prove me wrong. Alpha Centauri is made of chocolate. Prove me wrong. The ghost of Henry VII visited me last night. Prove me wrong. I am Superman. Prove me wrong. Dark Energy is created by the psychic fields of aliens working at Walmart. Prove me wrong.

I can't prove you wrong. But I don't have to. A claim has to be proved right.


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## KarinsDad (Oct 27, 2012)

Corathon said:


> See (coincidentally), Morris et al, 1988 (ref below) in which the authors speculate that a wormhole could be turned into a time machine. Such ideas may never be physically realizable, but they do exits in theory. General relativity is most definitely not my field, and maybe ideas have changed since I last paid much attention to it - but ideas for time travel have been discussed in physics journals.
> 
> 
> Morris, Michael S., Kip S. Thorne, and Ulvi Yurtsever, Wormholes, Time Machines, and the Weak Energy Condition, Physical Review Letters, 61, 1446, 1988.




Kip and the boys were sitting around the pool smoking some pretty strong stuff when they thought this up.

*"Hey, I know. We can take a wormhole, yeah, yeah, I know, we have no evidence that they exist, hear me out, and we find both ends of this microscopic object in different parts of the galaxy. Well, we just do, ok? Then, we make it really really big on the inside and both ends. Of course that will take tons of energy. Yes, I don't know how to do that. But listen, listen. Then, we rotate one end near the speed of light in order to slow time down on that end. No, we'll rotate it so that the wormhold doesn't break or disintegrate or something. Just pretend for a minute that we are billions of years in the future and have all of this cool tech, ok? Then we send a spaceship into the end that is moving at normal time and it goes through and comes out the other end where time was slowed up. So, we start this thing up at 1 PM on both sides, send the ship through at 2 PM on the normal time side and on the other side, the ship comes out at 1:01 PM. And Hey! I know where we get the gazillion dollars to do this. We'll get a grant from the federal government."*

Just because mankind can think of an idea does not make that idea doable, even in theory.

There is a difference between the word "theory" in the fantasy sense (like Star Trek), and "theory" in the scientific sense (like reality). This idea wasn't anywhere near the scientific sense of the word. It just made for amusing TV science shows.


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## freyar (Oct 27, 2012)

I've skimmed this thread with some interest and a bit of bewilderment for a while but, like Umbran, haven't had time to comment yet.  I can give a few comments right now (as a practicing physicist working in gravitation).

One is that Many Worlds Theory in physics is not really about time travel.  It's about measurements in quantum theory as time goes forward in the usual sense.  It's also a bit of a misnomer: there is still just one world but with an increasingly complicated description (wavefunction).  Nonetheless, you could speculate that some quantum theory of gravity would relate that to time travel, but I'm not aware of any serious (ie, mathematically formulated) theory that does that.

As for what general relativity says: yes, there are spacetimes in general relativity, including wormholes, that have what are called "closed timelike curves," trajectories you can travel into your future that would allow you to come back and see yourself leaving.  So that's time travel in the usual sense.  On the other hand, these spacetimes tend to be unstable (meaning, you move any matter around, and they will collapse into a black hole or similar object).  I don't know of any theorem about that, but it's generally believed that closed timelike curves always get hidden behind black hole horizons or in some other way can never be used to time travel in actuality.


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## Janx (Oct 29, 2012)

One thing to ponder is the concept that Time does not exist as a discernable place to travel to.

Let's ignore the whole physics thing for a second (like traveling really fast slows time down).

At its core, the concept of time is acknowledging the sequence of events.  I sat down beneath a tree.  Then I ate a sandwich under the tree that I sat down under.  After the sandwich was gone, I pondered deep thoughts under neath the tree that I was still sitting under.  Then an apple from the tree fell down on my head as I sat under the tree with a full belly from the sandwich that I ate.

While we remember these events as if they were places we could revisit, they are simply sequences of events AT physical places.  time inherently moves forward because we observe the world and see things happen in sequence.

It is not probable that an intelligent life form exists that observes all reality and time before events happen.  This Omniscient being is unlikely to know an apple is going to fall on my head in 4.5 billion years and thus get the idea for Newton's laws of physics by knowing something WILL happen, but hasn't happened yet.  By "knowing" all things out of order, it literally can't move from one idea to the next because it quite literally must have all ideas at once by nature of being defined as "not experiencing time" or seeing all events at once.

Anything that observes or interacts with the world sequentially is bound to time, not as a place, but simply as the mechanic that events happen in sequence.

Now physicists can step in and mention that Time is wierder than I ascribe.  Given that we can launch a ship at near light speed and when it gets back, the clock on it will read that less time passed inside the ship, than we all experienced.

Though time itself didn't re-order anything.  It simply slowed down for those inside the ship.  And that's not the same as time existing as a place that one can travel backwards to.


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## the Jester (Oct 29, 2012)

Morrus said:


> Nobody disputes that physicists have been using the theory to discuss theoretical time travel for decades; it's your added *"can only go as far back as when the time machine was built"* part that we're disputing.




I've seen this asserted in a number of physics-for-the-layman books, speculative science programs, etc. It seems to be a fairly widely-accepted view amongst scientists who do scientific speculation about time travel.


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## Morrus (Oct 29, 2012)

the Jester said:


> I've seen this asserted in a number of physics-for-the-layman books, speculative science programs, etc. It seems to be a fairly widely-accepted view amongst scientists who do scientific speculation about time travel.




Which ones? 

We're not talking speculation here; we discussing the claim that General Relativity specifically asserts that time travel works in this manner.


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## Umbran (Oct 29, 2012)

Corathon said:


> Theoretically, general relativity allows one to create a time machine. (that doesn't mean that it will ever be possible in practice). Such a  time machine allows one to travel back in time, but no further back than the creation of the machine.






Morrus said:


> I don't know what version of General Relativity you've been reading, but that's not what it says about time travel at all.




Both of you are correct, and both incorrect.

Relativity says a couple things on the subject of time travel:

1) If you can travel faster than light (you can't, but if you could), then you can travel in time.  There's no "time machine" other than the FTL ship, and it can travel to times prior to its own creation.

2) There are certain curvatures of spacetime (usually around "compact objects", things like black holes and neutron stars) that could allow you to fly what *freyar* mentioned - a "closed timelike curve".  You're never moving faster than light, but you end the route before you started it.  This kind of time travel does not allow you to move back before the curvature created by the compact object existed.  So, if you are flying a weird course around a black hole to travel in time, you cannot go to times before the star collapsed to create the black hole.  There are possibly some compact objects that can be used that have existed since the beginning of the universe, or the first inflationary period of the universe, such that how far back you could go using them is not an issue for human terms.  You would, of course, have to reach the objects in the first place, and none of them are anything like "nearby".

When we get to wormholes, things get funky.

A wormhole is a shortcut through spacetime, and it is allowed by General relativity.  It is a shortcut through space_time_, so you can go to the place, and time, of either end of the wormhole.

However, there's a catch - the wormholes of general relativity are not stable.  Flying a spaceship through one is likely enough of a disturbance to cause the thing to snap shut, destroying you and your ship in the process.  One can theoretically stabilize a wormhole with, get this, "negative energy".

Now, general relativity doesn't tell you how to make negative energy.  Quantum mechanics does. But most of you probably know that quantum mechanics and general relativity as we know them don't get along very well.  They don't speak to each other.  So, can you use the combination to create a wormhole time machine that can go back to some arbitrary period in time?  Or would such a thing only be able to use its creation as the earliest end of the wormhole?  

Nobody can say.  The math to describe how to do this doesn't exist yet.

General relativity does not speak to the "many worlds" (or any other quantum mechanical) time travel.  In fact, "Many Worlds" time travel, in which you don't actually travel in time, but you travel to some other branch of the many-worlds tree, is as far as I understand it, a complete fabrication of science fiction.  It is a simple fictional crossing between the "many Worlds Hypothesis" and the idea of time travel, without any theoretical math supporting it, as far as I am aware.


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## Morrus (Oct 29, 2012)

Umbran said:


> Both of you are correct, and both incorrect.
> 
> Relativity says a couple things on the subject of time travel:
> 
> ...




Aha, but this makes me completely correct and not at all incorrect. You've kindly elaborated upon my point in more detail. Indeed, it's _exactly_ what I said - Relativity does not forbid time travel prior to the creation of the device; the fact that it may allow for some solutions which are so restricted does not negate the fact that it allows for solutions (FTL, for example) which are not.

I admit, though, I was being mildly stubborn in not bringing up potential solutions such as FTL because I was being obstinate about the burden of proof.


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## Bullgrit (Oct 29, 2012)

> However, there's a catch - the wormholes of general relativity are not stable. Flying a spaceship through one is likely enough of a disturbance to cause the thing to snap shut, destroying you and your ship in the process. One can theoretically stabilize a wormhole with, get this, "negative energy".



What does "destroying" a spaceship mean in this kind of context?

Bullgrit


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## Umbran (Oct 29, 2012)

Morrus said:


> the fact that it may allow for some solutions which are so restricted does not negate the fact that it allows for solutions (FTL, for example) which are not.




Except, of course, that both special and general relativity explicitly disallow FTL travel.  Bit of an issue there to you being right on that count.  

I mentioned it because it is the usual thing brought up in time travel discussions: when you put a speed greater than c into the equations of relativity, time travel falls out.  But let us be clear - this is not relativity "allowing" for the solution.  It is you putting in something that doesn't fit in the model.



> I admit, though, I was being mildly stubborn in not bringing up potential solutions such as FTL because I was being obstinate about the burden of proof.




You were, however, entirely correct on the burden of proof.  The burden of proof sits on the one who makes the assertion, not on those who question the assertion.


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## Umbran (Oct 29, 2012)

Bullgrit said:


> What does "destroying" a spaceship mean in this kind of context?




Well, honestly, this is something that's outside solid models and mathematically-grounded theory.  

The typical expectation is that the hole actually is perturbed enough to snap shut before you really enter it.  There is gravitational energy tied up in the wormhole's existence.  It will be released as the hole snaps shut, with a burst of energy in the form of photons and elementary particles from both ends of the wormhole*.  For a wormhole large enough to pass a ship through, that should be enough energy to simply vaporize your ship.

If you do manage to be deep inside the wormhole, far from either end, the common expectation is that the mass of your ship just joins with the energy tied up in the wormhole, so you get squirted out both ends of it as elementary particles and light.


*For those who know about Hawking radiation, the effect is similar to the final moments of a black hole's life.


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## Umbran (Oct 30, 2012)

Going back to address some of the other questions that cropped up....



Jhaelen said:


> But how about being able to observe the past (i.e. without being able to change anything)?
> 
> Is there any argument against that being possible?




There is at least one, and it is subtle...

It is thermodynamically impossible to transmit information without also transmitting energy.  Therefore, to get information out of the past, you also take energy out of the past.  That energy was in the past before you observed, and now it isn't - the past is thus changed (perhaps a miniscule amount, perhaps not in a way you intend to control, but change nonetheless) by your observation.  Ergo, there is no such thing as "observe, but not change".




Thotas said:


> I have this hypothesis, that I'd love to talk to a qualified physicist/cosmologist about someday, regarding a possible explanation of the increasing rate of the expansion of the universe.




The current wisdom is that this is explained by the presence of sufficient "dark energy" (which, I know, sounds like, "It's magic!!!1!").  This happens to be pretty consistent with General Relativity, and doesn't have profound implications for time travel.  



delericho said:


> The problem with that is that every single event (on a quantum level, no less) creates an entirely new universe - duplicating everything that exists. Where does all the energy come from?




There are a couple of ways to think of that answer, but here's a simple one: Let us consider the case where there are not Many Worlds.  Where does the energy for the one single Universe come from?

The energy for the Many Worlds ensemble of universes comes from the exact same place.  Of course, "place" is not the right word, and here we get into one of the places where cosmology and quantum mechanics give people headaches... 



> I suppose it's possible that there could be a finite (and fixed) number of parallel timelines, constantly splitting from and converging with one another, in some sort of ball of wibbly-wobbly timey-wimey _stuff_. But that does seem rather unlikely.




It does seem unlikely, but probably not for the reason you think it does.  Recall that the ideas of "where" and "when" (as in, "_Where_ does the energy come from _when_ the universe splits?") are *internal* to the universe.  The idea of conservation of energy (even the phenomenon of "energy") is likewise internal to the universe, and only holds within our spacetime.  Our clocks and rulers (by which we define energy) do not function, apply, or exist outside our spacetime.  

Basically asking where the energy comes from is rather like asking where time comes from, or where space comes from.  In a sense, the energy does not come from anywhere - it is a function of the mere existence of the universe.


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## Thotas (Oct 31, 2012)

Umbran said:


> Going back to address some of the other questions that cropped up....
> 
> The current wisdom is that this is explained by the presence of sufficient "dark energy" (which, I know, sounds like, "It's magic!!!1!").  This happens to be pretty consistent with General Relativity, and doesn't have profound implications for time travel.




Not so much "it's magic" but my understanding is that the reason it's called "dark" and put in quotations like that is that nobody really knows for sure what it is.  That's what my layman's/crackpot hypothesis addresses, although even if there is something to it I'm sure it would have implications I totally wouldn't understand.


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## Umbran (Oct 31, 2012)

Thotas said:


> Not so much "it's magic" but my understanding is that the reason it's called "dark" and put in quotations like that is that nobody really knows for sure what it is.




Close.  It is called dark for the same reason "dark matter" is called dark - it doesn't actively shine in a way we can see it.

As for what it is - it is energy that is in a form that causes no impact currently measurable on short distances, but that in aggregate has a repulsive effect over very large distances.


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## Nagol (Oct 31, 2012)

Umbran said:


> Close.  It is called dark for the same reason "dark matter" is called dark - it doesn't actively shine in a way we can see it.
> 
> As for what it is - it is energy that is in a form that causes no impact currently measurable on short distances, but that in aggregate has a repulsive effect over very large distances.




And short means "within the heliosphere".  There was an anomaly detected on the Pioneer craft's speed, but it was shown to be function of heat radiation.  http://en.wikipedia.org/wiki/Pioneer_anomaly


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## Umbran (Oct 31, 2012)

Nagol said:


> And short means "within the heliosphere".




Actually, I think short is still "within the galaxy".  The effects of Hubble expansion are not generally measured on things within the Milky Way, but only when you start looking out at the rest of the Universe.


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## Morrus (Oct 31, 2012)

Umbran said:


> Close. It is called dark for the same reason "dark matter" is called dark - it doesn't actively shine in a way we can see it.




I was given to understand that's the name which was catchy enough to catch no with the newspapers and the like and that there were dozens of names floating round until the media effectively made the call.


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## Umbran (Oct 31, 2012)

Morrus said:


> I was given to understand that's the name which was catchy enough to catch no with the newspapers and the like and that there were dozens of names floating round until the media effectively made the call.




There are dozens of fanciful names for the possible specific forms of dark matter.  I, personally, am a fan of WIMPs* and MACHOs**.  Physicists tend to be fanciful - we call things "quarks" and bet each other if we can get away with using odd words in papers, leading to the "penguin diagram".

But for the overall terms "dark matter" and "dark energy", I don't recall there were really any major contenders.  They were coined by Fritz Zwicky (in 1933) and Michael Turner (in 1998, recalling Zwicky intentionally) respectively.



*Weakly Interacting Massive Particles
**Massive Compact Halo Objects


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## Dannyalcatraz (Oct 31, 2012)

Physics needs more DAMNs.*






* Dorky Acronyms Meaning Nothing


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## Umbran (Oct 31, 2012)

And, it being All Hallow's Eve, this seems particularly relevant:

_"Benjamin K. Tippett has a theory. The University of New Brunswick mathematician believes that he’s figured out what, exactly, those insane sailors saw that night in 1928 when they encountered Cthulhu on a lost island in the Pacific. And so Tippett has written a hilariously deadpan paper explaining “non-Euclidean geometry” once and for all."_

Possible Bubbles of Spacetime Curvature in the South Pacific

The final conclusion - that only entities capable of FTL travel (and thus spacetime cloaking and time travel) could generate the observed phenomenon - is quite interesting...


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## Corathon (Oct 31, 2012)

Morrus said:


> I can't think of how to present the logical flaw here more clearly: I recognise it, but my literary style is not up to the task. But there's a fundamental logical flaw in those two sentences, located in the logical fallacy of demanding proof of the negative (impossible). All I can say is - demanding that I disprove you is not proof of your assertion. Don't say "prove me wrong"; prove you're right.




I think that proving I'm right might be a logical impossibility; the old "you can't prove a universal negative" thing.



> There's an alien from Neptune behind my couch. Prove me wrong.




Were I at your house I could look behind the couch. 




> Alpha Centauri is made of chocolate. Prove me wrong.




I think that spectroscopic evidence pretty well rules that one out.



> The ghost of Henry VII visited me last night. Prove me wrong.




I can't, as I wasn't with you last night.



> I am Superman. Prove me wrong.




If we were in the same room, that would be fairly easy; I could take a pin and poke you. Not that I'd do such a thing, because I'm pretty sure that you're not actually Superman. 



> Dark Energy is created by the psychic fields of aliens working at Walmart. Prove me wrong.




I can't prove you wrong on that one - but "dark energy" is pretty much a label for ignorance anyway.



> I can't prove you wrong. But I don't have to. A claim has to be proved right.




As I said above, one counterexample proves me wrong. I guess that we just have to agree to disagree on this.


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## Thotas (Oct 31, 2012)

Umbran said:


> As for what it is - it is energy that is in a form that causes no impact currently measurable on short distances, but that in aggregate has a repulsive effect over very large distances.




Right, but isn't that more a description of the situation than an explanation for the effect?  "Oh, look, there's a repulsive effect adding up over large distances that is unnoticeable at (cosmic scale) small distances."  And since that's work, there has to be an energy responsible because the potential to do work is more or less the definition of energy in physics.  Last I heard, it was pretty well agreed that there's more to find out there ... of course, with science, we always find out there's more to find out there.


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## Morrus (Oct 31, 2012)

Corathon said:


> As I said above, one counterexample proves me wrong. I guess that we just have to agree to disagree on this.




On the burden of proof? Yes, I guess so. You made the claim. You may have heard the phrase "If this responsibility or burden of proof is shifted to a critic, the [logical] fallacy of appealing to ignorance is committed".


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## Umbran (Oct 31, 2012)

Thotas said:


> Right, but isn't that more a description of the situation than an explanation for the effect?




Yes, but that's most of science.  The difference between describing the situation and explaining the effect is how much information you have - when you have enough information, the description *is* the explanation.


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## Thotas (Oct 31, 2012)

Umbran said:


> Yes, but that's most of science.  The difference between describing the situation and explaining the effect is how much information you have - when you have enough information, the description *is* the explanation.




And again, I get that ... I guess what I'm saying is that it's my understanding that physicists don't think they have enough info to unify description and explanation into one thing on this issue.  I'm one of those guys who reads a lot of lay science books and wishes news media did a better job of reporting on science both in terms of quality and quantity, but a long way from being any kind of expert.  But when the experts are saying something needs better explanation, I tend to think it must.


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## Umbran (Oct 31, 2012)

Corathon said:


> As I said above, one counterexample proves me wrong. I guess that we just have to agree to disagree on this.




The point, though, is that you don't get to make an unsupported assertion, and say, "I am right unless you prove me wrong."  In general, burden of proof sits with the person who makes the assertion - if you can't support it, you shouldn't make the assertion.  

Let us see if we can put this one to bed: General relativity allows some forms of time travel, in the sense that it doesn't itself actively prohibit them.  GR allows for a solution that has a wormhole from point A to point B, for example.  

GR doesn't speak to how that wormhole is created, however.  GR _does not actively prohibit_ a box with a big red button on it that creates a wormhole to a specified point in the past.  Thus, GR allows time travel to arbitrary times before the machine was created, in the same sense that it allows time travel at all.


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## Umbran (Oct 31, 2012)

Thotas said:


> I guess what I'm saying is that it's my understanding that physicists don't think they have enough info to unify description and explanation into one thing on this issue.




And you'd be entirely correct in that.  No argument there.


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## Bullgrit (Oct 31, 2012)

Morrus said:
			
		

> On the burden of proof? Yes, I guess so. You made the claim. You may have heard the phrase "If this responsibility or burden of proof is shifted to a critic, the [logical] fallacy of appealing to ignorance is committed".



I think you and Corathon kind of got turned around in this back and forth about burden of proof. Corathon said a time machine couldn't go back to a time before it was created. You can't really expect him to prove that limitation anymore than you can prove a time machine *can* go back to a time before it existed.

It's like saying "Martians can't fly," and having someone say "prove it." One can't really prove they *can* fly, either.

Bullgrit


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## Morrus (Oct 31, 2012)

Bullgrit said:


> I think you and Corathon kind of got turned around in this back and forth about burden of proof. Corathon said a time machine couldn't go back to a time before it was created. You can't really expect him to prove that limitation anymore than you can prove a time machine *can* go back to a time before it existed.




If General Relativity said such a thing (which was the claim), I believe that locating a citation would be fairly trivial. He doesn't have to do the math himself; just cite where he got that information from - assuming it's a decent source. It's far from an unreasonable burden to cite where one found the information that led one to claim that a very well-known theory said a particular thing. Heck, if we're deeming something that trvial unreasonable or too difficult, let's just stop debating anything altogether!


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## Dannyalcatraz (Nov 1, 2012)

If only we could go back in time and clarify things so the disagreement never started...


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## Umbran (Nov 1, 2012)

Bullgrit said:


> Corathon said a time machine couldn't go back to a time before it was created. You can't really expect him to prove that limitation anymore than you can prove a time machine *can* go back to a time before it existed.




If some reasonable authority had made the claim, he could refer to that as support - it isn't proof, but it is support.

But, if he can't do the math himself, and he can't refer to another reasonable expert, perhaps he shouldn't make claims as to what General Relativity does or does not say.


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## Bullgrit (Nov 1, 2012)

> what General Relativity does or does not say.



I'm not trying to argue Corathon's point/assertion, here. I just have a question that comes up from Umbran's statement.

Does General Relativity say that reverse time travel is possible. I thought GR just said time slows down closer to c, and nothing can go faster than c. Does it say/assert/insinuate/prove that going over c (which doesn't GR say can't happen?) reverses time?

Bullgrit


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## Umbran (Nov 1, 2012)

Bullgrit said:


> Does General Relativity say that reverse time travel is possible. I thought GR just said time slows down closer to c, and nothing can go faster than c. Does it say/assert/insinuate/prove that going over c (which doesn't GR say can't happen?) reverses time?




Relativity says you can't go faster than c.  If you ignore what relativity tells you, and plug speeds faster than c into the equations of relativity anyway, the results effectively allow you to travel in time.  

But, on top of that, there are some oddball situations in GR that allow you to travel in time, even if you don't exceed c.  Like, if you have a infinite (possibly only a very, very long) cylinder made of neutronium (or something similarly dense), that is spinning on it's long axis such that it's surface is traveling near c, you can theoretically fly a ship around it in a specific path such that you return to your starting position before you left.

GR also allows for wormholes through spacetime, which have much the same effect as traveling faster than c, but you never actually exceed the speed of light in your travels.  GR "allows" wormholes, but note that nobody's ever observed one.


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## Nellisir (Nov 1, 2012)

Dannyalcatraz said:


> If only we could go back in time and clarify things so the disagreement never started...




Again?  You should've seen the first iteration of this thread!


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## Dannyalcatraz (Nov 1, 2012)

Nellisir said:


> Again?  You should've seen the first iteration of this thread!




I have always been in this thread.

I have never been in this thread before now.


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## RSKennan (Nov 1, 2012)

I'd like to assert for the record that General Relativity says that I'm sexier than any of my relatives.


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## freyar (Nov 1, 2012)

Thotas said:


> Right, but isn't that more a description of the situation than an explanation for the effect?  "Oh, look, there's a repulsive effect adding up over large distances that is unnoticeable at (cosmic scale) small distances."  And since that's work, there has to be an energy responsible because the potential to do work is more or less the definition of energy in physics.  Last I heard, it was pretty well agreed that there's more to find out there ... of course, with science, we always find out there's more to find out there.






Thotas said:


> And again, I get that ... I guess what I'm saying is that it's my understanding that physicists don't think they have enough info to unify description and explanation into one thing on this issue.  I'm one of those guys who reads a lot of lay science books and wishes news media did a better job of reporting on science both in terms of quality and quantity, but a long way from being any kind of expert.  But when the experts are saying something needs better explanation, I tend to think it must.






Umbran said:


> And you'd be entirely correct in that.  No argument there.




This is actually one of the things I've done some (research) work on.  It's not entirely true that experts can't "unify description and explanation" on dark energy.  It's I think more that there are some different possible explanations.  Let me explain the situation, and then you can decide if I'm splitting hairs. 

Within the context of general relativity, expanding space does not necessarily require work or reduce the total amount of energy.  For example, normal matter scattered through space causes space to expand (in a way that slows down as the expansion of space spreads the matter out).  In this case, if you were to define a region bounded by some set of galaxies, you'd find that the total energy in that region stays the same over time but gets less dense because that region has more volume.

What we see in our universe is that it is expanding and that expansion is actually speeding up.  What that requires is that our universe be filled with something that does not dilute as fast as normal matter.  The simplest possibility is that it is a constant energy density.  What this means is that, as space expands, each cubic centimeter always has the same energy in it, so the "total energy" always increases!  (I put that in quotes, though, because the "total energy" of a possibly infinite universe doesn't make a ton of sense in this context.)  In any case, that's the usual cosmological constant that Einstein first thought about.  But it's also possible that you have something like a cosmological constant that decreases slowly with time.  There are lots of models you can make that will do that.  I'd say that some make decent physical sense and a few just seem to be made up.

The other logical alternative is that Einstein's gravity is incorrect, so there is another theory of gravity.  In this case, the universe likes to expand faster and faster even with normal matter (or maybe even nothing) in it.

I guess to be fair is that there's one other possibility, too.  That possibility is that what we think we see in the measurements isn't correct and that the universe is actually slowing down.  So you have to give an argument about how that might happen.

So it's not like there aren't explanations beyond description.  The problem is more like having too many, and we will need more data to distinguish among them, so right now people just call the right one "dark energy."  I think the "mystery" of dark energy gets played up a bit much to the public because people are quite passionate about this.  A lot of it goes back to the cosmological constant.  From a particle physics point of view, it must have a very tiny value (if it is the dark energy, it is 10^-123 times the naive particle physics value).  For a very long time, people believed it must be zero and that the universe's expansion was slowing down.  Now that we think the expansion is speeding up, a lot of people still want the cosmological constant to be zero.

So there are plenty of explanations.  Of course, next we want to ask why does our favorite explanation come out the way it does.  For example, if you like the cosmological constant, why does it have such a small but nonzero value?  That's a bit trickier, worth a whole lot more than one post, and frankly controversial in a number of ways.


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## Nellisir (Nov 2, 2012)

freyar said:


> The simplest possibility is that it is a constant energy density.  What this means is that, as space expands, each cubic centimeter always has the same energy in it, so the "total energy" always increases!




So that's where the time travelers are going....


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## tomBitonti (Nov 2, 2012)

freyar said:


> Within the context of general relativity, expanding space does not necessarily require work or reduce the total amount of energy.  For example, normal matter scattered through space causes space to expand (in a way that slows down as the expansion of space spreads the matter out).  In this case, if you were to define a region bounded by some set of galaxies, you'd find that the total energy in that region stays the same over time but gets less dense because that region has more volume.




I've always wondered about that ... does expansion have no effect on a large but gravitationally bound system such as a galaxy?  If it doesn't, how are odd effects (anomalous curvature) prevented when transitioning from a uniform expansion in deep intergalactic space to a uniform lack of expansion inside of a galaxy?  But if expansion affects the region inside of a galaxy, that would be moving stars every so slightly but gradually further apart, in doing adding energry to the stars.

TomB


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## Umbran (Nov 2, 2012)

freyar said:


> For example, normal matter scattered through space causes space to expand (in a way that slows down as the expansion of space spreads the matter out).




Stop right there.  

There is no evidence that normal matter itself causes expansion.  My understanding is that stuff with positive energy density, like normal matter *cannot* be responsible for such expansion, and having normal matter actually cause expansion is, so far, apparently unnecessary to explain the phenomenon.  Much of the rest of what you say is close enough to current models, but this one bit is contrary to the current observations.



> I guess to be fair is that there's one other possibility, too.  That possibility is that what we think we see in the measurements isn't correct and that the universe is actually slowing down.  So you have to give an argument about how that might happen.




Might as well say you have to have an explanation for the possibility that, while I see an apple on my desk, can pick it up, and bite it, there actually *isn't* an apple on my desk.

You don't have to have explanations for things that aren't observed.  We observe an acceleration of the expansion.  We have double checked those results, corroborated them with several different research groups using different methods.  You don't have to explain how all of them might be wrong, until you see evidence they are, in fact, wrong.



> So it's not like there aren't explanations beyond description.  The problem is more like having too many, and we will need more data to distinguish among them




Yes, and that's exactly my point.  You can make up an oodle of cute stories about how the Universe works.  Data comes along and shows most of those stories are not consistent with reality.  Eventually, you have enough data to prove that all but a very small number of those stories are not consistent with reality.  This is where explanation and description become one and the same.

For "dark energy" we are nowhere near discarding most of the stories.  We only have a small amount of relevant data.  There are a ton of still possible explanations, but only a very small amount of description of what's happening.  The two are by no means one and the same.



> For example, if you like the cosmological constant, why does it have such a small but nonzero value?  That's a bit trickier, worth a whole lot more than one post, and frankly controversial in a number of ways.




No, it isn't.  At least, it is no trickier than why anything has its one particular value.  It is no trickier to explain this value than to explain the charge of the electron, for example.  That there is something weird about it being small is a figment of human perspective and ideas of scale, to which the Universe is by no means beholden.



tomBitonti said:


> I've always wondered about that ... does expansion have no effect on a large but gravitationally bound system such as a galaxy?




Not no effect.  Just so small that it doesn't impact things.

Within a galaxy, the density of matter is very high, so gravitation dominates over those distances.  The repulsion is there, but within a galaxy, the attraction of gravity is much stronger. So you have a strong pull, and a weak push, and the pull wins.

Out in the great vastness between galaxies, however, the normal matter is very, very thin, and the energy associated with the vacuum dominates, so the repulsion dominates.

There's nothing at all odd about this - take a bar magnet, and pick up a pin with it.  Locally, electromagnetic forces are overcoming the force of gravity on that pin.  Gravity is still there, but it isn't enough to pull the pin away from the magnet.  



> If it doesn't, how are odd effects (anomalous curvature) prevented when transitioning from a uniform expansion in deep intergalactic space to a uniform lack of expansion inside of a galaxy?




We aren't talking about sudden, sharp transitions.  There isn't some sharp wall at the edge of the galaxy, or something.  There's just a slow, smooth transition from the area around galaxies out into the deeps.


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## tomBitonti (Nov 2, 2012)

Umbran said:


> We aren't talking about sudden, sharp transitions.  There isn't some sharp wall at the edge of the galaxy, or something.  There's just a slow, smooth transition from the area around galaxies out into the deeps.




Yup, I would expect a smooth transition.  (Although, the curvature changes seem small.  Could the transitions be discrete after all?)

If the effect is as you imply --  present in (mostly) empty intergalactic space but absent inside of galaxies -- wouldn't that make for a "stretched" region of transition, with detectable affects on light passing through the stretched region?

Thx!

TomB


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## Umbran (Nov 2, 2012)

tomBitonti said:


> Yup, I would expect a smooth transition.  (Although, the curvature changes seem small.  Could the transitions be discrete after all?)




Insofar as these days we expect everything to have a representation in quantum mechanics, yes.  Otherwise, no.  

The force of gravity does not have a discrete cut-off with distance.  Neither does the "force" that results from the vacuum energy - both are continuous functions.  The sum of two continuous functions will be continuous, with no discrete discontinuities.  



> If the effect is as you imply --  present in (mostly) empty intergalactic space but absent inside of galaxies -- wouldn't that make for a "stretched" region of transition, with detectable affects on light passing through the stretched region?




No.  As Freyar noted, the vacuum energy (the simplest form of explanation, so we'll use that for explication purposes) is uniform throughout the entire universe.  It is a result of space itself.  Space is everywhere, the energy is everywhere, even within galaxies, and so it has a uniform effect *everywhere*.  It is an even background, on which locally sometimes gravity weighs in.  

What we do see is its effect on the rate of change of expansion of the space: not only is the universe expanding, but it will be expanding tomorrow just a tad faster than it is expanding today.


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## tomBitonti (Nov 2, 2012)

Umbran said:


> No.  As Freyar noted, the vacuum energy (the simplest form of explanation, so we'll use that for explication purposes) is uniform throughout the entire universe.  It is a result of space itself.  Space is everywhere, the energy is everywhere, even within galaxies, and so it has a uniform effect *everywhere*.  It is an even background, on which locally sometimes gravity weighs in.
> 
> What we do see is its effect on the rate of change of expansion of the space: not only is the universe expanding, but it will be expanding tomorrow just a tad faster than it is expanding today.




Then wouldn't you expect the space _inside_ a body such as a galaxy to expand?  For any small period of time the effect would be unmeasurable, but over time the effect would show as a continuous expansion (perhaps which is immediately consumed as a contraction and a small addition of energy).

I think that I've heard before that this doesn't happen.  if it doesn't, why not?  What is wrong with my naive approach that is creating this problem?

Thx!

TomB


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## freyar (Nov 2, 2012)

tomBitonti said:


> Then wouldn't you expect the space _inside_ a body such as a galaxy to expand?  For any small period of time the effect would be unmeasurable, but over time the effect would show as a continuous expansion (perhaps which is immediately consumed as a contraction and a small addition of energy).
> 
> I think that I've heard before that this doesn't happen.  if it doesn't, why not?  What is wrong with my naive approach that is creating this problem?
> 
> ...



The difficulty is that the effects of the overall expansion of the universe (whether the accelerating kind or a more mundane decelerating kind) do not just "add" with the effects of the matter in the galaxy on gravity.  (We say the equations of general relativity are "nonlinear" because the results are not additive.)  So, while I don't have the exact solutions to the equations at hand in this case, what you do expect to happen is that the gravitational attraction of the matter in the galaxy to "win" over the expansion of the universe, so the space inside the galaxy shouldn't stretch.  In other words, the galaxy is "gravitationally bound," so the distance across the galaxy shouldn't be increased by the overall expansion of the universe.  That will only affect distances between objects that aren't bound together.  

Another point to make is that, even if the expansion of the universe were happening in our galaxy, it would be a very small effect.  The expansion is only noticeable on very large distance scales (for example, the expansion rate between our Milky Way and Andromeda, the closest large galaxy, is quite slow).

What's interesting is when you get borderline cases, like galaxies that are gravitationally attracted to each other but also being swept apart by the expansion of the universe.  Then there is a legitimate competition between the two effects.

I hope this doesn't muddy the picture, but cosmology is a big and sometimes complicated subject.


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## Umbran (Nov 3, 2012)

tomBitonti said:


> Then wouldn't you expect the space _inside_ a body such as a galaxy to expand?




No.  Remember above, with the bar magnet?  Once the magnet has picked up the nail or pin, do you expect gravity to slowly, very slowly, pull the pin away anyway?  No.  It's there, it is stuck, and it isn't gonna move.

The forces of gravity are strong near matter. Very close to matter, space isn't expanding at all (very close to really big chunks of matter, space collapses - black holes!).  As you move away from matter, though, the force of gravity decreases.  Eventually, the force of gravity is small enough that it no longer counters the effective forces of the dark energy, and way out there, expansion accelerates.

In fact, the universe is mostly empty space - there's a whole lot of "force of gravity is small" out there.  So, on the whole, the universe is expanding, and that expansion is accelerating.  It is only near these tiny islands of galaxies where this is not true.


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## KarinsDad (Nov 3, 2012)

Until the late 80s when Voyager 2 passed Neptune, the then current scientific theory was that Neptune had very little in the way of winds due to its vast distance from the Sun and lack of solar heat for convection. However, Neptune has the fastest measured winds in the solar system, getting up over 2000 km per hour. Science was fairly advanced in the last 80s. Dark Matter theories had been around for 60 years. Einstein came up with most of his ideas 50 to 85 years earlier. How could scientists be so wrong on something so fundamental so recently?

The problem with Dark Energy (and Dark Matter) and some other theories where scientists are using a set of observations to come up with an educated guess, is that it's still pretty much a guess. Yes, these are educated and calculated guesses based on current data, but scientists really don't know yet. Until they do (requiring a lot more data, observation, and calculation), discussing theories like these is fun, but it's probably not going to lead to the actual truth (or at least when lay people such as myself discuss them on a web forum).

As an example, some scientists claim that the Large Hadron Collider can recreate conditions during the first few billionths of a second of the Big Bang. Well, maybe it can or maybe it cannot, but since nobody was around during those moments of the Big Bang, there's a chance that these scientists are wrong on this. They were wrong about Neptune. And, maybe scientists have found the Higgs Boson, but then again, even if they call it that, it might not actually be that.

Scientists have been searching for Dark Matter for nearly 80 years and still have not found it. Nor has any one set of Dark Matter equations actually worked for every galaxy that they used them on. Maybe part of the stellar attraction in galaxies is due to electromagnetic fields (which also influence at the square of the distance).

Dark Energy theory is only a little under 15 years old, so yeah, it's still fairly new. It might be wrong since it is primarily (but not solely) based on one type of observation and there is at least one alternative explanation, but it's still too early to know.

Interestly though, a lot of scientists think that the universal expansion is accelerating. Maybe it is. Maybe it isn't. But, we need other verification to be sure. If it isn't accelerating, then we get to throw out the most recent text books on at least part of the Lambda-CDM model.

The problem with many of our current theories of the universe are that many of them are built upon earlier theories that aren't necessarily 100% reliable or factual. Knock out one of the earlier ones and the current ones might collapse. For example, prove that the Big Bang never happened and many billions of dollars and lifetimes of research pretty much fall by the wayside.


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## Umbran (Nov 3, 2012)

KarinsDad said:


> Dark Matter theories had been around for 60 years. Einstein came up with most of his ideas 50 to 85 years earlier. How could scientists be so wrong on something so fundamental so recently?




Well, at the moment we are talking more about dark energy than dark matter.

For dark energy:  Einstein originally put in his cosmological constant, because he needed the term there in order to make the Universe static (neither expanding nor contracting).  Edwin Hubble's observations proved that the universe was expanding - Einstein, given that information, felt the constant was unnecessary to explain the observed phenomenon, so he removed it.

It is only more recently that our instruments have become precise enough to measure the not only the expansion, but the acceleration of expansion.  So, now with new data, we adjust again, and perhaps but Einstein's constant back in place.



> The problem with Dark Energy (and Dark Matter) and some other theories where scientists are using a set of observations to come up with an educated guess, is that it's still pretty much a guess. Yes, these are educated and calculated guesses based on current data, but scientists really don't know yet.




The acceleration of expansion was first observed in 1998.  In the years since, the fact that expansion is generally accelerating has been verified by different groups, using a wide variety of techniques.  That the expansion of the visible universe is accelerating is pretty well observed, and not really guesswork at this point.



> Until they do (requiring a lot more data, observation, and calculation), discussing theories like these is fun, but it's probably not going to lead to the actual truth (or at least when lay people such as myself discuss them on a web forum).




Are folks on this forum going to have some remarkable insight that clears up some fundamental issues of physics?  Probably not.  Can some folks learn a bit about the current thoughts of the universe?  Certainly.



> As an example, some scientists claim that the Large Hadron Collider can recreate conditions during the first few billionths of a second of the Big Bang. Well, maybe it can or maybe it cannot, but since nobody was around during those moments of the Big Bang, there's a chance that these scientists are wrong on this.




So what?

There is "a chance" for just about anything.  There's "a chance" that Morrus is actually Elvis Presley.  We cannot allow "a chance" that we might be wrong to stop us from trying and thinking.  The scientific method not only allows that there's a chance that we might be wrong, it absolutely requires we try anyway.  The whole process of science is, in the end, a big trial-and-error game.  Error is a major portion of the process, as error often reveals more information than being correct!

Beyond science, all of life is a process of taking the best shot with the information and tools at hand.  There's a chance that the next job you take will be a really, really bad career move, in ways that you will not be able to see.  Do you allow that to paralyze you?



> They were wrong about Neptune. And, maybe scientists have found the Higgs Boson, but then again, even if they call it that, it might not actually be that.




If you read the actual writing of the scientists involved in those experiments, you'll find that they didn't say they found the Higgs.  They said they found a _candidate for the Higgs_.  It has the right mass.  It decays in some of the right ways.  They'd like to find some of it's other decay modes, and verify its parity, before really claiming it to be a Standard Model Higgs. 



> Scientists have been searching for Dark Matter for nearly 80 years and still have not found it.




How long you've been looking isn't really a measure though.  Mankind was trying to fly for millennia (since Daedalus, at least), and it took the Wright Brothers to get us off the ground. 



> Interestly though, a lot of scientists think that the universal expansion is accelerating. Maybe it is. Maybe it isn't. But, we need other verification to be sure. If it isn't accelerating, then we get to throw out the most recent text books on at least part of the Lambda-CDM model.




See above - the acceleration of expansion in our visible universe is pretty well observed and accepted at this point.  The basic way out of invoking something exotic like Dark Energy is to have this be only a local phenomenon.  Historically, though, theories that have us living in a particularly special part of the universe have tended to be wrong.  



> For example, prove that the Big Bang never happened and many billions of dollars and lifetimes of research pretty much fall by the wayside.




Yes.  And?  So what?  Is there some preferable alternative?


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## Nellisir (Nov 3, 2012)

Umbran said:


> How long you've been looking isn't really a measure though.  Mankind was trying to fly for millennia (since Daedalus, at least), and it took the Wright Brothers to get us off the ground.




What Umbran said.  Looking does not mean that you're looking in the right places, or that you have the right instruments to see what you think you're looking for.  Extrasolar planets.  We've been looking for decades.  We know where they are.  We know what they are.  We know now that there are billions and billions and billions of them.  But we didn't actually find one until 1992.  We've been seeing planets around other stars longer than we've been able to see the accelerating expansion of space.


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## tomBitonti (Nov 3, 2012)

freyar said:


> The difficulty is that the effects of the overall expansion of the universe (whether the accelerating kind or a more mundane decelerating kind) do not just "add" with the effects of the matter in the galaxy on gravity.  (We say the equations of general relativity are "nonlinear" because the results are not additive.)  So, while I don't have the exact solutions to the equations at hand in this case, what you do expect to happen is that the gravitational attraction of the matter in the galaxy to "win" over the expansion of the universe, so the space inside the galaxy shouldn't stretch.  In other words, the galaxy is "gravitationally bound," so the distance across the galaxy shouldn't be increased by the overall expansion of the universe.  That will only affect distances between objects that aren't bound together.
> 
> Another point to make is that, even if the expansion of the universe were happening in our galaxy, it would be a very small effect.  The expansion is only noticeable on very large distance scales (for example, the expansion rate between our Milky Way and Andromeda, the closest large galaxy, is quite slow).
> 
> ...






Umbran said:


> No.  Remember above, with the bar magnet?  Once the magnet has picked up the nail or pin, do you expect gravity to slowly, very slowly, pull the pin away anyway?  No.  It's there, it is stuck, and it isn't gonna move.
> 
> The forces of gravity are strong near matter. Very close to matter, space isn't expanding at all (very close to really big chunks of matter, space collapses - black holes!).  As you move away from matter, though, the force of gravity decreases.  Eventually, the force of gravity is small enough that it no longer counters the effective forces of the dark energy, and way out there, expansion accelerates.
> 
> In fact, the universe is mostly empty space - there's a whole lot of "force of gravity is small" out there.  So, on the whole, the universe is expanding, and that expansion is accelerating.  It is only near these tiny islands of galaxies where this is not true.




If I may: A photon starts a billion light years away.  In the frame of an observer on the earth, is the photon already red shifted, or does the red shift occur as the photon travels to the earth?  If the red shift travels while the photon travels to the earth, what is different about the photon's experience as it travels to cause it to red shift, compared, say, to a photon in a perfect resonant cavity bouncing back and forth for a billion years?  Why does that photon never red shift, but the one that travels from a billion light years away does?

TomB


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## JustinAlexander (Nov 3, 2012)

Umbran said:


> There is "a chance" for just about anything.  There's "a chance" that Morrus is actually Elvis Presley.




I knew it!


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## KarinsDad (Nov 3, 2012)

Umbran said:


> Yes.  And?  So what?  Is there some preferable alternative?




No. Not at all.

I would just prefer to read a scientific article (and I read a lot of them, I just don't really have the background to discuss them rationally) and have the authors be a bit more circumspect about their discoveries. The tone of many articles is that what the scientists found is fact. I'd like to watch more lectures (which I also watch quite a few of) where the professors use the word hypothesis a bit more.

The reason is that the general public and even our scientific students are a bit misled into thinking that science is nearly 100% accurate in many non-bleeding edge areas and as we know, it is an evolving process where new data and experiments and observations and tools can and occassionally do, change current scientific theory.

With respect to the OP's question, I suspect that time travel is impossible. I have no proof, but to me, the burden of proof is upon those making the claim that it is. Some people say "extraordinary claims require extraordinary evidence". I don't go quite that far. I just say that "extraordinary claims require evidence". Time travel, teleporting, UFOs that contain extraterrestrials here on Earth, faster than light travel (and ghosts and bigfoot): most if not all of this probably does not exist at all except in the minds of people. Even some recent theories like superstring theory (and branes, multiverses, etc.) are probably just so much poppycock. I cannot prove that, but then again, the burden of proof is not on me. It's on the ones making the claim.

Just because mankind can think of something doesn't make it fact. Usually without some extremely solid evidence and multiple different experiments/observations to back it up, many of mankind's facts are nothing of the kind. Mankind has had a long and illustrious career of being wrong. I really wouldn't be surprised if Dark Energy and especially Dark Matter end up in the trash heap of scientific theories within this century even though they are being taught as basically fact today. We have zero scientific instruments located outside of our galaxy (and no really sophisticated ones located outside our solar system), so to claim that we understand how the macroscopic universe works (and how it started) based mostly on electromagnetic energy observations from thousands to billions of years ago is probably a bit naive. I keep going back to Neptune. When mankind can be wrong and doesn't have enough information, he probably is wrong.

One other note. A very large portion of current theory on the microscopic universe is based on particle accelerator experiments. Unfortunately, that's like determining the function of a computer inside a car based on an automobile wreck. Look at the pieces, determine what's going on. Although there is probably much validity to it, there might also be some misleading information from it that scientists pretty much regard as fact today, just based on the type of experiment it is. It will be interesting to see how different those current theories might become if we ever come up with a way to generate particles smaller than protons, electrons, etc. in order to peer more closely without colliding subatomic particles.

The nice thing about our current society is that science has made some deep strides in the last few centuries and the pace has been picking up significantly due to computers and advanced manufacturing techniques. But, don't be surprised if many of the concepts that scientists take for granted today (especially in the macroscopic and extremely small microscopic levels where our viewing windows are so limited and possibly distorted, not so much in the areas like chemistry) are drastically modified or even eliminated over the next few decades or so. I have no evidence for that, I just have mankind's track record.


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## Morrus (Nov 3, 2012)

KarinsDad said:


> I would just prefer to read a scientific article (and I read a lot of them, I just don't really have the background to discuss them rationally) and have the authors be a bit more circumspect about their discoveries. The tone of many articles is that what the scientists found is fact. I'd like to watch more lectures (which I also watch quite a few of) where the professors use the word hypothesis a bit more.




What are you reading? Articles by journalists _about_ science, or the actual papers? Because I don't believe there's such thing as a peer-reviewed scientific paper in existence which presents itself as fact (at least not one not by a crank of some kind); and popular science books written by scientists, in my experience, generally don't - to the point of overusing the word "hypothesis".



> The reason is that the general public and even our scientific students are a bit misled into thinking that science is nearly 100% accurate in many non-bleeding edge areas and as we know, it is an evolving process where new data and experiments and observations and tools can and occassionally do, change current scientific theory.




I don't think that's the fault of researchers. That's the fault of educators and media. Researchers do research and publish papers full of math. If the public and students are getting the wrong impression as to what those papers contain, blaming "scientists" is even less factual than the very articles you're criticizing.

But there are certainly vast swathes of scientific research where it can be stated that the theories make correct predictions nearly 100% of the time; I would suggest that calling these, as you put it "nearly 100% accurate" is a perfectly reasonable turn of phrase to use.

To take a silly, simple example - there's nothing at all wrong with a schoolteacher telling their students that speed is equal to distance divided by time.  That's "nearly 100% accurate" for any normal purpose. Sure, you can get all nit-picky with 12 year-olds about relativity and such, but at that point you stop educating them and start confusing them.


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## KarinsDad (Nov 3, 2012)

Morrus said:


> What are you reading? Articles by journalists _about_ science, or the actual papers? Because I don't believe there's such thing as a peer-reviewed scientific paper in existence which presents itself as fact (at least not one not by a crank of some kind); and popular science books written by scientists, in my experience, generally don't - to the point of overusing the word "hypothesis".




A lot of what I read comes from here:

http://phys.org/science-news/

Many of these articles are interviews with the scientists who write the papers. It could be journalistic bias, but then again, it happens quite a bit. I don't have the actual background to read (and understand) peer-reviewed scientific papers, just the synopses of those.

I also habitually read Scientific American.



Morrus said:


> I don't think that's the fault of researchers. That's the fault of educators and media. Researchers do research and publish papers full of math. If the public and students are getting the wrong impression as to what those papers contain, blaming "scientists" is even less factual than the very articles you're criticizing.




Agreed. Although I have read a few actual papers (mostly from grad students, not researchers in the field) where the scientific method is not rigorously followed, or assumptions are made. I tend to lump those under "I must not understand this completely because peer review should have caught this", assuming that I am the one in error.



Morrus said:


> But there are certainly vast swathes of scientific research where it can be stated that the theories make correct predictions nearly 100% of the time; I would suggest that calling these, as you put it "nearly 100% accurate" is a perfectly reasonable turn of phrase to use.




You mean like Mond Theory which is generally ignored by the scientific mainstream, but is nearly 100% spot on with regard to galaxy rotational observations? Mond puts all Dark Matter theories to shame in many areas. It's only real weakness is with regard to scales larger than a galaxy. Then again, I'm not sure if anyone has combined Mond with Dark Energy theories yet either. It could fall apart at larger scales because it doesn't take into account Dark Energy.

I'm not claiming that Mond Theory is correct, but I do think that scientists tend to be blind to theories which don't match what they've believed most of their lives.


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## Morrus (Nov 3, 2012)

KarinsDad said:


> but I do think that scientists tend to be blind to theories which don't match what they've believed most of their lives.




That's not a very nice thing to say. Scientists are _nothing_ like that; they're the very definition of the opposite. Scientists, in my experience, are pretty much the group _most _open to new things I've ever met; they're continually striving to uncover new stuff and break new ground. That's their job. That's what drives them. That's why scientific advancement happens. If they weren't generally open and very accepting of new ideas, we'd still think the sun revolved around the earth. 

Nah. I'm 100% in opposition to that view of "scientists" (even if it were possible to group them all in such a way). Just look around at what you can see near you right now and say that scientists are opposed to new stuff. Think about the many massive discoveries over the years which radically change the way we view the universe. Do you really think a group of people who were by nature opposed to new things would figure out gravity, quantum mechanics, relativity, the accelerating expansion of the universe, the Big Bang theory, heliocentrism, or a million other things? Stuff about which_ everyone else_ cried "it can't be true!"?

There are people blinkered against new theories.  Scientists are not those people.


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## KarinsDad (Nov 3, 2012)

Morrus said:


> I think that's fairly insulting thing to say. Scientists are _nothing_ like that; they're the very definition of the opposite. Scientists, in my experience, are pretty much the group most open to new things I've ever met. That's why scientific advancement happens. If they weren't generally open and very accepting of new ideas, we'd still think the sun revolved around the earth. The number of "new" ideas in the last century numbers in the hundreds of thousands in a thousand different scientific fields.  Just because they haven't en-masse adopted your pet theory doesn't make them "close minded".
> 
> No. Scientists are very much NOT " blind to theories which don't match what they've believed most of their lives". In fact their very job is to research new theories.




Actually, I should qualify that statement. I do think that SOME scientists tend to be blind to theories which don't match what they've believed most of their lives. In fact, the reason that science took so long to get off the ground (so to speak) is because of firmly held belief of many scientists throughout time. The ancient Greeks could have gotten us maybe a half millenium more advanced today (course, none of us would actually be here) if they would have been as concerned about practical applications of science as they were about "abstract truth" of science (and philosophy and math). The steam engine could have easily been invented over two thousand years ago because steam powered toys existed.

There are many examples of scientists totally disagreeing on the exact same data. Look at Steady State vs. Big Bang. Both theories have had a ton of modifications since they were first proposed to match additional data (which is a reasonable thing to do), but obviously, they both cannot be correct. One of these ideas was only really dropped completely by almost all scientists in the last two decades. And in fact, it's possible that they are both incorrect and that the scientific community is misinterpreting the results being found.

There are also examples of peer review that dismissed research that was perfectly valid, but controversial.

Scientists are people too and can be just as dogmatic as any other person. They also get research funds for specific projects and if those projects start to falter, just like many other people, they can distort their conclusions of the data to more closely match the expectations. Not necessarily the falsification of data and maybe not even intentionally, but skewing conclusions or omitting inconsistencies to match current theory or to match a specific theory.

As Morris Kline wrote (talking about mathematicians): "Many were modest; others extremely egotistical and vain beyond toleration. One finds scoundrels such as Cardan, and models of rectitude. Some were generous in their recognition of other great minds; others were resentful and jealous and even stole ideas to boost their own reputations. Disputes about priority of discovery abound."

Do you really think that scientists are really that morally superior than anyone else? Do you think that anyone whose job is dependent on them acquiring results will not get results?

And even open mindedness in the scientific community means viewing new claims through the filter of established knowledge. Not true open mindedness, but open mindedness with hard wired caveats. And, this is how it should be. But, it does lead to a certain level of closed mindedness when doing so.

Obviously, not all scientists are close minded. Many are very open minded and try new things. But some do new experiments to support their theories, not to disprove them. And, there are examples of not just closed mindedness, but actual intellectual dishonesty.

Diederik Stapel
Jan Hendrik Schön
Hwang Woo-suk
Emil Rupp
Michael Bellesiles
Luk Van Parijs
Scott Reuben
Jon Sudbø


Just like all people, there are good scientists and bad ones. Open minded ones and ones who consider anyone who disagrees with them to be a fool. Ones who will argue with you and ones who will go "Hmmm, I hadn't considered that. Let's do an experiment.".

I don't think you can paint the entire scientific community with the broad brush of most scientists being the most open minded individuals around.


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## Morrus (Nov 3, 2012)

KarinsDad said:


> Actually, I should qualify that statement. I do think that SOME scientists tend to be blind to theories which don't match what they've believed most of their lives. In fact, the reason that science took so long to get off the ground (so to speak) is because of firmly held belief of many scientists throughout time. The ancient Greeks could have gotten us maybe a half millenium more advanced today (course, none of us would actually be here) if they would have been as concerned about practical applications of science as they were about "abstract truth" of science (and philosophy and math). The steam engine could have easily been invented over two thousand years ago because steam powered toys existed.
> 
> There are many examples of scientists totally disagreeing on the exact same data. Look at Steady State vs. Big Bang. Both theories have had a ton of modifications since they were first proposed to match additional data (which is a reasonable thing to do), but obviously, they both cannot be correct. One of these ideas was only really dropped completely by almost all scientists in the last two decades. And in fact, it's possible that they are both incorrect and that the scientific community is misinterpreting the results being found.
> 
> ...




By that logic, I may as well say "messageboard posters are dishonest". What you're saying has nothing to with science or scientists at all; it's just people. Why did you choose to attach it to statement about scientists in particular, as opposed to football players or chess players?

I mean, it sounds like you have a beef with "scientists". That's three posts in a row where you've levelled different accusations at them. First it was their fault that educators and media portrayed hypothesis as fact; when I pointed out that it was unfair to blame them for that, you moved on to claiming they were all narrow-minded and opposed to new theories; and when I pointed out that they're not generally opposed to new theories, you say you didn't mean all of them, and switched to accusations of dishonesty. And before all that, you were attacking perfectly reasonable behaviour regarding hypothesis and interpretation of data because "it's possible" something could be wrong (which is obvious, but not a useful statement in any way). It really, really sounds like you hold some kind of grudge against the broader scientific community; I can't imagine what, though.

It's kinda odd. You've leapt into the thread blasting accusations at scientists around in this wild shotgun scatter pattern, with the accusations changing with each post. You've even implied that scientists are the ones responsible for slow progression of science, rather than the ones solely responsible for any progression at all. 

If you think there's resistance to scientific progress out there - you're damn right there is! It's not the scientists though. There are plenty of other high profile obvious candidates for that particular problem.

Yeah, sure, there are bad apples in every vocation. So what? That's not a useful commentary on science, scientists, or scientific practice in any meaningful sense. I mean if that's all that those posts were building too - yeah, sure.


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## the Jester (Nov 3, 2012)

Going back a few days, here's one example of the "not before the time machine was built" stuff:


[ame]http://www.youtube.com/watch?v=oRWwI61so5Q[/ame]


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## Morrus (Nov 3, 2012)

the Jester said:


> Going back a few days, here's one example of the "not before the time machine was built" stuff:




I only watched the first few seconds but - is that Tom Baker?  If so, awesome!


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## KarinsDad (Nov 3, 2012)

Morrus said:


> By that logic, I may as well say "messageboard posters are dishonest". What you're saying has nothing to with science or scientists at all; it's just people. Why did you choose to attach it to statement about scientists in particular, as opposed to football players or chess players?
> 
> I mean, it sounds like you have a beef with "scientists". That's three posts in a row where you've levelled different accusations at them. First it was their fault that educators and media portrayed hypothesis as fact; when I pointed out that it was unfair to blame them for that, you moved on to claiming they were all narrow-minded and opposed to new theories; and when I pointed out that they're not generally opposed to new theories, you say you didn't mean all of them, and switched to accusations of dishonesty. And before all that, you were attacking perfectly reasonable behaviour regarding hypothesis and interpretation of data because "it's possible" something could be wrong (which is obvious, but not a useful statement in any way). It really, really sounds like you hold some kind of grudge against the broader scientific community; I can't imagine what, though.
> 
> Yeah, sure, there are bad apples in every vocation. So what? That's not a useful commentary on science, scientists, or scientific practice in any meaningful sense. I mean if that's all that those posts were building too - yeah, sure.




I think that open discourse is a staple of scientific integrity and that includes how scientific knowledge gets disseminated to the public at large. I think that there are some scientific fields (ones where we cannot experiment in the lab, or are limited in the types of lab experiments that we can perform) where the theories are susceptible to both more rigid dogma and to a higher probability of incorrectness. Sorry, but having (possibly) hundreds of scientists looking for Dark Matter for 80 years (seriously looking for 40 years) and failing, and having every Dark Matter equation ever thought of only working for some galaxies and not for others, should give someone a clue to look for something else. That to me IS a definition of close mindedness.

Interestingly, there has been quite a bit in the literature recently about more and more astronomers siding with the idea that some form of modification to Einstein's general relativity and/or Newton's gravity equations as more reasonable explanations than Dark Matter.

With regard to educators and the media, scientists should hold their feet to the fire. Scientists should especially not let the media misrepresent their work in any way. I doubt scientists can do anything about educators since a lot of that tends to be hidden from their view. Course, a scientist who writes an educational book does get a share of the $100 to $200 per book sold, so they should be motivated to keep their books up to date with the latest theory and should indicate it as such.

As for my second post, you ignored most of it and concentrated on one specific sentence and decided to continue to only discuss it.

With regard to science, I think a lot of science is very solid. We wouldn't be having this conversation (or heating our dinner or doing hundreds of everyday things) without technology that resulted directly from solid science.

But, I do think that some of the more theoretical and not quite proven (or hard to prove or disprove) science tends to be a bit more dogmatic (when it shouldn't) and resistant to change. And some of this is the way it should be. We should not throw out the baby with the bath water every time a glitch comes along. But, we should also explore other alternatives because glitches do come along.

Anyway, I'm not really trying to attack anyone, but you seem to be taking it that way, so I'll just drop it.


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## Morrus (Nov 3, 2012)

KarinsDad said:


> Sorry, but having (possibly) hundreds of scientists looking for Dark Matter for 80 years (seriously looking for 40 years) and failing, and having every Dark Matter equation ever thought of only working for some galaxies and not for others, should give someone a clue to look for something else. That to me IS a definition of close mindedness.




Is that your _scientific_ opinion? That sounds like a statement that you'd need to really understand the science and the math to make. I wouldn't dream of proclaiming that "scientists" (again, there are many and they all do different things) are all wrong to be pursuing the theories they are, because I know I don't understand their theories - or, more specifically, what might be wrong with their theories. 

And you've said that you don't, either. Yet you know they're being close-minded in pursuing them? What qualifies you - above them, no less - to proclaim the correct allocation of research resources?

I'm honestly not trying to get into a fight here. I just literally cannot fathom your perspective. I feel like I've encountered an alien species whose mind works in an utterly baffling way!



> Interestingly, there has been quite a bit in the literature recently about more and more astronomers siding with the idea that some form of modification to Einstein's general relativity and/or Newton's gravity equations as more reasonable explanations than Dark Matter.




Huh? So now you're saying they _are_ considering other theories?

Yikes, man. I can't even follow this conversation any more.

That said - why do you keep returning to dark matter in particular? It seems to be your point of contention, despite not being the subject of this thread (or even dark energy, which the thread later drifted into) - that you personally feel aggrieved because the majority of tiny number of scientists working on that particular field of research are not focused on a different theory of which you're enamoured? Is that your data set?


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## Umbran (Nov 3, 2012)

tomBitonti said:


> If I may: A photon starts a billion light years away.  In the frame of an observer on the earth, is the photon already red shifted, or does the red shift occur as the photon travels to the earth?




Um, neither?  Doppler shifting is best modeled by thinking of the wave nature of light, not the particle nature.  Then, it the shifting Hubble observed is a result of relative motion between the source and observer.

Expansion of space only comes into it as a cause of that relative motion. 

In the cavity example, the source and observer are not moving relative to one another - they are both pretty much at rest with respect to the resonant cavity (I assume), so no redshift is observed.


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## KarinsDad (Nov 3, 2012)

Morrus said:


> Huh? So now you're saying they are considering other theories?
> 
> Yikes, man. I can't even follow this conversation any more.




The point I was trying to make is that only RECENTLY has there been more interest in non-Dark Matter theories. They have been around for about 30 years out of the 40 years that scientists have been actively trying to pursue DM, but mainstream scientists have called them radical theories for decades. Now, not so radical as more and more scientists are slowly starting to examine them more carefully. Part of the reason for this is lack of results for DM. Part of the reason for this might be because as more elderly / well known scientists who were a bit more rigid in their thinking that DM was the only possible answer have moved on (retired, died, etc.), similar to what happened with the few holdouts of the Static State theory.

Yes, some scientists are slowly getting around to being open minded on this, but for several decades, not so much. Here is just one example of where your claim to open mindedness by scientists doesn't hold much water if it takes three or four decades for them to consider other explanations as maybe not so radical after all. And, many scientists still will not consider anything other than DM.

To be considered open minded, one should examine different possibilities, not just pick the most popular one and stick to it.

This might be what you were not following from what I posted. This is an example of resistance to other ideas that has been happening for decades (since the birth of science actually since science is done by people). It disagrees completely with your claim of how most scientists function.



Morrus said:


> That said - why do you keep returning to dark matter in particular? It seems to be your point of contention, despite not being the subject of this thread (or even dark energy, which the thread later drifted into) - that you personally feel aggrieved because the majority of tiny number of scientists working on that particular field of research are not focused on a different theory of which you're enamoured? Is that your data set?




I'm not enamored by any specific theory. I was using this as an example. DM is just one of several different theories that didn't seem to have evidence to support it. It never has. Galaxies have observational effects which it tries to fill the hole for, but no evidence. The observations are real (or at least as real as our tools allow us to observe). The theory, maybe, maybe not. And, I don't know if it is Mond or Electromagnetic attraction or something else. All I know is the DM might not be the answer based on what some scientists are now saying. I take what I understand from them. I don't have an agenda (other than discussing it).

It's like Dark Energy. Scientists can state the observational effects that they use it to try to explain, but an explanation for what causes it is still pie in the sky. Until they can explain exactly what it is and what is causing it and they have evidence to illustrate this, there is a fair chance that it might be an incorrect theory.

When one comes up with a theory for science, one has to do experimentation to support or disprove the theory. In both cases here, there is an effect (galaxy rotational speed/lensing and apparent acceleration of universal expansion). But, the explanations for why these are occurring have no real teeth. There is DM. What is it? Don't actually know. There is DE. What is it? Don't actually know. Scientists have tried to come up with the actual answers, but so far, zip, zilch, nada. Some theories. Few real answers.

As an example, NASA scientists have stated that Dark Matter accounts for 23.3 percent of the cosmos, and Dark Energy accounts for 72.1 percent (to within various percentages). Well, NASA scientists don't actually know that, but they stated it. These are percentages presumably based on the "what if" scenario of if DM and DE are correct explanations. But now, this is public information that may or may not be quite accurate, but it has been presented to the public for all intents and purposes as fact. By NASA.

I've seen several "scientific programs" (e.g. Through the Wormhole as one example) where this percentage claim has also been made by people on the show. The public watches the show and many probably considers it fact. Argumentum ad verecundiam is very compelling in our society.


Look at quantum mechanics. All of our evidence for how quarks and other elementary particles work comes from one source. Smashing particles together at high speed. In any other branch of science, having only one basic type of experiment for your theories without other observations or experiments to verify against would be considered shoddy science. But, we have no other way yet to peer into protons, neutrons, and electrons, so science uses what it has. Does it make it correct? Maybe. Maybe not. It's pretty much considered fact in the scientific community and at this point in time, rightfully so. But, there's a fair chance that it is wrong. As Feynman once stated "I think I can safely say that nobody understands quantum mechanics" (granted, he said that over 45 years ago, but it still pretty much holds, we have more knowledge, but it's still a big mystery).


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## tomBitonti (Nov 4, 2012)

Umbran said:


> Um, neither?  Doppler shifting is best modeled by thinking of the wave nature of light, not the particle nature.  Then, it the shifting Hubble observed is a result of relative motion between the source and observer.
> 
> Expansion of space only comes into it as a cause of that relative motion.
> 
> In the cavity example, the source and observer are not moving relative to one another - they are both pretty much at rest with respect to the resonant cavity (I assume), so no redshift is observed.




Isn't the billion mile away emitter technically at rest (more-or-less) relative to us?  I thought the passage through the billion light years was what caused the red shift.  (This all is confusing because of the seeming difference between everyday velocity and apparent velocity due to space expanding.)  Then, the red shift is caused by a curvature along the way.  That curvature must not be uniform, otherwise, the light in the resonant cavity should experience a similar red shift.  Is the effect imperceptible because it adds non-linearly (in a manner to reduce the effect) in a region already curved due to gravity?

Note: This is all at the edge of my understanding.  I'm prepared for any one of these statements to be utterly wrong.  What I'm interested in as much as the correct answer (to current levels of understanding) is a correct approach to obtaining the answer.

TomB


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## Nellisir (Nov 4, 2012)

tomBitonti said:


> Isn't the billion mile away emitter technically at rest (more-or-less) relative to us?  I thought the passage through the billion light years was what caused the red shift.  (This all is confusing because of the seeming difference between everyday velocity and apparent velocity due to space expanding.)  Then, the red shift is caused by a curvature along the way.  That curvature must not be uniform, otherwise, the light in the resonant cavity should experience a similar red shift.  Is the effect imperceptible because it adds non-linearly (in a manner to reduce the effect) in a region already curved due to gravity?
> 
> Note: This is all at the edge of my understanding.  I'm prepared for any one of these statements to be utterly wrong.  What I'm interested in as much as the correct answer (to current levels of understanding) is a correct approach to obtaining the answer.
> 
> TomB




I think this falls over the edge of my understanding, but here goes.  Keep in mind that this is the first time I've ever actually tried to make sense of redshifting and wavelengths, so it's very possible I've gotten something totally wrong or backwards.

Redshift isn't a property of the photon itself.  The photon doesn't age or turn red as it travels.  Redshifting is a property of one object travelling away from another object, and the wave (not particle) of light between the two.  If the two objects are static in relation to each other, there is no redshift, regardless of the time or distance.

In our universe, however, *on a intergalactic scale*, everything is moving away from everything, in all directions.  We are all dots on a balloon, and the balloon is inflating. (If this gets you going about curvature, then it's a flat piece of balloon being pulled from the perimeter).  Because the expansion is speeding up, older things are moving faster,  and are more redshifted.

Since time = distance, and time = speed, therefore further = older = faster = more redshift.

Light is photons, which are particles, but it is also a wave.  The wavelength is the distance over which the wave's shape repeats.  Imagine that there's a line between two objects, and that line has a repeating wave pattern.  Let's say that there are....30 waves between Object A and Object B.  The length between each wave equals X.  If the objects are static, X is unchanged.  If the objects begin to move together,the waves get tighter and X becomes shorter (X-, aka blueshift).  If the objects move apart, X becomes longer (X+, aka redshift). The distance between waves is increasing, but the quantity of waves remains the same.  There's still 30 waves, but they have to stretch to accommodate the increasing distance. That's redshift.  The wavelength (distance between the same point on two adjoining waves) of light increases as objects move apart.


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## tomBitonti (Nov 4, 2012)

Nellisir said:


> In our universe, however, *on a intergalactic scale*, everything is moving away from everything, in all directions.  We are all dots on a balloon, and the balloon is inflating. (If this gets you going about curvature, then it's a flat piece of balloon being pulled from the perimeter).  Because the expansion is speeding up, older things are moving faster,  and are more redshifted.




That part I think I get.  But, I think this answers my question by building it into the presises of your answer.  Why *only* on an intergalactic scale?  Is that because the effect is too small to be measured on a smaller scale (similar to the precession of Mercury, which is barely measurable, compared to the presession of Earth, which I gather is not.  Or does the effect not happen at all within a galaxy?  Does the wave stay unshifted until it reaches a very flat part of intergalactic space and then start to shift?  (With a suitable smoothing in transition?)

If we model this as a set of rigid discs on an elastic surface, say, lots of CDs on a trampoline with sequins sprinkled here and there in-between, then we tighten the trampoline, then the distance between each of the rigid CDs and sequens grows, but the trampoline stretches, sliding, beneath the wide CDs.

But, if that happens, will there be an extra effect applied to light, similar to gravitational lensing, but not accounted for entirely by gravity, as we measure light passing galaxies from distant sources?

TomB


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## Umbran (Nov 4, 2012)

tomBitonti said:


> Isn't the billion mile away emitter technically at rest (more-or-less) relative to us?  I thought the passage through the billion light years was what caused the red shift.




No, merely having the wave (or photon) travel a long distance won't cause a red shift.  The Doppler Effect comes from motion of the source or receiver.

However, I find that at 12:30 AM, I'm not so good at explaining.  I will take a stab at it after I've gotten some sleep, if that's okay.


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## Morrus (Nov 4, 2012)

KarinsDad said:


> And, many scientists still will not consider anything other than DM.
> 
> To be considered open minded, one should examine different possibilities, not just pick the most popular one and stick to it.




This is a good summation of all those paragraphs. I disagree for the following reasons:

1) You are levelling your accusations at "scientists". Those studying dark matter are a tiny, tiny, tiny percentage of scientists.

2) Describing that process as "picking the most popular one and stick to it" is insulting to them and utterly inaccurate.

As far as I can tell your definition of "close minded" is "an expert who doesn't agree with (the non-expert) KarinsDad on how to conduct research of galactic rotation". Which is a definition, I suppose, but not one that I would ever care to use.



> It disagrees completely with your claim of how most scientists function.




Okay, okay. How many scientist do you _know_? Let's have a list of examples. Science is a big, big area - let's have a dozen major theories which are being inadequately and incorrectly researched in your opinion. 



> It's like Dark Energy. Scientists can state the observational effects that they use it to try to explain, but an explanation for what causes it is still pie in the sky. Until they can explain exactly what it is and what is causing it and they have evidence to illustrate this, there is a fair chance that it might be an incorrect theory.




No scientist has ever claimed to understand dark energy. This is a complete strawman and a misleading line of thought.



> In both cases here, there is an effect (galaxy rotational speed/lensing and apparent acceleration of universal expansion). But, the explanations for why these are occurring have no real teeth. There is DM. What is it? Don't actually know. There is DE. What is it? Don't actually know. Scientists have tried to come up with the actual answers, but so far, zip, zilch, nada. Some theories. Few real answers.




This is the OPPOSITE to what you've been saying. 



> Look at quantum mechanics. All of our evidence for how quarks and other elementary particles work comes from one source. Smashing particles together at high speed. In any other branch of science, having only one basic type of experiment for your theories without other observations or experiments to verify against would be considered shoddy science.




Jeezooos holy mofo, man! So we've progressed from accusations that scientists are narrow-minded, through dishonesty, claims that it is they who are holding scientists back (as opposed to... ice cream salesmen who are progressing it? Who?) to.... now *scientists are doing shoddy science?*

Really?

Based on_ your opinion_ of how they should be conducting this science, you deem it shoddy? You, who admits he _doesn't understand it_?

Do you really, honestly not see the problem here?


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## KarinsDad (Nov 4, 2012)

Morrus said:


> Do you really, honestly not see the problem here?




Yes I do. You and I cannot appear to have a rational discussion on this (Jeezooos holy mofo???) because you seem to be nitpicking anything I write. I suspect that I am hitting one of your buttons, similar to if I talked to someone about their religion. For some people, science is religion.  I do not know about you, but it appears that way since you have not once discussed science here, but merely leveled accustations at me of attacking scientists. That is the main thing that you have focused on. You appear to be mostly ignoring anything else I write and I haven't seen anything to indicate that you have actually researched those areas that I have discussed yourself. Science is religion for me, but one that I try to be objective about and not just believe everything I read about it.

And yes, I know several scientists in several different fields. The one I talk to the most has a PhD from MIT in chemistry. My wife's cousin has a PhD in molecular genetics and microbiology from Duke. All of the ones I know are good hard working people. I even had an Email correspondence with a physicist for over a year that I did not know at all. Great guy who helped me a lot by pointing me to papers that I never would have ever read. He didn't need to do that, but he did. I wouldn't actually mind starting a correspondence with Umbran because he is always level headed here. And, it's extremely likely that my daughter will become a (probably PhD) scientist. Within a bit over a decade, I will most likely have a scientist in the family who will have scientist colleagues in at least her field (whose brains I might one day get to pick  ).

I actually think that the vast majority of branches of science are very very well researched (not well funded, but well researched) and I wasn't really discussing those. I was discussing the more theoretical areas (like Kip Thorne and his silly time machine proposal which started this discussion). I apologize that by using the term scientist, I misled you to infer that I was talking about all scientific branches. I was trying to be more specific with my examples.

Thousands of years ago (even without Pythagoras), anyone with a brain could climb a tower or a mountain and tell that the world was curved by looking out over the ocean. Many learned people (and not just in Greece) knew this at the time (some seafarers knew that the world was at least curved by seeing just the masts of other ships at a distance). But, the prevailing theory was that the Earth was flat nearly everywhere around the world.

Dark Matter theories (and some other theoretical aspects of science, possibly like string theory) are the Flat Earth theories of our day and age. The signs are there, but some people refuse to see them. You can disagree with that, but it's probably just a matter of time before they fall by the wayside. I base this on my biased unlearned but heavily read opinion. One just needs to do a lot of reading and read between the lines. And, my opinion doesn't mean squat. But, I'm a patient person and can wait to see how it all shakes out.

I'll leave it at that.


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## jonesy (Nov 4, 2012)

KarinsDad said:


> Thousands of years ago (even without Pythagoras), anyone with a brain could climb a tower or a mountain and tell that the world was curved by looking out over the ocean. Many learned people (and not just in Greece) knew this at the time (some seafarers knew that the world was at least curved by seeing just the masts of other ships at a distance). *But, the prevailing theory was that the Earth was flat nearly everywhere around the world.*



That is called the Myth of the Flat Earth, and is simply not true. Are these scientist friends of yours members of the Flat Earth Society by any chance?


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## Nellisir (Nov 4, 2012)

tomBitonti said:


> Why *only* on an intergalactic scale?




Because gravity is stronger than dark energy on a local (galactic) scale, just as other forces are stronger than gravity at different scales.

We think of gravity as this mighty force that holds the sun and the planets together, but you overcome it every time you pick something up off the floor.  If other forces weren't stronger than gravity, we'd just be random atoms sprinkled over the planet, unable to come together.  We can overcome gravity with simple chemical rockets.  Water molecules overcome gravity by getting hot and converting to steam.  

On small scales, the electromagnetic force is much more powerful than gravity and at atomic scales, the strong force is much more powerful than electromagnetism.  (and there's also the weak force, which also works at an atomic level).

So, dark energy is still present in galaxies, just like gravity is present in molecules, but gravity is stronger than dark energy at the galactic scale and keeps things (scientifical terms!) bound together.


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## Morrus (Nov 4, 2012)

I'm suffering from what can only be described as an irony overload. But the old science=religion canard (which I could see was coming three pages back, but hoped wasn't) pretty much concludes this debate for me.


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## Umbran (Nov 4, 2012)

KarinsDad said:


> Yes I do. You and I cannot appear to have a rational discussion on this (Jeezooos holy mofo???) because you seem to be nitpicking anything I write.




I don't think he's nitpicking what you write.  To me, it looks like he's questioning the grounds upon which you are criticizing the competence and work ethics of those who have made their life's work of something that's pretty darned difficult to do.



> Thousands of years ago (even without Pythagoras), anyone with a brain could climb a tower or a mountain and tell that the world was curved by looking out over the ocean. Many learned people (and not just in Greece) knew this at the time (some seafarers knew that the world was at least curved by seeing just the masts of other ships at a distance). But, the prevailing theory was that the Earth was flat nearly everywhere around the world.




Here's the thing - that's not true.  Yes, in grade school, they may have taught you that the learned people of Columbus' day thought the Earth was flat.  But that was incorrect.  And, these days, we know it is incorrect.  

What you've just proven is that sometimes laymen didn't/don't know a whole lot about what scientists think.  Which, as Morrus points out, is kind of ironic.


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## Nytmare (Nov 5, 2012)

KarinsDad said:


> For some people, science is religion... Science is religion for me, but one that I try to be objective about and not just believe everything I read about it.





Science adjusts its beliefs based on what is observed.

Religions deny observation so that belief can be preserved.


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## GMforPowergamers (Nov 5, 2012)

Umbran said:


> I don't think he's nitpicking what you write.  To me, it looks like he's questioning the grounds upon which you are criticizing the competence and work ethics of those who have made their life's work of something that's pretty darned difficult to do.
> 
> 
> 
> ...



This is far from the first time I hqve learned that poor Mrs cullen lied to me, it is the first i have heard about columbus not seting out to prove that the eqrth is not flat... so p,ease tell me some moreabout this, and where can I find out more?


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## Nellisir (Nov 5, 2012)

GMforPowergamers said:


> This is far from the first time I hqve learned that poor Mrs cullen lied to me, it is the first i have heard about columbus not seting out to prove that the eqrth is not flat... so p,ease tell me some moreabout this, and where can I find out more?




Um...seriously?  Columbus set out to find a route to the China & the East Indies so he could make money.  For more information, try Wikipedia.


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## jonesy (Nov 5, 2012)

The issue with Columbus and the false things said was that Washington Irving published a biography of him in 1828 where he made claims about the intents of the voyage and people thought that the biography was accurate when most of it was made up. In reality the journey wasn't about the shape of the world, but its width and the distances to other continents.


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## GMforPowergamers (Nov 5, 2012)

Nellisir said:


> Um...seriously?  Columbus set out to find a route to the China & the East Indies so he could make money.  For more information, try Wikipedia.




Yes, seriusly, I know he was looking for a route for spices... but my understanding (from 3rd grade history) was that most people thoughthe was crazy and going to fall off the edge of the earth... and that he though(correctly) that the earth was round.and.he.would travel around the world to the east and end up in the west...


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## Morrus (Nov 5, 2012)

GMforPowergamers said:


> it is the first i have heard about columbus not seting out to prove that the eqrth is not flat




Are you being serious? You guys are actually being taught that?

No, Columbus did not set out to prove the Earth was not flat.  As Russell said, "no educated person in the history of Western Civilization from the third century B.C. onward believed that the earth was flat" and other historians say "there was scarcely a Christian scholar of the Middle Ages who did not acknowledge [Earth's] sphericity and even know its approximate circumference".

Then again, this thread has revealed far more bizarre viewpoints than that to me.


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## jonesy (Nov 5, 2012)

Some of Colombus' opponents were of the opinion that according to their calculations the trip to Asia would be far too long for his crew to survive it without starving to death. I think that might have been where the whole 'fall off the face of the Earth' thing started from. The interesting thing is that they weren't wrong. If the Americas hadn't been there the trip really would have been too long.


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## Umbran (Nov 5, 2012)

GMforPowergamers said:


> so p,ease tell me some moreabout this, and where can I find out more?




As Jonesy said, it is referred to as the "Myth of the Flat Earth".  Wikipedia has an article on it, which may be a good place to start:

http://en.wikipedia.org/wiki/Myth_of_the_Flat_Earth



			
				Wikipedia said:
			
		

> The issue in the 1490s was not the shape of the Earth, but its size, and the position of the east coast of Asia, as Irving in fact points out. Historical estimates from Ptolemy onwards placed the coast of Asia about 180° east of the Canary Islands. Columbus adopted an earlier (and rejected) distance of 225°, added 28° (based on Marco Polo's travels), and then placed Japan another 30° further east. Starting from Cape St. Vincent in Portugal, Columbus made Eurasia stretch 283° to the east, leaving the Atlantic as only 77° wide. Since he planned to leave from the Canaries (9° further west), his trip to Japan would only have to cover 68° of longitude.
> 
> Columbus mistakenly used a much shorter length for a degree (he substituted the shorter 1480 m Italian "mile" for the longer 2177 m Arabic "mile"), making his degree (and the circumference of the Earth) about 75% of what it really was. The combined effect of these mistakes was that Columbus estimated the distance to Japan to be only about 5,000 km (or only to the eastern edge of the Caribbean) while the true figure is about 20,000 km. The Spanish scholars may not have known the exact distance to the east coast of Asia, but they believed that it was significantly further than Columbus' projection; and this was the basis of the criticism in Spain and Portugal, whether academic or amongst mariners, of the proposed voyage.
> 
> The disputed point was not the shape of the Earth, nor the idea that going west would eventually lead to Japan and China, but the ability of European ships to sail that far across open seas. The small ships of the day (Columbus' three ships varied between 20.5 and 23.5 m – or 67 to 77 feet – in length and carried about 90 men) simply could not carry enough food and water to reach Japan. The ships barely reached the eastern Caribbean islands. Already the crews were mutinous, not because of some fear of "sailing off the edge", but because they were running out of food and water with no chance of any new supplies within sailing distance.


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## GMforPowergamers (Nov 5, 2012)

Morrus said:


> Are you being serious? You guys are actually being taught that?
> 
> No, Columbus did not set out to prove the Earth was not flat.  As Russell said, "no educated person in the history of Western Civilization from the third century B.C. onward believed that the earth was flat" and other historians say "there was scarcely a Christian scholar of the Middle Ages who did not acknowledge [Earth's] sphericity and even know its approximate circumference".
> 
> Then again, this thread has revealed far more bizarre viewpoints than that to me.




I hate to tell you the lies we teach... columbus discovered america well trying to prove that the earth was round and he could get to china.

We are also tought (in public hs in 1996 anyway) that the civil war was fought becuse aberham lincon wanted to free the slaves,  in collage I learned of the social economics that may also play into it

In 2002 a friend was told by a chemistry teacher in hs that there are only 3 stages of mattter, and another friends older brother went to the school (he is a chemical engener) that she was just plain wrong.

In my older sister's HS she was told aids was an african deases that started in monkies, 4 years later my teacher laughed at the idea that it was from a monky, but it was from africa ???

Even today with a 1st grade nephew we teach basic but wrong things.then correct them 4-5 years later but still misslead until collage gives you all the information.


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## Dannyalcatraz (Nov 5, 2012)

Part of the reason for some of the jaw-dropping oddities in American education is that schools in the USA are controlled at the state level- IOW, almost no nationwide standards exist.  Each state gets to choose how and what its students learn about math, history, science, etc.  It is as if there were not objective standards!

in addition, the process of choosing books has become politicized.  I live in Texas, and there are so many students in our system, that other states often use our approved books list, and you may have seen articles about the shenanigans that goes on here.

One of the ones approved by Texas a few years ago talked about how we _won_ the _Korean War _ by dropping the _Atomic Bomb._

But Texas isn't alone- I'm an army brat, and I got to go to public schools in many states.  I once had a school book that talked about how we may someday land a man on the Moon (this was in the mid-1970s).

Thank God I'm a "reader" whose grandparents & Mom were ALL educators (from K to grad school), whose Dad is a MD- no way my education was going to be compromised by the idiocy of others.


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## Umbran (Nov 5, 2012)

Morrus said:


> Are you being serious? You guys are actually being taught that?




Well, I don't think many of your users are currently in grade school, Morrus.  They were teaching that in American schools in the 1970s.  I don't know about now.[/QUOTE]



GMforPowergamers said:


> In my older sister's HS she was told aids was an african deases that started in monkies, 4 years later my teacher laughed at the idea that it was from a monky, but it was from africa ???




Current theory is that HIV had its origins in non-human primates in west-central Africa.

http://en.wikipedia.org/wiki/HIV/AIDS#Origins


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## GMforPowergamers (Nov 5, 2012)

Umbran said:


> Well, I don't think many of your users are currently in grade school, Morrus.  They were teaching that in American schools in the 1970s.  I don't know about now.






Current theory is that HIV had its origins in non-human primates in west-central Africa.

http://en.wikipedia.org/wiki/HIV/AIDS#Origins[/QUOTE]

Now going back to 92-93 my sister was tought that, and 96-97 I was tought that was prajadice propaganda. NEW ENGLAND middle school and grade schools are horrable with it, and High schools were only a little better, I graduated in 1998 from high school and still to this day find things like this columbus thing.


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## freyar (Nov 5, 2012)

Wowsers, get busy for a couple of days and watch the thread explode!  I'm a bit tired and in the middle of some work, but I want to address a couple of points quickly.




tomBitonti said:


> If I may: A photon starts a billion light years away.  In the frame of an observer on the earth, is the photon already red shifted, or does the red shift occur as the photon travels to the earth?  If the red shift travels while the photon travels to the earth, what is different about the photon's experience as it travels to cause it to red shift, compared, say, to a photon in a perfect resonant cavity bouncing back and forth for a billion years?  Why does that photon never red shift, but the one that travels from a billion light years away does?
> 
> TomB






Umbran said:


> Um, neither?  Doppler shifting is best modeled by thinking of the wave nature of light, not the particle nature.  Then, it the shifting Hubble observed is a result of relative motion between the source and observer.
> 
> Expansion of space only comes into it as a cause of that relative motion.
> 
> In the cavity example, the source and observer are not moving relative to one another - they are both pretty much at rest with respect to the resonant cavity (I assume), so no redshift is observed.






tomBitonti said:


> Isn't the billion mile away emitter technically at rest (more-or-less) relative to us?  I thought the passage through the billion light years was what caused the red shift.  (This all is confusing because of the seeming difference between everyday velocity and apparent velocity due to space expanding.)  Then, the red shift is caused by a curvature along the way.  That curvature must not be uniform, otherwise, the light in the resonant cavity should experience a similar red shift.  Is the effect imperceptible because it adds non-linearly (in a manner to reduce the effect) in a region already curved due to gravity?
> 
> Note: This is all at the edge of my understanding.  I'm prepared for any one of these statements to be utterly wrong.  What I'm interested in as much as the correct answer (to current levels of understanding) is a correct approach to obtaining the answer.
> 
> TomB






Nellisir said:


> I think this falls over the edge of my understanding, but here goes.  Keep in mind that this is the first time I've ever actually tried to make sense of redshifting and wavelengths, so it's very possible I've gotten something totally wrong or backwards.
> 
> Redshift isn't a property of the photon itself.  The photon doesn't age or turn red as it travels.  Redshifting is a property of one object travelling away from another object, and the wave (not particle) of light between the two.  If the two objects are static in relation to each other, there is no redshift, regardless of the time or distance.
> 
> ...






tomBitonti said:


> That part I think I get.  But, I think this answers my question by building it into the presises of your answer.  Why *only* on an intergalactic scale?  Is that because the effect is too small to be measured on a smaller scale (similar to the precession of Mercury, which is barely measurable, compared to the presession of Earth, which I gather is not.  Or does the effect not happen at all within a galaxy?  Does the wave stay unshifted until it reaches a very flat part of intergalactic space and then start to shift?  (With a suitable smoothing in transition?)
> 
> If we model this as a set of rigid discs on an elastic surface, say, lots of CDs on a trampoline with sequins sprinkled here and there in-between, then we tighten the trampoline, then the distance between each of the rigid CDs and sequens grows, but the trampoline stretches, sliding, beneath the wide CDs.
> 
> ...






Umbran said:


> No, merely having the wave (or photon) travel a long distance won't cause a red shift.  The Doppler Effect comes from motion of the source or receiver.
> 
> However, I find that at 12:30 AM, I'm not so good at explaining.  I will take a stab at it after I've gotten some sleep, if that's okay.




That's a lot of words about the Doppler effect and cosmological redshift!  Going through that, I'm finding some good explanations and a little bit of confusion, so I'll just try my best at explaining rather than pick through all the previous posts (I will try to reference them at appropriate points if I can keep them all in mind).

OK, as Umbran says, the usual Doppler effect is not something that "happens" to the photon but is due to the relative motion of the photon's source and receiver.  At low relative speeds, you can think that the receiver moves past the peaks of the light wave (photon) more quickly/slowly if the receiver is moving toward/away from the source.  At these low speeds, the relative change in the light's wavelength = v/c, the relative speed over the speed of light (assuming head-on motion).  This picture isn't quite right at relativistic speeds (a significant fraction of the speed of light), but it is a reasonable picture if you also throw in some words about "relativistic time dilation" (the equation changes a lot, though).  A proper explanation would traditionally come after a couple of years of university physics, but I could try to give you something better in a few days (when I'm less busy) if someone wanted to start a different thread.  I'm happy to do physics Q&A when I'm not swamped at work.  Anyway, Nellisir also, I think, gave something similar to this explanation but may have been mixing in some of what I'm about to say next.

All that said, the normal Doppler effect is _not quite_ what happens in the cosmological expansion of the universe, at least not in Einstein's general relativity, although you can think about it that way when you talk about light moving from galaxy to galaxy.  However, it's more useful (and generalizes better mathematically) to think that the galaxies are really not moving with respect to some grid you've laid down in space (at least not due to the expansion of the universe).  Instead, it's the space inside the grid lines that's getting bigger.  In this way of thinking, the photon/light wave itself really does get stretched out as space grows between successive peaks of the wave.  In my work, I find this to be the clearest way of thinking about it, especially in the very early universe, which is just a big plasma without galaxies (or much other structure), so it's very hard to think about objects moving apart unless you go down to the single particle level.  On the other hand, it's very easy to think about the overall plasma being at rest and just diluting as the universe expands --- and the photons redshift as it does so.


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## freyar (Nov 5, 2012)

The other thing I wanted to talk about (hope I'm coherent now, 'cause I'm way past bedtime) is KarinsDad's discussion with Morrus and others.  I don't want to get into the whole thing about open-mindedness other than to make one point: I am a physicist by profession and know a lot of other physicists.  With very few exceptions, we try to be open-minded in the sense of weighing every new idea.  We might make quick judgements sometimes, we might disagree with each other's opinions, and sometimes we (either individually or as a community consensus) are wrong, but we have well-thought out reasons for those decisions nearly all the time.  And, frankly, those reasons have to do with the data at hand.

But I do want to say a bit more about dark matter and alternatives, since that came up and it's an area of my research.  Dark matter was first discovered about 80 years ago, like KarinsDad mentioned, but most scientists didn't pay much attention at the time because there wasn't enough data to demand it --- the observations could have been fluky in some way.  So the first big "demand" for dark matter was the discovery that galaxies rotate faster than they ought to if (1) gravity works the way Newton and Einstein said and (2) there's just normal matter.  But it works if you add a invisible, nearly pressureless form of matter that interacts via gravity with about 5 times as much total mass as normal matter.  That's the how we define "dark matter."  You can also describe galaxy rotation well if you change gravity.  This is the idea of MOdified Newtonian Dynamics (MOND) and similar theories (I'll lump them together).  However, you can also look at galaxies moving in clusters of galaxies.  They also move faster than they "should" based on normal gravity and just normal matter.  If you add 5 times as much dark matter, you get a prediction consistent with observation.  If you use MOND, you also have to add some kind of invisible, nearly pressureless matter usually identified by MOND enthusiasts as neutrinos (from the Standard Model of particle physics).  So MOND doesn't work perfectly on its own.  Next, you can look at the cosmic microwave background light (CMB), which is the oldest light that's possible to see (any older, and the universe was opaque).  It looks very uniform, but there are tiny variations in its temperature over the sky.  Based on dark matter, people made detailed predictions for the patterns of the CMB years in advance of their measurement. These predictions match the measurements very well.  For  MOND to make these predictions, you need a relativistic version, and there are 2 problems: (1) it's not clear that the relativistic version of MOND is a self-consistent theory of physics as it looks pretty ugly and (2) there's controversy -- meaning conflicting calculations -- about whether it can predict (postdict now, maybe) the pattern of the CMB.

So, dark matter makes good predictions, MOND, not so much.  Nonetheless, MOND is an intriguing option, so there are physicists still working on it, getting grants, etc.  As long as there is a way it might fit the data, people will talk and think about it.  But I hope I've explained why dark matter is the prevalent idea. It just works with the data, and it has the added bonus of being something that makes sense in terms of particle physics (I haven't gotten into that).  I also hope you can see that it's not like what KarinsDad has said about Neptune's winds; it's not that we think we know what's going on and make a prediction in the absence of data (though that is supposed to be one part of the scientific method).  There really is a lot of data consistent with and predicted by the dark matter hypothesis.


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## KarinsDad (Nov 5, 2012)

Umbran said:


> Here's the thing - that's not true.  Yes, in grade school, they may have taught you that the learned people of Columbus' day thought the Earth was flat.  But that was incorrect.  And, these days, we know it is incorrect.
> 
> What you've just proven is that sometimes laymen didn't/don't know a whole lot about what scientists think.  Which, as Morrus points out, is kind of ironic.




Columbus didn't live thousands of years ago (i.e. a minimum of two thousand years ago), the time frame that I mentioned. So, your example here is a strawman at best.

What you just proved is that many people do not read what is actually written, but put their own spin on it.


----------



## KarinsDad (Nov 5, 2012)

freyar said:


> However, you can also look at galaxies moving in clusters of galaxies.  They also move faster than they "should" based on normal gravity and just normal matter.  If you add 5 times as much dark matter, you get a prediction consistent with observation.  If you use MOND, you also have to add some kind of invisible, nearly pressureless matter usually identified by MOND enthusiasts as neutrinos (from the Standard Model of particle physics).  So MOND doesn't work perfectly on its own.




Thank you for your more detailed and thought provoking discussion. As I mentioned earlier, MOND does have weaknesses when discussing clusters of galaxies.

But, have Dark Matter theories never been adjusted as more data is acquired? The fact that adjustments have to be made to MOND appears to be what good science should do. Adjust theories to match observations.

As for cluster issues, doesn't LCDM some times have problems with clusters? For example, in the Bullet Cluster, shouldn't the relative velocity between the two clusters be faster using Dark Matter? The lensing works, but the velocity doesn't? Isn't the Bullet Cluster a problem for both types of theories?



freyar said:


> So, dark matter makes good predictions, MOND, not so much.




Actually from what I've read, MOND makes very good predictions in many areas (and not just galaxy speed), not so good ones in others. To focus on just those where it falters and say that it is probably wrong because of those isn't objective science. No doubt, MOND is minimally incomplete and even possibly incorrect because it cannot explain everything. Dark Matter also has areas where it does not make good predictions (or at least consistent predictions from one galaxy to the next).

There are a lot of recent articles that are starting to support MOND more and more, but some of the more (apparently, who can actually tell) objective sources that I've recently read seem to indicate that it is not winning the fight quite yet. And, this is how you appear to view it.

But, I wouldn't be surprised if the final answer is a combination of a few current theories. The gravitational equations might be wrong and there might be invisible matter out there. There also might be other forces at work beyond just gravity or gravity might not work exactly as thought.

http://blogs.scientificamerican.com...atter-a-glimpse-of-a-deeper-level-of-reality/

But, when someone like Physics Nobel Prize winner Martinus Veltman (who helped architect the standard model of particle physics) states that he doubts that Dark Matter exists at all, other scientists should at least listen:

http://www.scientificamerican.com/article.cfm?id=lindau-dark-energy

He might be wrong. Only time and a lot of hard work will tell.



freyar said:


> I also hope you can see that it's not like what KarinsDad has said about Neptune's winds; it's not that we think we know what's going on and make a prediction in the absence of data (though that is supposed to be one part of the scientific method).  There really is a lot of data consistent with and predicted by the dark matter hypothesis.




An absence of data in the case of Neptune? Actually, this analysis is a bit flawed.

Scientists did have data. They knew the distance from the Sun to Neptune. They had an idea of its mass and chemical composition. They had information on the amount of solar wind that arrived at Earth and models to predict how much would arrive at the other planets. They did have some information on wind speed on other gas giants in the solar system like Jupiter and Saturn and had even more wind speed data once Voyager 1 and 2 got to Jupiter and Saturn, a decade before Voyager 2 got to Neptune.

To insinuate that scientists had no data and then "Wow, now we have data" is incorrect. They had quite a bit of data there. Their theories and models were just incorrect based on the data that they had at the time. But, there wasn't a total lack of data there.


If one does a matrix diagram of Dark Matter theories and Gravity Modification theories, one finds that both types of theories cover some observations very well and other observations, not so well.

The problem that has been creeping more and more into at least the literature (and opinions like Martinus Veltman's) is a) DM has its flaws just like MOND or other theories, but more importantly b) DM just hasn't been found yet. Period. Not even a hint of it. At least to the lay person, DM sounds like magic. We don't know what it is, we've spent many hundreds of thousands of manhours of some of the brightest people on the planet, and many millions of dollars trying to figure out what it is and/or detect it, but we are teaching our students that it must be true because the equations tell us that it is true.

Well, of course one is going to have thousands of scientists the world over that believe it is true if that is what they were taught in school and there is no alternative theory that explains it all better. Just look at how quickly the students in the video above were ready to defend DM. Why? Because that is what they were taught as true science. They know about alternative theories, but they discard them out of hand without putting any real work into them.


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## Janx (Nov 5, 2012)

KarinsDad said:


> Columbus didn't live thousands of years ago (i.e. a minimum of two thousand years ago), the time frame that I mentioned. So, your example here is a strawman at best.
> 
> What you just proved is that many people do not read what is actually written, but put their own spin on it.




well duh!

Case in point, is it possible, that you have put your own spin on what you've read?

You don't like Dark Matter.

Right now, Dark Matter is just a placeholder that makes the math work out.  In turn, that math turns out to correctly account and predict more things than not.

In another thread, I once challenged Umbran on whether any of this quantum mechanic stuff existed or was applicable to the rest of the world.  Because we can't see or prove a quark or other sub-atomic particle actually exists.  He pointed out that things like computer processors were designed using the math and concepts from quantum mechanics.

If the ideas were wrong, then the math wouldn't work and the processor wouldn't process.

It's kind of like what we know about basic chemistry and how atoms try to pair up with other atoms to get 8 electrons in their outer orbit (I suppose some chemist will tell me that all changed since high school).  While I can't see an atom or its parts, I kinda gotta assume its accurate when the model tells me what bonds to what and I can take those chemicals and produce the effect the model describes.

All without actually seeing the protons, neutrons and electrons to prove they exist.

frankly, I'm not sure why you give a monkey's arse so much about this Dark Matter topic.  It is or isn't real.  Don't bloody matter.  the idea of it will change, as scientists futz with it.  I suspect its more of a place-holder concept anyway.  By the time scientists actually discover it, it'll get rebranded as  [MENTION=177]Umbran[/MENTION]ium or something.


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## Umbran (Nov 5, 2012)

KarinsDad said:


> Columbus didn't live thousands of years ago (i.e. a minimum of two thousand years ago), the time frame that I mentioned. So, your example here is a strawman at best.
> 
> What you just proved is that many people do not read what is actually written, but put their own spin on it.




No, sir. I was giving you some benefit of the doubt that your "thousands of years ago" was hyperbole.

If it wasn't hyperbole - citation needed.  At this point, I don't accept your assertion of what scientists or scholars of any period believe or believed, much less what was believed by those who left no written records.


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## KarinsDad (Nov 5, 2012)

Umbran said:


> No, sir. I was giving you some benefit of the doubt that your "thousands of years ago" was hyperbole.
> 
> If it wasn't hyperbole - citation needed.  At this point, I don't accept your assertion of what scientists or scholars of any period believe or believed, much less what was believed by those who left no written records.




Benefit of the doubt??? You change the minimum time frame I was discussing by 1500 years, you then ridicule the notion in the time frame that you chose, and then claim you were giving me the benefit of the doubt?


Here's a citation with references:

http://en.wikipedia.org/wiki/Flat_Earth

A lot of people do not like wikipedia as a strict reference, but if you want to investigate the topic beyond that, please do your own research. I won't be doing it for you. I wrote what I intended and then you ignored what I wrote and changed it to make me sound "less informed" in some way. That's a fallacy in logic. Sorry, it doesn't work that way.


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## Umbran (Nov 5, 2012)

KarinsDad said:


> Benefit of the doubt??? You change the minimum time frame I was discussing by 1500 years, you then ridicule the notion in the time frame that you chose, and then claim you were giving me the benefit of the doubt?




My opening note is very simple - I am making zero effort to ridicule anyone or anything.  I am, as usual, concerned about scientific accuracy, not in making anyone look foolish.  Until you understand that, we aren't going to get very far.

I was giving you the benefit of my doubts, yes.  

We were talking about scholars and scientists, and their behavior.  I took it to mean that your "prevailing view" in context meant "prevailing among scholars".  You brought up Flat Earth as a demonstration of those who *should* have known better sticking to an outmoded theory.  Given that there's a known and documented case in American education where folks we actively taught that example, you should by no means be surprised my mind went there.

I, personally, don't fault anyone who doesn't have the necessary tools for not knowing a thing.  Development of the math to show the Earth is round coincided with (and is generally required for) the building of those towers and masts you mentioned, tall enough to regularly observe the phenomenon.  Which is why Pythagoras and Aristotle worked out the round Earth, as you would say, thousands of years ago.

So, can you demonstrate that those who *should* have known the Earth was round frequently stuck to Flat Earth?  If not, then Flat Earth is not really relevant to our discussion.  Except...

"Dark matter," is exactly analogous to, "look at masts coming over the horizon, and figure out the world is round."  You look at galaxies - you can estimate the amount of mass that's visible.  You can also measure their motions.  As Freyar noted, the two don't match.

That there's extra mass that you cannot see at intergalactic distances (thus "dark" or "missing") is the absolute simplest explanation for the observed phenomenon.  And it is hardly a leap, given that we all live on a chunk of matter that would be "dark" in that sense.  

If you figure anyone who can climb a mountain should guess that the world is round, you should also figure that anyone who can measure the motion of galaxies should figure there's mass present that we cannot see.


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## Janx (Nov 5, 2012)

KarinsDad said:


> Benefit of the doubt???




ya know dude, you sound like you are much more knowledgeable about physics and more read up on it than I am.

Possibly more than Morrus, who is also pretty well versed in physics stuff.  He probably could have watched the tone of his responses a little better.

But reading this thread, you came on a little hot and from an odd angle from the rest of the folks on this thread.  

Ask yourself, could you have entered and handled this thread more smoothly? 


Ignore whatever beef you had with Umbran and Morrus's responses.  Could you yourself have expressed your ideas differently?

I say yeah.  Why are we talking about Columbus?  Or if the guys behind Dark Matter are wrong?  The whole topic is a thought experiment.  Ain't none of it actually scientifically provable yet.  And some theories are quackier than others.  The limit on time frame of the machine's existance just isn't backed up by logic that it MUST be that way.

That's how this whole thing started.  Some dude said something matter of factly, that was restrictive and outside of the just as likely solutions some science dudes said.  I'm inclined to side with the science dudes when they say a thing is possible, it more likely is.


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## Janx (Nov 5, 2012)

Umbran said:


> "Dark matter," is exactly analogous to, "look at masts coming over the horizon, and figure out the world is round."  You look at galaxies - you can estimate the amount of mass that's visible.  You can also measure their motions.  As Freyar noted, the two don't match.
> 
> That there's extra mass that you cannot see at intergalactic distances (thus "dark" or "missing") is the absolute simplest explanation for the observed phenomenon.  And it is hardly a leap, given that we all live on a chunk of matter that would be "dark" in that sense.




That makes sense to me.

Personally, I'd be wondering how a dude could know how much mass is in a galaxy.  At best, with a great telescope, you can make out all the stars.  Assign an average mass per star type and add it all up.

I'm also guessing that you have math that says if you know how much mass these objects have, you can calculate their orbit, speed, etc.  Something that us layman might not realize, as we figure that space stuff is positioned and moving at whatever speed it coagulated at.  this would be by nature of the bowling ball on a bed effect of gravity, I assume.

It sounds like that basic math came out a little short, and the preferred reason was that they guessed wrong on the total mass.  The difference between the "required" mass to match reality and the calculated mass being the existancce of "dark matter"

It's also possible the formula just doesn't scale to galactic proportions yet.  I reckon that's what the MOND theory stuff is about, finding a different formula instead.

Which might really be matter the scientists forgot to count (maybe every star has way more heavy atoms at the center), or the existance of a mystery matter that is invisible, or planets, as we sure haven't found all of them yet.  Given that we can't get there to just scoop some up, I reckon it's understandable on why it remains a mystery.

I see no reason to get cranky about Umbran's dark matter thing.  I trust that if he finds a better explanation, he'll switch to that instead.


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## Umbran (Nov 5, 2012)

Janx said:


> Personally, I'd be wondering how a dude could know how much mass is in a galaxy.  At best, with a great telescope, you can make out all the stars.  Assign an average mass per star type and add it all up.




They don't go about noting every single star - a typical galaxy can have hundreds of billions of stars, so taking them individually is not tractable.  Instead, it is done by measuring overall brightness, and knowing enough about stars to estimate how much mass is required per unit of brightness.



> I'm also guessing that you have math that says if you know how much mass these objects have, you can calculate their orbit, speed, etc.




You can pick out individual stars at various points in the target galaxy, and work out their speeds via noting their doppler shifts.



> It sounds like that basic math came out a little short, and the preferred reason was that they guessed wrong on the total mass.  The difference between the "required" mass to match reality and the calculated mass being the existancce of "dark matter"




The basic math came out more than a little short.  It came out a lot short. The mass of glowing, visible matter is much smaller than you'd expect watching how quickly the objects are orbiting.  Now, if it were a little off, you'd just say that the estimation due to brightness was a little off.  But the difference is so great, most agree there needs to be another explanation.

Moreover, there's an oddity.  In a normal case of objects orbiting a central mass (like, say, in our solar system), you expect things near the center to be moving quickly, but objects far out to be moving slowly.  But, in most galaxies, you see stuff farther out orbiting just as quickly as those things closer to the center.

You can get a model to act like this, if you adjust it to assume there's a halo of mass distributed differently than the mass you see.  Thus, dark matter.



> It's also possible the formula just doesn't scale to galactic proportions yet.  I reckon that's what the MOND theory stuff is about, finding a different formula instead.




Yes.  The idea is that Newtonian gravitation has only been tested when the acceleration due to gravity is relatively large.  MOND (MOdified Newtonian Dynamics) says Newton didn't get it quite right.  Newton says acceleration is directly proportional to force (F=ma).  MOND says that, for small acceleration, it isn't directly proportional to the force.  

MOND is not the only modification of Newtonian mechanics to try to explain the effect.  It is merely (I think) the first and simplest of them.


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## KarinsDad (Nov 5, 2012)

Umbran said:


> I am, as usual, concerned about scientific accuracy




Then we are concerned about the same thing.


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## freyar (Nov 6, 2012)

KarinsDad said:


> Thank you for your more detailed and thought provoking discussion. As I mentioned earlier, MOND does have weaknesses when discussing clusters of galaxies.




You're welcome.  Sorry for the delay on this response, busy day.



> But, have Dark Matter theories never been adjusted as more data is acquired? The fact that adjustments have to be made to MOND appears to be what good science should do. Adjust theories to match observations.
> 
> As for cluster issues, doesn't LCDM some times have problems with clusters? For example, in the Bullet Cluster, shouldn't the relative velocity between the two clusters be faster using Dark Matter? The lensing works, but the velocity doesn't? Isn't the Bullet Cluster a problem for both types of theories?




The basic framework of DM models, specifically, "there exists a non-luminous, nearly pressureless matter with approximately 5x as much total mass as normal matter," has not been changed, no.  The reason is that it hasn't needed to be changed.  The error bars have gotten smaller regarding how much DM you need as the data has gotten better, but that's it.  It has worked very well.  There is a variation in the ratio of normal to dark matter in different galaxies, but you'd expect that the very complicated evolution of structure in the universe would lead to that, and the best computer simulations agree in broad terms.  I should say that structure formation isn't completely understood in either DM or MOND theories.

As for the Bullet Cluster, the observations agree very well with what you expect for DM and not what you'd expect for MOND.  In particular, you expect the DM to go through, like the galaxies (ie, to miss everything), while the gas piles up in the middle.  From what I've read on the MOND description of it, you'd expect extra long-range attraction in MOND, so it's MOND where you'd expect the galaxies to be moving faster than they are.




> Actually from what I've read, MOND makes very good predictions in many areas (and not just cluster speed), not so good ones in others. To focus on just those where it falters and say that it is probably wrong because of those isn't objective science. No doubt, MOND is minimally incomplete and even possibly incorrect because it cannot explain everything. Dark Matter also has areas where it does not make good predictions (or at least consistent predictions from one galaxy to the next).




MOND makes very good predictions for galaxy rotation curves but is not so good for other things.  I want to stress the measurement of the CMB.  The reason that is so important is that the universe was much simpler back when the CMB was formed (looking at the CMB is looking very far back in time).  What that means is that, given a theory (either DM or MOND), you can make precise predictions with very few assumptions and compare them to precise experiments, and there aren't many confounding factors.  So CMB measurements carry a lot of weight because we know what's going on.  There isn't a complicated history to things, no astrophysics (like stars blowing up, etc).  And this is where DM works very well and MOND does not.  Well, last I saw, there was argument about how to do this calculation even.  EDIT: Let me clarify this last statement based on re-reading what I'd remembered, which is worse for MOND than I'd recalled.  Even a fairly strong proponent of MOND agreed that the standard theory taken as the relativistic version of MOND can't reproduce the CMB and furthermore that it's hard to imagine a modified theory of gravity that could if you didn't include dark matter.  Link

I have to go but want to address the rest of this later.


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## freyar (Nov 6, 2012)

KarinsDad said:


> Actually from what I've read, MOND makes very good predictions in many areas (and not just galaxy speed), not so good ones in others. To focus on just those where it falters and say that it is probably wrong because of those isn't objective science. No doubt, MOND is minimally incomplete and even possibly incorrect because it cannot explain everything. Dark Matter also has areas where it does not make good predictions (or at least consistent predictions from one galaxy to the next).




I want to come back to this again briefly.  From studying the CMB, we know that there must be some kind of non-normal dark matter in essentially the right amount to explain the speeds of galaxies in clusters, rotation of stars in galaxies, etc.  Even some strong MOND advocates agree.  Is it possible that MOND is necessary to explain the motions of, for example, stars in galaxies?  Yes.  I haven't heard anyone to argue that it's not possible (that is, another physicist make this case).  However, it seems reasonable to most people that dark matter works well with what data we have, so there is no reason *yet* to add a new ingredient of MOND until we see something that dark matter predicts incorrectly.



> There are a lot of recent articles that are starting to support MOND more and more, but some of the more (apparently, who can actually tell) objective sources that I've recently read seem to indicate that it is not winning the fight quite yet. And, this is how you appear to view it.



I'm not sure about "a lot" of recent articles, at least not scientific research. The vast majority is really on dark matter.  Think of it like a company.  You might spend some of your capital on a risky but high-payoff kind of project (MOND), but you want to invest almost all of it on a less risky but also pretty high-payoff project (DM).  I should also mention that there are strong motivations to believe that there is particle physics beyond the Standard Model of particle physics (for reasons independent of wanting dark matter) which themselves often include possible dark matter particles.



> But, I wouldn't be surprised if the final answer is a combination of a few current theories. The gravitational equations might be wrong and there might be invisible matter out there. There also might be other forces at work beyond just gravity or gravity might not work exactly as thought.
> 
> http://blogs.scientificamerican.com...atter-a-glimpse-of-a-deeper-level-of-reality/



It's possible.  There are people following up on the papers discussed in this blog post.  But some of the history not mentioned in that post is that Verlinde's idea is very similar to work done a long time ago (10-20 years, I think) and has never produced much.  Could it be right?  Yes, but it just doesn't have a good track record yet.  The logic in the papers also seems a bit circular in places, if I recall.



> But, when someone like Physics Nobel Prize winner Martinus Veltman (who helped architect the standard model of particle physics) states that he doubts that Dark Matter exists at all, other scientists should at least listen:
> 
> http://www.scientificamerican.com/article.cfm?id=lindau-dark-energy
> 
> He might be wrong. Only time and a lot of hard work will tell.




I'm listening to the clip.  And people do respect that (listen to the woman talking to the other students at around 11:30).  But I can tell you that he makes some inaccurate statements (in particular, saying that we believe in dark energy because of one experiment -- it was a number of things together that convinced people).




> An absence of data in the case of Neptune? Actually, this analysis is a bit flawed.
> 
> Scientists did have data. They knew the distance from the Sun to Neptune. They had an idea of its mass and chemical composition. They had information on the amount of solar wind that arrived at Earth and models to predict how much would arrive at the other planets. They did have some information on wind speed on other gas giants in the solar system like Jupiter and Saturn and had even more wind speed data once Voyager 1 and 2 got to Jupiter and Saturn, a decade before Voyager 2 got to Neptune.
> 
> To insinuate that scientists had no data and then "Wow, now we have data" is incorrect. They had quite a bit of data there. Their theories and models were just incorrect based on the data that they had at the time. But, there wasn't a total lack of data there.




OK, I think maybe I wasn't entirely clear in what I was saying.  They did not have direct data on the wind speeds at Neptune.  What you're saying is that they had a lot of indirect information to build a model of Neptune's climate -- true -- but that model turned out to be incorrect.  (By the way, solar wind is not the same as a planetary wind.)

This is very different than the case of dark matter.  First off, the idea of dark matter was prompted by data (stellar motion in galaxies).  It wasn't a case of thinking about something we already saw (Neptune) and trying to model something we hadn't directly observed yet (the winds).  Then, people started asking about predictions or consequences of the idea of dark matter.  And these predictions have been verified, in at least one case (the CMB) very very well.






> If one does a matrix diagram of Dark Matter theories and Gravity Modification theories, one finds that both types of theories cover some observations very well and other observations, not so well.
> 
> The problem that has been creeping more and more into at least the literature (and opinions like Martinus Veltman's) is a) DM has its flaws just like MOND or other theories, but more importantly b) DM just hasn't been found yet. Period. Not even a hint of it. At least to the lay person, DM sounds like magic. We don't know what it is, we've spent many hundreds of thousands of manhours of some of the brightest people on the planet, and many millions of dollars trying to figure out what it is and/or detect it, but we are teaching our students that it must be true because the equations tell us that it is true.
> 
> Well, of course one is going to have thousands of scientists the world over that believe it is true if that is what they were taught in school and there is no alternative theory that explains it all better. Just look at how quickly the students in the video above were ready to defend DM. Why? Because that is what they were taught as true science. They know about alternative theories, but they discard them out of hand without putting any real work into them.




You have more than one point here, and I'm going to address them out of order.  Last one first: I think it's really quite unfair to the astrophysics/cosmology/particle physics community to say that it "discards" alternative theories because of "what they were taught as true science."  First of all, there are definitely people working on alternatives, like MOND, and the division of labor is the result of an optimization process: each physicist deciding (1) what seems like a promising avenue based on current evidence and (2) where he or she can make a good contribution.  Next, science has proven to be very good at self-correction.  As you've cited, there are "contrarian" scientists, and people do listen when the give solid arguments.  (Veltman's in that video were not well-articulated IMO, but maybe he has stronger reasons than what came out in the clip.)

Back to the first point.  I hope I've made it clear that the most important, cleanest observation, the CMB, points strongly to the existence of matter outside the Standard Model.  It's also pretty difficult for theories without dark matter to explain things like the Bullet Cluster.

And now, the grand finale .  Why haven't we discovered DM in a lab yet?  After all, there are lots of experiments looking for it as it passes through earth.  Well, first off, there are some experiments that claim to have detected it (though there is a lot of scepticism about those results for various reasons).  But even leaving that aside, should we expect to have discovered DM in those experiments?  Certainly, the models studied the most should be in range of detection, at least.  But why are they the most studied?  At least partly because people wanted to be ready in case they were detected!  Remember, all cosmology tells us is that there's some kind of non-luminous nearly pressureless type of matter not in the Standard Model.  The only way it absolutely has to interact with normal matter -- like our experiments -- is through gravity.  If that's it, our experiments can't possibly detect dark matter passing through the earth.  And I might add that there are some good possibilities for dark matter in well-motivated extensions of the Standard Model that would not be detectable.  So this is what we'd call a model-dependent question.  Yes, it would be disappointing if we can't find DM in a lab.  Would it be a waste of money?  Well, millions of dollars is chump change compared to some experiments, the profits of some corporations, etc.  I tend to think of it as fulfilling our curiosity.


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## Nellisir (Nov 6, 2012)

KarinsDad said:


> Well, of course one is going to have thousands of scientists the world over that believe it is true if that is what they were taught in school and there is no alternative theory that explains it all better.



I....
I mean....




> ...there is no alternative theory that explains it all better.



Do you want them to believe in a theory that explains it all worse?


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## Summer-Knight925 (Nov 6, 2012)

Backwards would be the only theoretical way to travel. The future has endless possibilities and thus cannot be a location.

Backwards, however, means that you have already traveled back in time, and thus cannot change anything, for it would have been changed in your life already.

We can argue about the science all day, but in the end, the flow of time will not be changed.

If I go back in time and kill Hitler, that means he was dead from me already, and I hadn't changed a thing.

if I had gone back and killed me, well that raises all sorts of questions, but nevertheless, that too would already had to have happened.


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## Jhaelen (Nov 6, 2012)

Summer-Knight925 said:


> The future has endless possibilities and thus cannot be a location.



Why do you think so? What evidence do you have that there's actually more than one possible future?

Hopefully you aren't under the delusion that you have something like 'free will' and would therefore be able to influence the future in any way


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## Umbran (Nov 6, 2012)

KarinsDad said:


> Well, of course one is going to have thousands of scientists the world over that believe it is true if that is what they were taught in school and there is no alternative theory that explains it all better. Just look at how quickly the students in the video above were ready to defend DM. Why? Because that is what they were taught as true science. They know about alternative theories, but they discard them out of hand without putting any real work into them.




You know how, when discussing games, we ask folks to not assign motives to others?   That isn't just for game discussions.  It is a general precept.  An internet video or two do not stand as a good gauge of how an entire group of people think.  



freyar said:


> I think it's really quite unfair to the astrophysics/cosmology/particle physics community to say that it "discards" alternative theories because of "what they were taught as true science."




Specifically: Those reading popular reportage of science should not assume they know what's being taught to the students and grad students in the classroom, or discussed among the practitioners.



> The only way it absolutely has to interact with normal matter -- like our experiments -- is through gravity.  If that's it, our experiments can't possibly detect dark matter passing through the earth.




Well, some of the suggestions have ways to interact other than gravity.  WIMPs, for example, also interact through the Weak nuclear force - and with a large enough detector, you might catch them through such interactions.


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## KarinsDad (Nov 6, 2012)

Umbran said:


> Specifically: Those reading popular reportage of science should not assume they know what's being taught to the students and grad students in the classroom, or discussed among the practitioners.




There are lots of online video classroom lectures and other presentations on the Internet these days. Obviously, nobody is up to speed on everything being discussed, but then again, it's not a complete vacuum either.



Umbran said:


> Well, some of the suggestions have ways to interact other than gravity.  WIMPs, for example, also interact through the Weak nuclear force - and with a large enough detector, you might catch them through such interactions.




Or one could catch something else and because the interaction is so weak, one could assume it was what they were looking for as opposed to what it really is. It's an extremely hard science at this point.


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## Janx (Nov 6, 2012)

Jhaelen said:


> Hopefully you aren't under the delusion that you have something like 'free will' and would therefore be able to influence the future in any way




that's a topic for another thread I been meaning to start.  Namely, why don't people like the idea of NOT having Free Will.  We should let this thread finish out though.  Somebody's bound to say something insightful or inciteful yet.


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## KarinsDad (Nov 6, 2012)

freyar said:


> However, it seems reasonable to most people that dark matter works well with what data we have, so there is no reason *yet* to add a new ingredient of MOND until we see something that dark matter predicts incorrectly.




How about the Cuspy Halo problem? CDM is the simplest explanation for DM, but the theoretical solutions to this problem require at least some type of an adjustment to CDM models. MOND doesn't have the Cuspy Halo problem TMK because it's a direct outcome of CDM. Pilipenko, Lukash, et al just released a new model a few months back that might solve the problem, but that doesn't mean that the problem is definitively solved yet, correct?


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## Umbran (Nov 6, 2012)

KarinsDad said:


> There are lots of online video classroom lectures and other presentations on the Internet these days. Obviously, nobody is up to speed on everything being discussed, but then again, it's not a complete vacuum either.




But those video lectures are not representative of the experience of being a grad student in physics, or any of the hard sciences.  You were questioning how actual practitioners thought, and go about their business - video lectures don't show you what and how we actually learn, or how we are taught the activity of doing science.


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## KarinsDad (Nov 6, 2012)

Umbran said:


> But those video lectures are not representative of the experience of being a grad student in physics, or any of the hard sciences.  You were questioning how actual practitioners thought, and go about their business - video lectures don't show you what and how we actually learn, or how we are taught the activity of doing science.




No, but they are representations of what is being taught to grad students, the statement of yours which I was responding to.


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## Janx (Nov 6, 2012)

KarinsDad said:


> No, but they are representations of what is being taught to grad students, the statement of yours which I was responding to.




Are you basically debating about the nature of the ACTUAL content being taught to college physics students?

In that you think it presents a biased view favoring Dark Matter?

I have no doubt that any college professor is going to promote his favorite theory over others, as college profs are prone to be quite verbose when going over their primary field of expertise.

So yes, it is possible that some professors are going to hammer on Dark Matter = real, especially if that's what they own published works favor.

But I'd also be wary against saying science professors are close minded and teach close mindedness to their students.  While they can be guilty of some myopic hypocrisy, as a general rule, the sciences are known to produce people with more open mindedness than any other demographic of humans on the planet.

that's hyperbole on my part, but there ain't any other degrees that teach a methodology for testing your own ideas (the scientific method).

I have no doubt you can find video lectures that prove your point.  Just as Umbran could fiind video lectures that are more open minded.  I think a better test would be to audit a bunch of physics courses at top univerities.  that's where the top new physicists are being made, and that's where they are or are not getting brainwashed.


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## freyar (Nov 6, 2012)

Quoting away, sometimes out of order....

First, about detection of dark matter:


Umbran said:


> Well, some of the suggestions have ways to interact other than gravity.  WIMPs, for example, also interact through the Weak nuclear force - and with a large enough detector, you might catch them through such interactions.




Absolutely!  Although, to be honest, people use WIMP to mean anything with a weak-strength interaction, not just the weak nuclear force any more.  And these interactions with the Standard Model are reasonably well-motivated.  But that part has always been a bit of a guess; maybe there's a whole dark sector with weak-strength interactions and even weaker interaction with the Standard Model.  Or DM might interact relatively strongly with the Standard Model but not with the detectors we've built for some reason (yes, I can give some good ones if you want to know).  My point is that, while experiments are currently excluding a very interesting part of parameter space, interactions with the Standard Model are not at all required for cosmological dark matter to work.  In any case, this is a very very active area because there are a number of hints that we may actually be starting to see some signs of dark matter.



KarinsDad said:


> Or one could catch something else and because the interaction is so weak, one could assume it was what they were looking for as opposed to what it really is. It's an extremely hard science at this point.




Yes, it is, and the claims of possible detections, some of them very strong, face even stronger scepticism.  People know how hard it is and that there are a lot of pitfalls.  We're talking about extremely small signals, and miniscule amounts of radiation, etc, cause problems.  No one knows this better than the people running the experiments.  The results are subject to A LOT of scrutiny.



KarinsDad said:


> How about the Cuspy Halo problem? CDM is the simplest explanation for DM, but the theoretical solutions to this problem require at least some type of an adjustment to CDM models. MOND doesn't have the Cuspy Halo problem TMK because it's a direct outcome of CDM. Pilipenko, Lukash, et al just released a new model a few months back that might solve the problem, but that doesn't mean that the problem is definitively solved yet, correct?




Problem, what problem?   Kidding, kidding...

Let me explain this for our other readers before I respond, since it's clear you've read a lot about dark matter.  I should also mention that the wikipedia page on it isn't very good in that it relies on just a few authors (including at least one who is a noted MOND proponent) and doesn't really get at the consensus.  The issue is that simulations of the formation of galaxies using plain vanilla dark matter (and other standard assumptions) suggest that dark matter becomes extremely concentrated at the center of galaxies.  This is called a cusp.  However, observations of some galaxies suggest that there is no cusp, rather that the density tops out at some maximum value.  This is the origin of the "cuspy halo problem."

Here's the issue with the problem.  First, it's tricky to do observations of dark matter halos (since, you know, dark matter is invisible), and, while people agree that there is no evidence yet of cusps, most observers don't think the observations are good enough to say there is evidence against cusps yet.  At least not in all types of galaxies -- small galaxies called dwarfs are typically accepted not to have cusps.  But the observational situation for big galaxies, like ours, is far from clear, and there could, in fact, be strong cusps still.  On the other side of the coin is that the idea of cuspiness comes from simulations which take millions of CPU hours but do not yet include all the physics.  In particular, it's only recently that simulations have started to go beyond just the gravity of dark matter and to add normal matter, which does things like make stars, cool down, etc.  This can have different effects on dark matter, which might end up making the dark matter more or less cuspy, depending on exactly what happens.  It hasn't been worked out well enough yet.




KarinsDad said:


> There are lots of online video classroom lectures and other presentations on the Internet these days. Obviously, nobody is up to speed on everything being discussed, but then again, it's not a complete vacuum either.






Umbran said:


> But those video lectures are not representative of the experience of being a grad student in physics, or any of the hard sciences.  You were questioning how actual practitioners thought, and go about their business - video lectures don't show you what and how we actually learn, or how we are taught the activity of doing science.






KarinsDad said:


> No, but they are representations of what is being taught to grad students, the statement of yours which I was responding to.




I have to go do some work now, so just briefly: compared to the number of graduate programs in the world, there are very very few graduate course lectures available on-line, and only a percentage of those would relate to dark matter or anything similar.  Also, the most important part of graduate education doesn't take place in the classroom but in meetings with supervisors and on your own reading, doing homework, or doing research (yes, grad students are important researchers).  What that means is that, while even cutting-edge subjects might be presented in a "here it is" kind of way in some lectures, the evidence is also presented, analyzed, and critiqued.  That's part of what you learn to do, hopefully even before grad school.


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## KarinsDad (Nov 6, 2012)

Janx said:


> Are you basically debating about the nature of the ACTUAL content being taught to college physics students?




Not so much the content. Go watch that video I posted above and it appeared that the research students were a bit surprised and even a bit argumentative about what Martinus Veltman was saying. Additionally, George Smoot acknowledged the fact that students believe what they are taught in the video ("Because what your professors taught you, you think that is the true science.") as did Veltman ("they are absolutely convinced, and because that's something that they got taught in their early period"). From the videos I've seen online (and granted, that's a very tiny subset of class and lecture material), I haven't seen much push back by students questioning what they are taught.

My daughter and I have discussed this on many occasions. It's not a matter of merely learning the ideas and theories, especially in theoretical areas, it's a matter of learning where these ideas come from and also on how to evaluate these ideas (i.e. learn how to think). I try to influence her strongly to not only keep an open mind on what she is taught, but find out the reasons behind what she is being taught (who discovered it, how it came about, what competing theories existed, etc.). Being open minded is a bit of a nebulous concept at best, but one that I feel is essential to scientists (considering that she is currently planning to become one).



Janx said:


> In that you think it presents a biased view favoring Dark Matter?




Somewhat.



Janx said:


> While they can be guilty of some myopic hypocrisy, as a general rule, the sciences are known to produce people with more open mindedness than any other demographic of humans on the planet.




You have any evidence to support this claim?

I see lines of strongly opinionated scientists from many different branches of science on both sides of the global warming topic. If the evidence for mankind's influence on it is even somewhat substantial, how come so many scientists from unrelated branches are declaring a side? There either should only be one side that most of them line up on, or these scientists should say that we don't yet have enough data.

Instead, strong opinions, to the point that they are made public, are formed. 

I could conjecture that mathematicians might have more open mindedness than any other demographic of humans on the planet. Course, I would probably be wrong.



Janx said:


> I think a better test would be to audit a bunch of physics courses at top univerities.  that's where the top new physicists are being made, and that's where they are or are not getting brainwashed.




Actually, Stanford has several grad school level lectures online. There are also free classes online from a few top universities. A guy I work with sometimes takes these and I have been considering the idea.


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## tomBitonti (Nov 7, 2012)

A problem in the original discussion space (time travel) is that we really are asking the wrong question first: Not, is time travel possible, but, what would it look like it was possible?  That is, at a fine, measurable, level, what would we see if time travel were actually happening?

Why this is important is because folks are talking about the possibility of time travel without figuring out the basic consequences.

As an example, there is an interesting device in Baxter's Time Ships.  I don't remember what they called it in the book, so I'll just call it Plattnerite Pool.  What you have is a pool table fitted with time machines in the pockets.  A ball dropped into a pocket travels back in time a few seconds or perhaps a minute, then is spat out of a different pocket.

If time travel of a form allowing travel to one's own past is possible, then Plattnerite Pool is possible, at least from a particle interaction point of view, if not as an actual pool table.

If such a device were available, and one had a chance to play with it for a while, the question arises as to what would be the stable behavior of the table?  A globally consistent behavior must arise as a fixed point from the physics and one's interaction with the device.  What possible behavior is there?

As an example: A ball is placed on the table near one corner.  Waiting for a minute or so, a ball appears on the far side of the table, rolls forward to display the first ball, then slowing to take the first ball's place.  The first ball rolls into a pocket and disappears with a flash.

Upon examination, the second ball is seen to be identical, in all ways, to the first ball, except having two very slight smudges where the ball structk itself.

This example would perhaps be impossible: There is a causal loop with no initial cause.

As a second example, one decides to do the simplest experiment, and push a ball into a pocket.  One approaches the table, watches satisfyingly as a ball appears out of one of the pockets at the predicted time, then deposits the ball (in hand) into a pocket.

As a third example, one is clever and decides to mix up the experiment.  Pocket A goes to Pocket C while Pocket B goes to pocket D.  One decides to drop a ball into pocket A if the ball is seen to appear in pocket D, and to drop a ball into pocket B if the ball is seen to appear in pocket C.  What happens?  Does a ball ever appear?

Thx!

TomB


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## Tequila Sunrise (Nov 7, 2012)

No.


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## Umbran (Nov 7, 2012)

tomBitonti said:


> If such a device were available, and one had a chance to play with it for a while, the question arises as to what would be the stable behavior of the table?  A globally consistent behavior must arise as a fixed point from the physics and one's interaction with the device.  What possible behavior is there?




That there *must* be a consistent behavior is an assumption.  It may well be that no consistent behavior is strictly required.  Of course, if it isn't required, one can quickly come up with scenarios where perpetual motion machines and other previously impossible situations become possible.  

It is imaginable that we live in a universe where time travel is not strictly prohibited, but that it can be used in such ways that actually destroy the cosmos.


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## Nellisir (Nov 7, 2012)

freyar said:


> Also, the most important part of graduate education doesn't take place in the classroom but in meetings with supervisors and on your own reading, doing homework, or doing research




I can testify that no amount of sitting in your room, watching internet videos about landscape architecture, is going to give you the experience of sitting here in studio at 2am in the morning for the third night straight beating your project into shape for a 9am presentation.  Along with half your class.


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## Summer-Knight925 (Nov 7, 2012)

Jhaelen said:


> Why do you think so? What evidence do you have that there's actually more than one possible future?
> 
> Hopefully you aren't under the delusion that you have something like 'free will' and would therefore be able to influence the future in any way




Time is often seen as a river, a single flowing length of something.
Many like to believe it has many possible branches and roots and pathways and there are many different 'times'


But really, time is like a big ball of wibbly-wobbly stuff.


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## Nellisir (Nov 7, 2012)

KarinsDad said:


> I haven't seen much push back by students questioning what they are taught.



I can guarantee that much, much, much more of this "pushback" occurs off-camera.  A lecturing instructor has a written plan of the material they want to cover in the 90 minutes they have available.  They usually answer questions that are on topic, but it's not a 90-minute free-for-all about whatever theory you feel like bringing up today.  There is always a small crowd around the instructor afterwards with students seeking clarification, answers, commentary, and an excuse for missing the last class.  Free-for-alls are for office hours.



> Actually, Stanford has several grad school level lectures online. There are also free classes online from a few top universities. A guy I work with sometimes takes these and I have been considering the idea.



Watching a prerecorded lecture online is to sitting in a real classroom, with a real instructor, as watching online porn is to actually having sex.  If that's good enough for you, then so be it, but it's not even close to the same.


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## Janx (Nov 7, 2012)

KarinsDad said:


> Not so much the content. Go watch that video I posted above and it appeared that the research students were a bit surprised and even a bit argumentative about what Martinus Veltman was saying. Additionally, George Smoot acknowledged the fact that students believe what they are taught in the video ("Because what your professors taught you, you think that is the true science.") as did Veltman ("they are absolutely convinced, and because that's something that they got taught in their early period"). From the videos I've seen online (and granted, that's a very tiny subset of class and lecture material), I haven't seen much push back by students questioning what they are taught.




Bear in mind, when it comes to physics, there's been some discovery that many students, who do great at the math, don't actually understand it, and fail the question on if you drop a 1 pound and 2 pound ball at the same time, which one hits the ground first question.

They just weren't getting it, and merely parrot or calculate their way through the material, without really grasping it.

NPR had an article with a physics prof from standford or one of those big schools where he talked about the problem.  One of the things he realized was that the way a 20 year physics veteran explains a concept he has undertstood forever to a student is not the same as how a fellow student who just "figured it out" will do so.

So, he changed up his teaching to present a concept, have the students work out a problem, and then have the students talk and self-correct as the kids who figured it out correctly transmit way of thinking to the rest of the students.

In any case, I'm still wary that a video of a lecture shows the whole picture on interaction with a professor or even fellow students.




KarinsDad said:


> You have any evidence to support this claim?
> 
> I see lines of strongly opinionated scientists from many different branches of science on both sides of the global warming topic. If the evidence for mankind's influence on it is even somewhat substantial, how come so many scientists from unrelated branches are declaring a side? There either should only be one side that most of them line up on, or these scientists should say that we don't yet have enough data.
> 
> Instead, strong opinions, to the point that they are made public, are formed.




Of course I don't.  I was being hyperbolically exagetative like I said I was.

There are always scientists who stick to their theory, and belittle other theories.  Archaeology has the same problem, when some scientist has proof that the presuumed chain of evolution happened differently due to his new fossil.

But, the sciences as a whole, tend to produce people who do tend to change their mind.

As opposed to people who think the world is only 6000 years old.  Those people will die holding that belief their entire lives.  Since humans as a general rule, tend to NOT be able to change their beliefs, scientists are downright openminded as a subset of the human demographic.

You can also probably test this by reviewing the public record on each scientist and see if they later change their position on something the rest of the science community disproved.  I suspect, you will find that most scientists do shift their position as new information is discovered, though they may cling to certain areas more tenaciously than others.


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## tomBitonti (Nov 7, 2012)

Umbran said:


> That there *must* be a consistent behavior is an assumption.  It may well be that no consistent behavior is strictly required.  Of course, if it isn't required, one can quickly come up with scenarios where perpetual motion machines and other previously impossible situations become possible.
> 
> It is imaginable that we live in a universe where time travel is not strictly prohibited, but that it can be used in such ways that actually destroy the cosmos.




Yeah, the *must* is strong, but, the alternative seems to be a universe where science doesn't work.  Quantum mechanics, although random for individual events, fits distributions, and (as I gather) is a consistent theory. When attempting to understand time travel, should we start by assuming that it completely breaks other principles?

Thx!

TomB


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## KarinsDad (Nov 7, 2012)

Umbran said:


> It is imaginable that we live in a universe where time travel is not strictly prohibited, but that it can be used in such ways that actually destroy the cosmos.




One would be hard pressed to imagine the amount of energy required to destroy the cosmos, assuming that it cannot just unravel at the seams. It seems more likely that the energy required to go back in time (i.e. to force everything everywhere to reverse itself) would be infinitely great (or at least so great that it is more than all of the energy in the universe) and hence, not achievable.


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## Umbran (Nov 7, 2012)

Site maintenance prevented me from posting this yesterday:



freyar said:


> Absolutely!  Although, to be honest, people use WIMP to mean anything with a weak-strength interaction, not just the weak nuclear force any more.  And these interactions with the Standard Model are reasonably well-motivated.  But that part has always been a bit of a guess; maybe there's a whole dark sector with weak-strength interactions and even weaker interaction with the Standard Model.




Oh, of course.  The primary characteristic for the stuff is "doesn't interact much with the rest of the Universe, except by gravity" - outside of that, we can imagine all sorts of exotic behavior.  You just try the basics first, is all.



> Or DM might interact relatively strongly with the Standard Model but not with the detectors we've built for some reason (yes, I can give some good ones if you want to know).




I'd be interested in hearing, yes.  



> I have to go do some work now, so just briefly: compared to the number of graduate programs in the world, there are very very few graduate course lectures available on-line, and only a percentage of those would relate to dark matter or anything similar.




And, if you found one, you'd likely find that if you didn't already have the equivalent of an undergraduate degree in math or physics, you'd likely not be able to follow the presentation - grad school lectures in the area are very math-heavy.



KarinsDad said:


> From the videos I've seen online (and granted, that's a very tiny subset of class and lecture material), I haven't seen much push back by students questioning what they are taught.




And, you take this to mean that nobody questions?

The process of hard sciences is dominated by peer review.  That's _PEER_ review, not students reviewing teachers.  The students generally don't know the details well enough to question them on the spot in lecture - remember that each notable model or result will probably take the student some to many hours of work through the math before they have a real handle on it. 

And that's assuming that interrupting a speaker in the middle of the presentation (a presentation that's being recorded for public consumption, even) was the socially acceptable way to go about it.  



> It's not a matter of merely learning the ideas and theories, especially in theoretical areas, it's a matter of learning where these ideas come from and also on how to evaluate these ideas (i.e. learn how to think).




The bulk of a graduate student's career is building up just that.  One topic builds on the next - you don't see that looking at isolated lectures.  



> If the evidence for mankind's influence on it is even somewhat substantial, how come so many scientists from unrelated branches are declaring a side? There either should only be one side that most of them line up on, or these scientists should say that we don't yet have enough data.




You picked an example that's hard to discuss without touching politics.  

But, let's consider it this way: how many individuals are actually responsible for the Edition Wars?  Modern communications make it possible for a small number of very vocal individuals to make it seem as if there's a fundamental controversy, when none really exists.


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## Umbran (Nov 7, 2012)

tomBitonti said:


> When attempting to understand time travel, should we start by assuming that it completely breaks other principles?




We should assume it *might*, yes.  Do remember that time travel implies events coming before their causes - there's not much more fundamental-principle-breaking than that.  




KarinsDad said:


> One would be hard pressed to imagine the amount of energy required to destroy the cosmos, assuming that it cannot just unravel at the seams.




Well, I am not convinced that the assumption that the universe cannot "unravel at the seams" is at all outlandish, once we start considering time travel at all.

Plus, with a time machine, you can have perpetual motion, which means you have access to infinite energy.  And, with that same time machine, you can have that energy all in one time.  It might, subjectively, take a number of loops through time to get the ball rolling, but once that ball is rolling, from the point of view of the rest of the universe, the energy all just pops out of nowhere.

Again, with the "not much more fundamental-principle-breaking than that".


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## tomBitonti (Nov 7, 2012)

Umbran said:


> We should assume it *might*, yes.  Do remember that time travel implies events coming before their causes - there's not much more fundamental-principle-breaking than that.
> 
> Well, I am not convinced that the assumption that the universe cannot "unravel at the seams" is at all outlandish, once we start considering time travel at all.
> 
> ...




Perhaps, but all that seems very unlikely.  Even the advances of relativity and quantum mechanics play nice at the macro level.  I'm more inclined to imagine folks tripping themselves over misunderstandings of very basic principles and common notions which don't carry very well into either relativity or quantum mechanics, and probably don't serve very well, either, when looking more closely at causality.

From a pure physical point of view, what happens when field equations are extended through a closed time-like curve?  If the field is subject to a device which projects the field if the field is absent and is quiet when the field is present, and that is placed on a point on the curve, that seems to cause problems.  Would the field reject placement of the device?

Thx!

TomB


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## Umbran (Nov 7, 2012)

tomBitonti said:


> If the field is subject to a device which projects the field if the field is absent and is quiet when the field is present, and that is placed on a point on the curve, that seems to cause problems.  Would the field reject placement of the device?




This is logically equivalent to the Grandfather Paradox - we don't know the answer.  The equations of physics don't handle recursive conditional statements.


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## KarinsDad (Nov 7, 2012)

Umbran said:


> We should assume it *might*, yes.  Do remember that time travel implies events coming before their causes - there's not much more fundamental-principle-breaking than that.




We should assume that it *might not*.

Why?

Because the universe has not yet been destroyed. 

Even time travel where the observer can go back and observe, but not interact, would imply that at least light leaves the past in order to be observed in the future (which also on some level infers that s/he is interacting anyway). This has some minor possibility of not breaking actual laws (who knows if energy cannot be created or destroyed in a single instant of time, or over all time).


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## Umbran (Nov 7, 2012)

KarinsDad said:


> Even time travel where the observer can go back and observe, but not interact




As I noted upthread, this is not possible.  Thermodynamically, transfer of information requires transfer of energy - taking information from the past means taking energy from the past - so if you observe and come back, you did interact.

Unless, of course, the laws of thermodynamics don't hold - and then we're back with the perpetual motion machines.


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## Janx (Nov 7, 2012)

Umbran said:


> As I noted upthread, this is not possible.  Thermodynamically, transfer of information requires transfer of energy - taking information from the past means taking energy from the past - so if you observe and come back, you did interact.
> 
> Unless, of course, the laws of thermodynamics don't hold - and then we're back with the perpetual motion machines.




Conceptually, when we observe stars, we are looking at their state from millions of years ago (1 year in the past, per light year).

technically, the star was broadcasting the info, and we just happen to catch it in the future.

Though by Umbran's more technical assertion, we did not actualy go back in time and capture that info (thereby removing that energy).

I suppose what Umbran means my Information = Energy, and what most of us mean might also be different.

If I could go back and watch Licoln's assasination, the rays of light in the room are bouncing around and hitting me and my eyes, where previously there was an empty gap.  I can assert that my brain is merely copying the information of what happened, but technically, I was blocking somebody's view of the play, and "taking" energy from the event.

Us layman see that as a pretty passive event.  Especially if I stood someplace out of the way.  Scientists who pay attention to observation protocols would probably nitpick it based on the concept that the act of observing tends to interfere, if only slightly in the event being observed.  Plus, I recall some phenomenon in quantum physics where you tend to find what you were looking for, but that trended toward hippy quantum physics.


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## tomBitonti (Nov 7, 2012)

*Some interesting links*

http://arxiv.org/pdf/gr-qc/0703100.pdf

28-May-2007

Stability of Closed Timelike Curves in the G¨odel Universe

Val´eria M. Rosa
Departamento de Matem´atica, Universidade Federal de Vi¸cosa, 36570-000 Vi¸cosa, M.G., Brazil

Patricio S. Letelier†
Departamento de Matem´atica Aplicada-IMECC, Universidade Estadual de Campinas, 13081-970 Campinas, S.P., Brazil

PACS numbers : 04.20.Gz, 04.20.Dw, 040.20 Jb

Stability of Closed Timelike Curves in the G¨odel Universe

From the top of page 2:

"All our experience seems to indicate that the physical laws do not
allow the appearance of CTCs. This is that, essentially, says the
Chronology Protection Conjecture (CPC) proposed by Hawking in 1992
[16]."

http://physics.stackexchange.com/qu...ke-curves-to-be-impossible-in-quantum-gravity

There are a lot of other links which I haven't scanned.  That second one has this:

These are some notes to complement previous answers.

Your concerns are sound: a rigorous definite way to rule out CTCs has
not been found. What we have is arguments (and quite nice looking
ones) to illustrate that every known universe with CTCs looks
unphysical.

Second, there are two nicely-written pedagogical letters written by
Kip Thorne addressing your question [1],[2]. They mainly focus on
physical aspects of the known CTC solutions, and three popular
mechanisms that could prevent CTCs: violation of the averaged null
energy conditions (the first argument cited in the post), classical
instabilities of chronology horizons, and quantum field instabilities
(following the notation of [2], section 4). Although he does not seem
to believe in CTCs personally, at the end of [2] he states that this
is still an open question:

    It may turn out that on macroscopic lenghscales chronology is not
    always protected, and even if chronology is protected
    macroscopically, quantum gravity may well give finite amplitudes
    for microscopic spacetime histories with CTCs [29].

    [29] Friedman J 1992 in Proceedings of the 4th Canadian Conf. on
    General Relativity and Relativistic Astrophysics eds G Kunstatter
    et al (Singapore: Word Scientific) pp. 183-199.

Finally, regarding the argument against CTCs that uses logical
paradoxes, which has already appeared in the post: it is not clear to
many people whether CTCs inevitably lead to causal paradoxes. Several
studies have pointed out that causal-paradoxes of time travel could
disappear once one takes quantum mechanical effects; or maybe their
meaning could simply change [3],[4],[5],[6]. For instance, in the
framework used in the first reference the grandfather's paradox does
not violate causality. In connection with this, although it is known
that some of these models of CTCs [7],[8] lead to counter-intuitive
collapes of computational complexity classes, this is not exactly the
same as a causal paradox.

The link [1] to a Kip Thorne PDF seems to be the jackpot:

http://www.its.caltech.edu/~kip/scripts/ClosedTimelikeCurves-II121.pdf
"Closed Timelike Curves" Kip S. Thorne

One of the questions reviewed: Do the laws of physics prevent CTCs
from ever forming in classical spacetime?

From page 2:

"However, the combination of general relativity's laws and the laws of
quantum fields in curved spacetime may well provide a chronology
protection mechanism, though we might be sure of this until we
understand the laws of quantum gravity much more deeply than today."

Lots of complicated details to the PDF, which I am not qualified to
present.  There is this end note:

"In summary, these studies are giving us glimpses of how CTSs
influence physics; but whether those glimpses are teaching us
something deep and important or just playing fun mental games, is far
from clear."

Note: The paper is from Feb, 1993.


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## KarinsDad (Nov 7, 2012)

Umbran said:


> As I noted upthread, this is not possible.  Thermodynamically, transfer of information requires transfer of energy - taking information from the past means taking energy from the past - so if you observe and come back, you did interact.
> 
> Unless, of course, the laws of thermodynamics don't hold - and then we're back with the perpetual motion machines.




Precisely.

So, if the laws of thermodynamics are limited to "3 dimensions" (i.e. not time if one considers time a dimension) then backwards time travel in any sense is not possible.


----------



## freyar (Nov 7, 2012)

tomBitonti said:


> <snip...>




Right, that's what I mentioned (way) upthread.  There's not a proof of it at the moment (that I'm aware of), but most physicists believe that CTCs, even if they can exist, can't allow time travel.  The examples we know tend to be unstable, meaning that sending matter along them makes that part of spacetime pinch off.


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## Umbran (Nov 8, 2012)

KarinsDad said:


> So, if the laws of thermodynamics are limited to "3 dimensions" (i.e. not time if one considers time a dimension) then backwards time travel in any sense is not possible.




I don't believe that follows, and wasn't the point I was trying to make.

Most folks, when considering time travel, have one thing that bakes their noodle - causality.  Being able to change the past means you can have paradox.  The typical defense is, "we can *observe* the past, but not interact with it."  I'm merely saying that, in and of itself, isn't possible.  If I observe the past, I must at least take energy out of the past, thus changing it.

If you don't mind changing the past, that's not an issue.  

Robert L Forward wrote a nice novel considering time travel* - his basic posit was that the Universe didn't care if you traveled in time, but the end result would be self-consistent - that was the one law.  You could violate whatever other laws you wanted, but there would be *no* logical paradox.  If you went back in time to kill your own grandfather before your father was born, then, well your genes would get together in some other manner.

The end result was that the time traveler had his free will removed - once he'd changed the past, he also became the vehicle for enforcing self-consistency.  It wasn't like the universe started falling apart if he didn't act, or anything.  He just acts - whatever he thought his choices might have been, in the end, they were choices that enforced the self-consistency.  


*_Timemaster_ - his science, as always, was good, but his characterization leaves a lot to be desired.


----------



## KarinsDad (Nov 8, 2012)

Umbran said:


> I don't believe that follows, and wasn't the point I was trying to make.
> 
> Most folks, when considering time travel, have one thing that bakes their noodle - causality.  Being able to change the past means you can have paradox.  The typical defense is, "we can *observe* the past, but not interact with it."  I'm merely saying that, in and of itself, isn't possible.  If I observe the past, I must at least take energy out of the past, thus changing it.
> 
> If you don't mind changing the past, that's not an issue.




If one considers the "Butterfly Effect", then removing any energy from the past violates causality as well as violating the first law of thermodynamics.

That was the point that I was trying to make.


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## Nytmare (Nov 8, 2012)

Umbran said:


> Robert L Forward wrote a nice novel considering time travel* - his basic posit was that the Universe didn't care if you traveled in time, but the end result would be self-consistent - that was the one law.




It's been a while since I read it, but if memory serves, one of Mark Hodder's 'Burton and Swinburne' novels, [ame="http://www.amazon.com/Strange-Affair-Spring-Heeled-Swinburne/dp/1616142405"]"The Strange Affair of Spring Heeled Jack"[/ame] centers around the follies of a time traveler who keeps jumping further and further back trying to right some wrong that he himself unwittingly created.


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## Umbran (Nov 8, 2012)

KarinsDad said:


> If one considers the "Butterfly Effect", then removing any energy from the past violates causality as well as violating the first law of thermodynamics.




Actually, in my example, the laws of thermodynamics hold - it is their holding that leads to causality violation.  

The laws of thermodynamics say you can't move the information without moving energy.  So, fine, you move the energy (the laws of thermodynamics hold).  But now, whatever that energy did in the past doesn't happen.  Even that doesn't automatically violate causality - causality is only violated if your future is not consistent with that energy not reaching its original destination.


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## KarinsDad (Nov 8, 2012)

freyar said:


> I want to come back to this again briefly.  From studying the CMB, we know that there must be some kind of non-normal dark matter in essentially the right amount to explain the speeds of galaxies in clusters, rotation of stars in galaxies, etc.




Well, if this is true, then there would be an experiment that could (more or less) prove it once and for all.

The planets revolve around the sun in ellipses. These ellipses are distorted ever so slightly by the gravity of other planets (and to some degree, other objects in our solar system).

There should be a way with our current technology to exactly measure the speed and shape of these ellipses as planets go around the sun and determine how much they are distorted by the gravity of the galactic core since there is considerably more gravity in that direction than in the opposite direction.

If DM exists, then the distortions should match those predicted by DM theories.

If DM does not exist, then the distortions should match those predicted by a total lack of DM.

And, of course, it could be somewhere in between.

It would be a complex analysis (having to take into account the speed of light and many other factors to determine the exact position of the planets over an extended period of time), but it should be doable.


This type of experiment (done properly and with the proper precision) would seem to be a long pole in the tent to decide the matter one way or another.


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## tomBitonti (Nov 8, 2012)

KarinsDad said:


> There should be a way with our current technology to exactly measure the speed and shape of these ellipses as planets go around the sun and determine how much they are distorted by the gravity of the galactic core since there is considerably more gravity in that direction than in the opposite direction.




Wouldn't that distortion only measure the mass within the radius of the sun's orbit through the milky way?  The field strength is determined by the total mass within that radius, not by the distribution of the mass.

Probably, the ongoing surveys will help to provide more accurate measurements of stellar velocities, so to tell the mass distribution.

E.g.:

http://www.latimes.com/news/science/sciencenow/la-sci-sn-milky-way-star-map-20121025,0,4431104.story

http://www.astronomynow.com/news/n1201/09APOGEE/

This image from the ESA is simply fantastic:

http://www.eso.org/public/images/eso1242a/zoomable/

If you play with that image, when you zoom give it time to fill in the details!

Thx!

TomB


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## KarinsDad (Nov 8, 2012)

tomBitonti said:


> Wouldn't that distortion only measure the mass within the radius of the sun's orbit through the milky way?




Not the sun's radius, the planet's radius. When planets are closer to the galactic center, the radius of where the planet is located around the galaxy is smaller than if they are away from the galactic center. If DM is a halo around the entire galaxy with or without a higher density towards the center, then a smaller radius of the planet on one side of the sun should have a more distorted ellipse. The velocity when heading towards the galactic center should be greater (by a slightly greater amount) than if DM did not exist, and the velocity away from the galactic center should be less. The amount this is influenced would require extreme precision to measure.

Thinking about it some more, this could disprove DM (or minimally create observational problems for the theory), but it couldn't prove it. If there were other reasons for attraction (like electromagnetic fields), those attractions might still exist.


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## Nagol (Nov 8, 2012)

Our instruments aren't sensitive enough for us to detect the minute variation in something as small as a planetary orbit.

I saw a report of a similar experiment using stellar motions of nearby stars.  http://www.eso.org/public/news/eso1217/


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## Umbran (Nov 8, 2012)

KarinsDad said:


> There should be a way with our current technology to exactly measure the speed and shape of these ellipses as planets go around the sun and determine how much they are distorted by the gravity of the galactic core since there is considerably more gravity in that direction than in the opposite direction.




The term "considerably more gravity" does not have a well-defined technical meaning.

There's a whole lot more mass out there, yes.  But it is very, very far away.  And the force exerted by that mass drops off with the square of the distance.  So, the acceleration due to gravity from the galaxy is actually small.  

Now, what you're actually suggesting we look for is the difference in forces on the planet - when it is close to the galactic core in its orbit, and when it is farther away.  Compared to the distance to the galactic core, the distance across the orbit is very, very small.  The planetary orbit is really just a dot by comparison.

Hm.  Let me see...

It is about 27,000 light years to the galactic core.  Meanwhile, the Earth's orbit is about 8 light minutes in radius.  That's about 1.5x10^-5 light years.  So, the differences are going to be very small indeed.

I think you overestimate our technical abilities, if you think we should, with current tech, be able to measure such differences.


----------



## freyar (Nov 8, 2012)

KarinsDad said:


> Well, if this is true, then there would be an experiment that could (more or less) prove it once and for all.




The results of the CMB measurements are true and publicly available (even the raw data is for many of the independent experiments which have done this measurement).  Furthermore, it's a fairly easy calculation to show that normal matter by itself could not produce those measurements.  Trying to revise gravity to make normal matter make those patterns would require something even more bizarre than MOND (see the link I provided in post 170).  The point is, to physicists trained in this subject, the CMB measurements are far cleaner and more definitive than the experiment you propose.



> <KarinsDad's planet experiment>




This is actually a nice idea but with a few problems.  One, as Umbran points out, I don't think we have the technical capacity to make those measurements.  Two, we know from stellar rotation curves in other galaxies that something affects orbits, be it dark matter or something like MOND.  Well, presumably MOND would affect planetary orbits in some way similar (if not identical) to the way dark matter would.

You might also ask about the gravity from dark matter that is in our solar system.  But there's just not enough of it to make a difference.  The way dark matter ends up being so much more than stars, etc, is that it fills the vast empty regions between stars.




Nagol said:


> I saw a report of a similar experiment using stellar motions of nearby stars.  http://www.eso.org/public/news/eso1217/




Yes, looking at the motions of stars is how we measure how much DM we think is near the sun, which is of course important for experiments on the earth that hope to see DM particles hitting normal stuff.  The paper you mention there was a big deal earlier this year. It uses a new method to look at stellar motions (which is good and interesting) but claimed that there isn't really any dark matter!  That's why it got a lot of press.  However, a reanalysis of their study was done by other authors within a month and found a very serious mistake in how the data was interpreted.  A correct interpretation actually finds agreement with previous studies of DM near the sun.  And the authors of the original paper agreed.  So this is a good example of self-correction in science and how scientists can be open-minded that they were in fact wrong about something.

......
A while back I mentioned that there are models of dark matter that could interact reasonably strongly with normal matter (well, strongly enough that you'd naively expect to have "caught" some DM in an experiment by now) but would not actually turn up in experiments looking for DM on earth.  Umbran said he was interested in hearing about some.  Here are a couple:

1) Assume that some of the experiments which are claiming possible DM detections are right.  Then why don't the other experiments detect DM?  Well, the experiments use different types of atoms --- particularly the nuclei --- to look for DM.  What if DM interacts differently with protons and neutrons?  Then it's possible that interactions with the protons and neutrons in particular nuclei could nearly cancel out.

2) Maybe dark matter has excited states.  Then it's entirely possible that DM interacting with normal matter can't stay in the same state but must always jump between the ground state and excited state.  If the excited state is only slightly more energetic than the dark matter itself, then there is simply not enough kinetic energy for DM hitting a nucleus to jump that excitation gap.  So no scattering is possible.


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## Umbran (Nov 8, 2012)

freyar said:


> 1) Assume that some of the experiments which are claiming possible DM detections are right.  Then why don't the other experiments detect DM?  Well, the experiments use different types of atoms --- particularly the nuclei --- to look for DM.  What if DM interacts differently with protons and neutrons?  Then it's possible that interactions with the protons and neutrons in particular nuclei could nearly cancel out.




That makes sense.  Especially if it is interacting through the Weak nuclear force, the composition of the targets will matter a great deal - and the differences in observed interactions between various detectors should tell you something about the stuff, even.



> 2) Maybe dark matter has excited states.  Then it's entirely possible that DM interacting with normal matter can't stay in the same state but must always jump between the ground state and excited state.  If the excited state is only slightly more energetic than the dark matter itself, then there is simply not enough kinetic energy for DM hitting a nucleus to jump that excitation gap.  So no scattering is possible.




This one seems a bit more arbitrary.  Generally, you expect collisions with things from off-planet to be pretty high energy.  How often would we expect the relative velocities to be so low that there's not enough for a bit of excitation.  Not impossible, but odd.


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## KarinsDad (Nov 8, 2012)

Umbran said:


> I think you overestimate our technical abilities, if you think we should, with current tech, be able to measure such differences.




No?

I was thinking of actually measuring the motion of Neptune against the background of distant galaxies. The Hubble telescope or alternatively, the James Webb or Large Synoptic Survey telescopes (once working) could take photographs (wavelength dependent on telescope) of it at regular intervals over 6 or 8 months of each year. The timing of the photographs would need to be precise, but they could be done with an atomic clock or alternatively, many photographs could be taken over an extended time, but an atomic clock might need to be used to determine exactly when they were each taken. Since the data might need to be collected over many decades, it might take a long time to get a precise answer. But, even early observations would give us a good idea, and it should be technically feasible. There would be problems with the brightness of Neptune versus the brightness of background galaxies, retrograde motion, wobble due to Triton, calculating how much DM affects the Earth itself, etc., but nothing that is technically or mathematically insurmountable.

As for your calculation of Earth, the average diameter of Neptune's orbit around the Sun is ~9 billion km. It is nearly a circular orbit as well (hence, the acceleration / deceleration due to the sun's gravity is very tiny). The amount of dark matter in a spherical halo within the galactic radius when it is closest to the galactic core versus when it is away from the galactic core would be ~99.999990 percent (assuming an equal distribution, a more densely packed distribution would result in a higher percentage). The distance to the galactic core from closest to furthest would be ~99.9999963 percent. But by collecting a lot of data over large time scales, the precision of the measurements increases (by comparing data collected a long time ago with recent data).

Now, this assumes that the sun doesn't significantly change its distance from the galactic core over these time frames. Considering that it takes the sun 225+ million years to orbit the galaxy, even if it has an elliptical orbit, the change in distance would be extremely small in a time frame of decades. But, it should also be measured as best as possible and taken into account for the calculations.

This also wouldn't prevent us from measuring Mars (or other planets) instead, or in addition. The amount of time observing for Mars would be a lot less (3 to 4 years, maybe double that to get more data), but the precision would need to be a lot greater (probably at least two orders of magnitude). Course, since Mars is closer, precision might be greater because of it.

If we can measure that supernovas are accelerating away from our galaxy based on extremely precise temporary brightness from them and hence, calculate that 72% of the universe is Dark Energy, we can measure this. It's a lot closer, easier to see, we can view at optimal times, etc.

Note: I did not state that such an experiment would not be expensive or time consuming, I merely stated that it should be technically feasible. There are also faster and more precise, but even more expensive but technically feasible ways, to measure this (e.g. multiple telescopes in orbit on various sides of the Earth viewing the planet with different angles to acquire a more precise change in background, distance from Earth/sun, etc.). But, it should still be doable with technology already built or in the works today.


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## Nagol (Nov 8, 2012)

Neptune, at 30 AU, is still insignificantly far away compared to galactic core.

When Nepture is closest to the core it is 99.9999997%  (26999.999984 / 27000.000016) of its maximum distance. Or it has a variance of 0.00000012% (1.2x10**-9).

So at its closest, Neptune experiences 100.0000002% of the force felt at its furthest.

Call the mass of the Milky Way 10**12 solar masses.  

That means the difference in force is about the equivalent of 2400 solar masses across 27,000 LY... or half a solar mass at 400 LY Or about 1 tonne about 100 km over the planet.


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## KarinsDad (Nov 9, 2012)

Nagol said:


> Neptune, at 30 AU, is still insignificantly far away compared to galactic core.




Ok, I ran the numbers (using Newton's gravity equation) and it worked out that the difference between DM and no DM over 40 years of observing Neptune heading towards the galactic core as it swung around the Sun (1/4th of an orbit) was just under a kilometer. So, I give on this one.


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## Umbran (Nov 9, 2012)

KarinsDad said:


> Ok, I ran the numbers (using Newton's gravity equation) and it worked out that the difference between DM and no DM over 40 years of observing Neptune heading towards the galactic core as it swung around the Sun (1/4th of an orbit) was just under a kilometer. So, I give on this one.




Yeah.  

And, here's another point - assume you can take the measurement.  It tells you how much total mass is in a shell 27,000 ly radius, and the thickness of the diameter of Neptune's orbit.*  Note that is total mass, not dark-matter-mass.  You're measuring with gravity, so you can't tell the difference.

Do you happen to otherwise know how much normal matter also resides in that shell?  If not, you can't speak to how much of the effect is dark matter.



*For those of you who like perspective games - assume that the sphere is an apple.  The shell is about 2 nanometers thick.  Not the thickness of the skin of the apple.  Not the thickness of the outer layer of cells.  The thickness of one DNA molecule.


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## Nellisir (Nov 9, 2012)

Umbran said:


> *For those of you who like perspective games - assume that the sphere is an apple.  The shell is about 2 nanometers thick.  Not the thickness of the skin of the apple.  Not the thickness of the outer layer of cells.  The thickness of one DNA molecule.




I don't.  My brain hurts.  Please stop.


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## freyar (Nov 9, 2012)

Umbran said:


> That makes sense.  Especially if it is interacting through the Weak nuclear force, the composition of the targets will matter a great deal - and the differences in observed interactions between various detectors should tell you something about the stuff, even.




Actually, the weak nuclear force interacts (roughly) equally with protons and neutrons, so that won't work.  DM and normal matter must interact through two different forces.  I'll give you a simplified model that illustrates the idea but wouldn't actually work (I think).  Imagine that DM interacts with both protons and neutrons equally via the weak force but that DM also carries an electric dipole moment (still electrically neutral, so ok).  The dipole interacts with protons (charged) but not neutrons.  If the sign of the two interactions is different, then the scattering can depend a lot on the target nucleus.  I wouldn't say that working models are more complex but do require a bit more background to explain.  These ideas were pretty popular a year or so ago but have dropped off a bit recently partly because none of the experiments are coming out with new data right now --- it's kind of a wait-and-see phase.



> This one seems a bit more arbitrary.  Generally, you expect collisions with things from off-planet to be pretty high energy.  How often would we expect the relative velocities to be so low that there's not enough for a bit of excitation.  Not impossible, but odd.




The collisions are shockingly low energy, actually.  The earth's orbital speed is only about 1/1000 the speed of light, which is similar to what we believe is a typical DM speed in the galaxy.  But that means the kinetic energy available to excite the DM in a collision with something on earth is only about (1/2) mv^2, or about 1 part in a million compared to the mass of the DM particle (energy mc^2).  So only a small relative gap between DM states is enough to prevent the scattering.  (For typical WIMP masses of around 100 GeV, you're only talking about a 100 keV splitting, which is a fraction of an electron mass).


Since we've been talking dark matter so much (in a time travel thread, oddly enough), I figure you all might be interested in the latest news.  The big news is that, about 6 months ago, analysis of data from the Fermi gamma ray telescope discovered a signal of high energy gamma rays with a set energy coming from the galactic center.  People have for years thought this would be a "smoking gun" for dark matter annihilating very rarely into photons just because there aren't astrophysical mechanisms that can do this easily.  So there's quite a lot of work going on right now to (1) figure out if the signal is real (not a problem with the instrument or how the data is analyzed), (2) see if it has other characteristics consistent with what we'd expect from dark matter, and (3) what models of dark matter can create this signal.  Very exciting times right now in the field; if this holds up, it will mark the first known non-gravitational detection of dark matter and effectively the discovery of a particle outside the Standard Model.  There are of course the usual caveats that this is being scrutinized very carefully as indicated above and that there's also a lot of thought going into whether some kind of standard astrophysics can produce this.  And everyone agrees that more data is needed.  So we'll see how this plays out over the next year or two.


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## Bullgrit (Nov 9, 2012)

Apropos to our the topic.
	

	
	
		
		

		
			





Bullgrit


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## Umbran (Nov 9, 2012)

freyar said:


> Actually, the weak nuclear force interacts (roughly) equally with protons and neutrons, so that won't work.




I know.  I was thinking more about the effect on cross section from the density and size of nuclei, and thinking back to my lectures on how materials for the various neutrino detectors were chosen - the only issue being how long ago those lectures were 



> The collisions are shockingly low energy, actually.  The earth's orbital speed is only about 1/1000 the speed of light, which is similar to what we believe is a typical DM speed in the galaxy.




Ah, yes, that'd do it.  

I'd recalled some reading a while back that suggested that what we'd likely see in detectors was the DM equivalent of a high-velocity star, that has come out of an interaction with a bunch more energy than the run-of-the-mill particle.  The piece included some cogent thoughts on the velocity distribution, and justification for thinking that such particles would be common enough to detect, and more likely to be seen due to the higher energy.


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## Bullgrit (Nov 9, 2012)

Reading these extreme scientific conversations is fascinating to me. But as a non-scientist, I can only understand [parts of] it in a vague sort of way. It's sort of like back in the 80s trying to watch porn on a scrambled cable channel.

Bullgrit


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## Dannyalcatraz (Nov 9, 2012)

> It's sort of like back in the 80s trying to watch porn on a scrambled cable channel.




That had an entirely different kind of dark matter, though.


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## Umbran (Nov 9, 2012)

Bullgrit said:


> It's sort of like back in the 80s trying to watch porn on a scrambled cable channel.




I believe this is the first time I have seen physics-geekery compared to porn.  

Anyone here watch the American version of "Whose Line Is it Anyway"?  They had a game called, "If you know what I mean...," where the players attempt to put together a dialogue using improvised innuendo and euphemisms.  

"Well, he got himself stuck in a closed timelike curve, if you know what I mean..."


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## freyar (Nov 9, 2012)

Bullgrit said:


> Reading these extreme scientific conversations is fascinating to me. But as a non-scientist, I can only understand [parts of] it in a vague sort of way. It's sort of like back in the 80s trying to watch porn on a scrambled cable channel.
> 
> Bullgrit



Yeah, I'm not entirely sure how we got on this subject myself.  It's also very difficult to know how to explain things here, since I like to make things understandable for as many people as possible while at the same time having a discussion on sometimes pretty technical issues with other science folks on the boards.


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## Aaron L (Nov 9, 2012)

Oh I think you're doing fine.  I get easily lost in the math of physics (genuinely having Attention Deficit Disorder tends to screw with ones advanced mathematical abilities, regardless of how intelligent one is otherwise ), but when it's talked about the way you and Umbran have been I find it fairly straightforward to follow.  And utterly fascinating.  Please, keep going.


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## the Jester (Nov 10, 2012)

Aaron L said:


> Oh I think you're doing fine.... I find it fairly straightforward to follow.  And utterly fascinating.  Please, keep going.




All of this!


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## Someone (Nov 12, 2012)

Umbran said:


> I believe this is the first time I have seen physics-geekery compared to porn.
> 
> Anyone here watch the American version of "Whose Line Is it Anyway"?  They had a game called, "If you know what I mean...," where the players attempt to put together a dialogue using improvised innuendo and euphemisms.
> 
> "Well, he got himself stuck in a closed timelike curve, if you know what I mean..."




The Cyberiad has a tale of an artificial poet that composes a poem about love in mathematical language (it makes more sense in context). It's absolutely hilarious.

Edit: I even found the quote:

Come, let us hasten to a higher plane
Where dyads tread the fairy fields of Venn,
Their indices bedecked from one to n
Commingled in an endless Markov chain!

I'll grant thee random access to my heart,
Thou'lt tell me all the constants of thy love;
And so we two shall all love's lemmas prove,
And in our bound partition never part.

Cancel me not — for what then shall remain?
Abscissas some mantissas, modules, modes,
A root or two, a torus and a node:
The inverse of my verse, a null domain.


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## Nellisir (Nov 19, 2012)

Perhaps very pertinent?  http://www.sciencedaily.com/releases/2012/11/121119094627.htm


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## Dannyalcatraz (Nov 19, 2012)

They say they are only certain to a 1 in 10 tredecillion (1043) or 14-sigma level of certainty, but they're getting their info from BaBar.  Why trust any elephant that much?  Especially a FICTIONAL one?


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## Umbran (Nov 19, 2012)

Dannyalcatraz said:


> Why trust any elephant that much?




The Elephant never forgets, of course.  



> Especially a FICTIONAL one?




If being fictional is the worst thing he's done, well, then he's a better entity than most of us, and therefore far more trustworthy.


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## Nellisir (Nov 19, 2012)

Dannyalcatraz said:


> They say they are only certain to a 1 in 10 tredecillion (1043) or 14-sigma level of certainty, but they're getting their info from BaBar.  Why trust any elephant that much?  Especially a FICTIONAL one?




Exactly.


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## Umbran (Nov 20, 2012)

Nellisir said:


> Perhaps very pertinent?




To answer this question without reference to elephants...

Um, maybe?

Here's the thing - this is quantum mechanical result.  The only real entrances we have into time travel are through General Relativity.  

Note how no theory to unite gravity and quantum mechanics has gotten very far?  We don't know how much that result bears on our only current path to an answer for the question.


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## freyar (Nov 20, 2012)

Nellisir said:


> Perhaps very pertinent?  http://www.sciencedaily.com/releases/2012/11/121119094627.htm






Umbran said:


> To answer this question without reference to elephants...
> 
> Um, maybe?
> 
> ...




I'll agree with Umbran as far as he goes on this, but I'd also add that there's probably nothing to do even with quantum time travel in this result.  Specifically, this experiment looks at a type of particle known as a "B0," which has the unusual (but not unheard of) ability to change into its own antiparticle.  _Now I'm going to oversimplify grossly._  What BaBar did _more or less_ is measure the speed at which B0 turns into its antiparticle compared to the speed at which the antiparticle turns back.  There are really a lot of technical details that make that not quite right, but the gist is that the two speeds are different.  So the one reaction is not the same speed if you run it forwards vs backwards in time.  This is called time-reversal violation, and it's been expected for a long time.  In fact, it's probably been measured before with other types of particles, but those earlier measurements have some more errors.  So there's really nothing about going back in time; it's about whether two reverse processes happen at the same speed going forward in time.

Another point to make is that there is actually always (mathematically proven) a way to reverse a process and get the same speed.  This is called CPT because it involves reversing the time (T) (ie, the order of the process), parity (P) (which is reversing all the spatial motion), and charge (C) (turning all particles to their antiparticles).  If you do that, you get two equal speeds; if not, well, you'll (1) have to subject your results to incredibly intense scrutiny and (2) win fame, fortune, and a Nobel prize if you're right (but you're probably not ).  

There's a reasonably nice not-too-technical explanation here.


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## KarinsDad (Nov 21, 2012)

freyar said:


> So there's really nothing about going back in time; it's about whether two reverse processes happen at the same speed going forward in time.




And that's where "the public perception" of physics can sometimes get out of sync with what can really be concluded from experiments.


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## Dannyalcatraz (Nov 21, 2012)

Umbran said:


> To answer this question without reference to elephants...
> 
> Um, maybe?
> 
> ...




So...you're saying that article might be irrelephant?


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## Nellisir (Nov 21, 2012)

Umbran said:


> To answer this question without reference to elephants...
> 
> Um, maybe?
> 
> ...




And thus the "perhaps"


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## freyar (Nov 21, 2012)

KarinsDad said:


> And that's where "the public perception" of physics can sometimes get out of sync with what can really be concluded from experiments.



True enough, and the media doesn't usually help.  In this case, even the press release from SLAC, the lab that hosts the BaBar experiment, makes a big error just for the sake of getting a splashier headline (see the link I posted above).


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## Umbran (Nov 21, 2012)

freyar said:


> Another point to make is that there is actually always (mathematically proven) a way to reverse a process and get the same speed.




Yah.  This is a case where natural language and physics language differ.  You gave a nice link, but maybe I can take a really short stab at it...

What he's talking about is not a physical process you can enact in the real world.  When you say "reverse a process" in natural language, it means actually reversing it - Bruce Banner turns into the Hulk, the Hulk turns into Bruce Banner.  Reversible.

When we talk about this in natural language with respect to time, and the arrow of time, we are usually talking about things on the line of "we stir sugar into water, and it dissolves".  This is not reversible - we cannot stir the other way and have all that sugar come out of solution and reform crystals.

Freyar is talking about a form of _mathematical _reversal - kind of like saying that if you flip _*all*_ the numbers to be their negatives, the thing runs the same way.  This is not equivalent to stirring the water the other way - it includes reversals that we cannot enact in the real world (yet?).


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## freyar (Nov 21, 2012)

Umbran said:


> Yah.  This is a case where natural language and physics language differ.  You gave a nice link, but maybe I can take a really short stab at it...
> 
> What he's talking about is not a physical process you can enact in the real world.  When you say "reverse a process" in natural language, it means actually reversing it - Bruce Banner turns into the Hulk, the Hulk turns into Bruce Banner.  Reversible.
> 
> ...



That's a pretty good stab at it!


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