# Travelling through a wormhole in space



## Bullgrit (Jun 9, 2015)

Movies and other visual media always show travel through wormhole-like portals as being a psychedelic trip through a long tunnel. I understand that no one knows for sure what wormhole travel would actually be like because it isn't possible at this time, but from what science does know/theorize, what is more likely:

A long flashy tunnel or an instantaneous popover?

Bullgrit


----------



## Morrus (Jun 9, 2015)

I'm sure Umbran will provide better info, but to my most recent reading about the subject in popular science literature, there isn't a theoretical way to travel as any wormhole collapses as soon as matter enters it (and wormholes have never been observed, of course - they're just a solution to an equation).

I wonder if time dilation and other weird effects would take place?  Would your perception of time be even relevant?

The_ Interstellar_ depiction of a wormhole is - as far as I know - what Kip Thorne (physicist) says one would look like.  A shiny sphere, rather than the funnel you see in _Deep Space 9_.


----------



## Scott DeWar (Jun 9, 2015)

Morrus said:


> I'm sure Umbran will provide better info, but to my most recent reading about the subject in popular science literature, there isn't a theoretical way to travel as any wormhole collapses as soon as matter enters it (and wormholes have never been observed, of course - they're just a solution to an equation).
> 
> I wonder if time dilation and other weird effects would take place?  Would your perception of time be even relevant?
> 
> The_ Interstellar_ depiction of a wormhole is - as far as I know - what Kip Thorne (physicist) says one would look like.  A shiny sphere, rather than the funnel you see in _Deep Space 9_.




Since I am curious what he would say, then [MENTION=177]Umbran[/MENTION]!


----------



## Deset Gled (Jun 9, 2015)

Assuming sci-fi style wormholes ever become a physical reality, I think travel-able ones would just be black.  They shouldn't emit their own light, since that would mean they're undergoing some high energy process that makes them unsuitable for human travel.  They shouldn't reflect light, since that would mean incomplete energy transfer through the wormhole.  And there would be light that comes through them, but if that light is being refracted/distorted in any artistic way, matter traveling through the wormhole probably wouldn't fare much better.


----------



## Joker (Jun 9, 2015)

I always imagined them as not having a visual representation.  Like one moment you're in galaxy A and then you move a bit forward and you're in galaxy B.  Or walking into a cave system on one planet and exiting on another planet in a different part of the universe.

Of course, I have nothing to back this up with.


----------



## Scott DeWar (Jun 9, 2015)

I have to admitt, I am watching Dr. Who; Collection 2, Episode 1: The mind robber part 1 and tho opening of the show looks to be something of what I  have heard to be an acid trip.


----------



## Olgar Shiverstone (Jun 9, 2015)

Let me just pop through this wormhole and I'll let you know ...


----------



## GMMichael (Jun 9, 2015)

I always kind of pictured a Subway restaurant, filled with sumo wrestlers giving you high-fives as you walk through.

More scientifically speaking, if a wormhole is a tunnel from one point of a universe to another, then one the one hand I don't see any special time effects on the grounds that time is not a separate element from space (each universe marks only one point in time).  On the other hand, if your position changes so radically that the time you spend traveling is time the rest of the universe spends aging, then wormhole travel could look like traveling into the future.


----------



## freyar (Jun 9, 2015)

Morrus said:


> I'm sure Umbran will provide better info, but to my most recent reading about the subject in popular science literature, there isn't a theoretical way to travel as any wormhole collapses as soon as matter enters it (and wormholes have never been observed, of course - they're just a solution to an equation).
> 
> I wonder if time dilation and other weird effects would take place?  Would your perception of time be even relevant?
> 
> The_ Interstellar_ depiction of a wormhole is - as far as I know - what Kip Thorne (physicist) says one would look like.  A shiny sphere, rather than the funnel you see in _Deep Space 9_.




From a not-quite-insider's view, that's what I have heard about wormholes as well, Morrus, ie, all the ones we know about are unstable.  Time dilation would almost certainly happen, but of course that's relative to someone who doesn't go through or too near the wormhole.  That means, like always, you see one second as being one second.  It's just your seconds are different than someone else's.  

I haven't seen Interstellar, but I thought that Kip Thorne provided the equations describing light going around a black hole rather than specifically a wormhole.  Many mathematical descriptions of black holes do include wormholes, which is probably what the movie uses.  But those are "eternal" black holes --- ones that just sat around from the dawn of time, rather than forming by the collapse of a star or something.  So that's still sci-fi.   Also, I think the shiny ring around that black hole is light from the accretion disk behind the black hole that gets bent around to our view by gravitational lensing.  If you just had a wormhole (or black hole) hanging around in space with no matter nearby, it would be like Deset Gled says.  Just black.


----------



## Morrus (Jun 9, 2015)

freyar said:


> I haven't seen Interstellar, but I thought that Kip Thorne provided the equations describing light going around a black hole rather than specifically a wormhole.




Both feature in the movie.  He provided consultancy on both.


----------



## Umbran (Jun 10, 2015)

Okay, let me try to address some points....



			
				bullgrit said:
			
		

> A long flashy tunnel or an instantaneous popover?




Popovers are not instantaneous.  I mean, you need tie to make the batter, and then the things have to bake...

But seriously...

I don't think there's a "most likely" here.  As far as I am aware, there's no particular theoretical constrain on how long the trip through the trip through the wormhole is.  I would presume that, should the engineering allow it, folks would make wormholes with very short paths, but maybe that's not possible.



			
				Morrus said:
			
		

> I'm sure Umbran will provide better info, but to my most recent reading about the subject in popular science literature, there isn't a theoretical way to travel as any wormhole collapses as soon as matter enters it (and wormholes have never been observed, of course - they're just a solution to an equation).




Yes, they are a solution to an equation, and none have ever been observed.  There is, to date, no _practical_ way to create or travel though such a thing.  But, as for theoretical ways, yes, there are ways that work, in theory.

Einsteinian relativity allows for space to be shaped such that it has a "shortcut" through it.  The problem is that all such solutions are not stable to small perturbations - which means that even small changes in gravitational fields in or near the wormhole tend to collapse it.  However, Kip Thorne and others have come up with various ways to stabilize wormholes against these small perturbations.  They all require application of materials or energies with unusual properties that, again do not seem to be prohibited by the Universe, but we have never observed in nature or made ourselves in anything like large enough quantities.  These are usually lumped under the term "exotic materials".  There are a few solutions I've seen that suggest that quantum effects could also be used - but since we don't have a good marriage between quantum mechanics and gravitational effects, that's very speculative.

Which amounts to there being ways in theory.  Which makes sense, as wormholes have not been observed, so they only exist in theory anyway 



			
				Joker said:
			
		

> I always imagined them as not having a visual representation.




Well, yes and no.  When you look at a wormhole, you aren't looking at a *thing*.  The wormhole is a path through space.  So, like when you look through a doorway, you'll see whatever is on the other side of the door.  If both ends of the wormhole are in basically open space, this won't look like much, no.  But, if one is in New York City's Central Park, and the other end is in the Sahara Desert, you're looking trough a door into a radically different environment.

Someone actually did an image rendering, taking the curvature of space involved, for one type of wormhole.  It looks like this:



(this is "Wurmloch" by CorvinZahn - Gallery of Space Time Travel (self-made, panorama of the dunes: Philippe E. Hurbain). Licensed under CC BY-SA 2.5 via Wikimedia Commons )

There are other, slightly different solutions for what we'd call a wormhole, that have somewhat different appearances to our eyes.  But I think it is this type that was used in "Interstellar".  



			
				freyar said:
			
		

> Time dilation would almost certainly happen, but of course that's relative to someone who doesn't go through or too near the wormhole.




I am not sure this is correct.  The actual path through the wormhole is pretty much normal, flat space. The wormhole has an odd appearance, but there is no singularity, and so no ever-increasing curvature of spacetime that creates time dilation as you see around a black hole.  

Now, if one end of the wormhole is *moving*, relative to the other end, then funny things can happen.  Since it is a deformation of spacetime, it can be moving in space and/or in time...


----------



## freyar (Jun 10, 2015)

Umbran said:


> I am not sure this is correct.  The actual path through the wormhole is pretty much normal, flat space. The wormhole has an odd appearance, but there is no singularity, and so no ever-increasing curvature of spacetime that creates time dilation as you see around a black hole.
> 
> Now, if one end of the wormhole is *moving*, relative to the other end, then funny things can happen.  Since it is a deformation of spacetime, it can be moving in space and/or in time...




Gravitational time dilation isn't only found near singularities or extremely high curvature.  In fact, it's measurable even in the weak gravity of the earth --- GPS units have to account for it.  Generically, meaning except in extremely special circumstances, there should be some degree of time dilation, and usually we expect weird situations to include some fairly substantial 

But I'll freely admit that I didn't look up any wormhole metrics before making that statement, so I went and looked just now.  One thing I found was a pedagogical paper by Michael Morris and Kip Thorne, who seem to have pushed the serious study of wormholes in the late 20th century.  They make a point of analyzing precisely how to reduce time dilation effects --- since that also reduces tidal forces.  There are indeed wormhole solutions with no time dilation, but they have "exotic matter" everywhere.  We live in a universe without much exotic matter, so a realistic wormhole solution requires matching onto a long-distance solution with normal matter.  Then you do get time dilation.  If you want to keep time dilation effects small and the exotic matter localized to the wormhole itself, you need a wormhole that's about 600 AU across.  Given that the exotic matter needed to create these things is pretty rare, naturally-occurring wormholes would seem likely to have significant time dilation effects.  On the other hand, maybe an advanced civilization could construct one without much time dilation.

That's of course depending on whether exotic matter with precisely the right properties to make wormholes exists.  It's certainly possible to violate the energy conditions that might look like they forbid exotic matter; indeed, some kinds exotic matter of that type looks to be perfectly fine.  But other types of exotic matter seem to cause mathematical inconsistencies, so I can't say about what's needed for wormholes.  Morris & Thorne also note that wormholes of the type they study are also necessarily time machines, so that may or may not be a clue that they're not allowed.

It's also worth noting that the Morris-Thorne wormholes and the eternal black hole wormholes (called Einstein-Rosen bridges) don't connect two points on the same part of space.  They connect two different parts of space.  In other words, I travel from earth to a wormhole, go through the wormhole, then travel to planet X.  If I want to get back to earth, I can't go the "long way around," I have to go through the wormhole (or some wormhole) again.


----------



## tomBitonti (Jun 10, 2015)

To say, Interstellar shows two different phenomena: The initial step of the journey is through a wormhole which orbits Saturn.  A journey through that wormhole takes you to the black hole, Gargantua.  There were depictions of both the wormhole and of the black hole.  

My understanding is that we have a pretty good idea of lensing effects around a black hole.  Here is a discussion of the black hole images from the movie:

http://io9.com/the-truth-behind-interstellars-scientifically-accurate-1686120318

Kip Thorne did evidently collaborate closely with the movie producers:

http://www.space.com/27701-interstellar-movie-science-black-holes.html

There is a nice view of the wormhole here:

http://www.quora.com/Why-did-the-wo...nding-of-light-at-the-periphery-of-the-sphere

I don't think that the wormhole in this depiction is actually "shiny": The apparent reflection at the edges is more an illusion of the way the light reaches us through the wormhole.

I would presume that there would be effects on the outside of the wormhole, too, but, these are not visible because they are too faint.  That would be lensing of the background field, but it would seem to be much fainter than what would be visible through the wormhole, and would be washed out of the view to the naked eye.

I would think that the image is correct for a specified topology and curvature, but very possibly inaccurate in that additional effects are not depicted, say, particle creation (the worm hole might be a strong particle emitter), or instabilities (the wormhole, as shown, is perfectly uniform).  Then there is the problem of a mass-energy distribution which could create the curvature (egad!)

In any case, I imagine the view is much more realistic than the "funnel in space" type view, for example, from Farscape:

http://www.foundation3d.com/index.php?categoryid=38&p13_sectionid=376&p13_fileid=461

I do wonder what the view would be for a non-spherically symmetric wormhole, e.g. Sliders or Stargate.  Could you actually move curvature to the edges and have simple uncurved flat transition at the center?

Here is a thought experiment: Would a spherically symmetric wormhole create a detached space representing the prior "interior"?  That is, in 2D, if you glued two sheets together and cut a hole in them, then stretched both sheets to make for a smooth transition around the edges, that would leave you with the two cut discs, which you could also smooth out to make a closed but unbounded space.  (I've used this in one my games as a prison for a Mythic Rune Giant  )

Edit: There is a nice animation of a black hole moving across an edge on view of a galaxy:

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

And if you are up for the maths, here is a discussion of wormhole lensing:

http://www.nipne.ro/rjp/2012_57_3-4/0736_0747.pdf

Thx!

TomB


----------



## Umbran (Jun 10, 2015)

freyar said:


> Gravitational time dilation isn't only found near singularities or extremely high curvature.  In fact, it's measurable even in the weak gravity of the earth --- GPS units have to account for it.




With respect, the fractions of a second involved there are not what laymen are thinking of when they hear the term.  They're thinking about the Interstellar "if we take this path, we will age little, but many years will pass on Earth," variety.  



> One thing I found was a pedagogical paper by Michael Morris and Kip Thorne, who seem to have pushed the serious study of wormholes in the late 20th century.




Kip Thorne wrote, "Black Holes and Time Warps: Einstein's Outrageous Legacy" in 1994.  It has been a while since I opened my copy, but on the other hand it is now 20 years old.  I'd be surprised if Kip hasn't gone well beyond that work by now.



> There are indeed wormhole solutions with no time dilation, but they have "exotic matter" everywhere.




The ones that *don't* use exotic matter require a particular modification to General Relativity, Gauss-Bonnet gravity, involving some extra spacial dimensions, to be correct.  Such solutions are often associated with brane theory.  So, unless we have enough branes, we need exotic matter to do the job.  Pun intended.   



> It's also worth noting that the Morris-Thorne wormholes and the eternal black hole wormholes (called Einstein-Rosen bridges) don't connect two points on the same part of space.  They connect two different parts of space.  In other words, I travel from earth to a wormhole, go through the wormhole, then travel to planet X.  If I want to get back to earth, I can't go the "long way around," I have to go through the wormhole (or some wormhole) again.




You might want to clarify what you mean by the "same part of space".


----------



## Umbran (Jun 10, 2015)

tomBitonti said:


> I do wonder what the view would be for a non-spherically symmetric wormhole, e.g. Sliders or Stargate.  Could you actually move curvature to the edges and have simple uncurved flat transition at the center?




Something like a Stargate wormhole presents difficulties - what happens at the edge of that circle?  

If you get your stargate by taking a sphere and squashing it flat, such that both "sides" (and the edge, technically), are still wormhole, then you're okay.  But if it has a "front" but no "back" so to speak, you have a discontinuity in space - what we'd tend to call a singularity, but in this case as a loop, rather than a point.

I think there are solutions with such, but they tend to have things like cosmic strings at the edge.  And you don't get much more exotic than that.



> Here is a thought experiment: Would a spherically symmetric wormhole create a detached space representing the prior "interior"?




Wild-arsed guess - you don't cut out a circle to make a stable wormhole.  You start with a pinpoint aperture, on the Planck scale of distances, and widen it.  So, the process is more like putting a needle through a sheet of clay and slowly deforming into a larger hole.


----------



## tomBitonti (Jun 11, 2015)

Umbran said:


> Something like a Stargate wormhole presents difficulties - what happens at the edge of that circle?
> 
> If you get your stargate by taking a sphere and squashing it flat, such that both "sides" (and the edge, technically), are still wormhole, then you're okay.  But if it has a "front" but no "back" so to speak, you have a discontinuity in space - what we'd tend to call a singularity, but in this case as a loop, rather than a point.
> 
> ...




Yeah.  Stargate conveniently avoids a problem by hiding the edges.

What I was wondering, though, is if you squashed a surface of a spherical wormhole flat, there would still be a big transition between the throat and flat spacetime.  The curvature up to the flat section of the throat would be uniform in two dimensions (perhaps), but not in a line moving away from the throat.

Thx!

TomB


----------



## Umbran (Jun 11, 2015)

tomBitonti said:


> What I was wondering, though, is if you squashed a surface of a spherical wormhole flat, there would still be a big transition between the throat and flat spacetime.




You shift from wondering (which implies a question) to a statement.

Would there be a big transition?  That depends on what you call "big".  In general, this thing is supposed to be traversable.  You need spacetime to be pretty flat on distance scales roughly on order of the size of the object that goes through - otherwise you either turn the item into spaghetti, or you have a problem of the subject's head being in today, while her feet are already in next week.  So, where you expect people to go, spacetime isn't terribly curved, and what curvature there is can't change a whole lot.

I think most of the traversable solutions have spacetime largely flat along the throat of the wormhole and out into "normal" space, and squeeze to very steep curves towards the edges/sides.  So, you can travel down the middle, but hitting the "wall" would be bad for you.


----------



## tomBitonti (Jun 11, 2015)

Umbran said:


> You shift from wondering (which implies a question) to a statement.
> 
> Would there be a big transition?  That depends on what you call "big".  In general, this thing is supposed to be traversable.  You need spacetime to be pretty flat on distance scales roughly on order of the size of the object that goes through - otherwise you either turn the item into spaghetti, or you have a problem of the subject's head being in today, while her feet are already in next week.  So, where you expect people to go, spacetime isn't terribly curved, and what curvature there is can't change a whole lot.
> 
> I think most of the traversable solutions have spacetime largely flat along the throat of the wormhole and out into "normal" space, and squeeze to very steep curves towards the edges/sides.  So, you can travel down the middle, but hitting the "wall" would be bad for you.




Sorry, writing too quickly.  I have a better understanding of the topology than the physics, so I'll approach from that direction.

Putting this in two dimensions: You could imagine a clean splice/join, with two sheets joined across a cut.  If you made the cut a line, then you would have a discontinuity at the ends of the slice, but a completely flat transition across the cut.

You can make the join a ring, which avoids the end effects, but you end up with a different problem.  The join shifts you from a concave segment to a convex segment.  (Imagine pushing a curved bar through the cut: It ends up curved the other way.)  Crossing the throat would be the same as a bend, and probably a big problem for a small ring.  The problem is the shift from heading in to the ring to heading away from the ring.  There is no adjustment to the join which avoids this problem (without changing the local metric).

If you use a ring and set the metric to make the join locally flat (not sure if that makes sense), then you have the problem of the transition from asymptotically flat space to the join, with curvature required by the transition.  Which means there are apparent forces and tidal stress, so still a problem.

The question, then, is how much does this line of thinking make sense: For a physical wormhole, what case occurs?  I'm thinking the third.  But then, Stargate or Sliders would have forces (potentially huge ones) near the throat.  Is there any way to avoid these?

Thx!

TomB


----------



## freyar (Jun 11, 2015)

Umbran said:


> With respect, the fractions of a second involved there are not what laymen are thinking of when they hear the term.  They're thinking about the Interstellar "if we take this path, we will age little, but many years will pass on Earth," variety.



Fair enough.  Please note, I've not seen the movie, so I'm just going on hearsay there.  However, the rest of the point is that Morris & Thorne have argued that, if you want to engineer a wormhole to have negligible time dilation and tidal forces (ie, comfortable for human travel), it has to be big.  Like 20x as big as the solar system.




> Kip Thorne wrote, "Black Holes and Time Warps: Einstein's Outrageous Legacy" in 1994.  It has been a while since I opened my copy, but on the other hand it is now 20 years old.  I'd be surprised if Kip hasn't gone well beyond that work by now.



I just did a literature search on the best research database for this kind of thing, looking for research papers by Kip Thorne about wormholes.  I don't find any evidence for such work between 1993 and 2015 (the 2015 papers were about visualization for _Interstellar_).  A number of other authors have worked on wormholes, of course, and I'm skimming through some of that literature for curiosity's sake.



> The ones that *don't* use exotic matter require a particular modification to General Relativity, Gauss-Bonnet gravity, involving some extra spacial dimensions, to be correct.  Such solutions are often associated with brane theory.  So, unless we have enough branes, we need exotic matter to do the job.  Pun intended.




I think I must not have explained properly.  Morris & Thorne weren't looking at wormholes without exotic matter.  They just didn't want exotic matter filling up the entire universe, so they wanted spacetimes that have a wormhole created by exotic matter surrounded by space with normal matter (including people, space stations, etc), like where we live.  To do that, they found a "comfortably traversable" wormhole needed to be very large.  Smaller than that would have large tidal forces and at least noticeable time dilation.  Keep in mind that Morris & Thorne were specifically trying to make wormhole spacetimes with small amounts of time dilation and were willing to postulate any characteristic of matter necessary.

Side point: Gauss-Bonnet gravity need not be associated with brane-world models.  The point is that the Gauss-Bonnet term doesn't change the Einstein equations in 4D; they only do that in 5D and above.  That's interesting for brane-worlds as well as other extra dimensional models.  Also as a side note, I've written a paper using 5D GB gravity myself.



> You might want to clarify what you mean by the "same part of space".




In technical terms, the wormhole solutions written down connect two distinct asymptotic regions.  In plain English, the two ends of the wormhole are connected to two distinct universes, or maybe more precisely two regions of space that don't connect to each other except through the wormhole.  In principle, I don't think there's a reason you couldn't make a wormhole connecting two parts of the same universe, but I haven't yet seen a metric that does that.  It's not what people typically study when the look at wormholes.




Umbran said:


> You shift from wondering (which implies a question) to a statement.
> 
> Would there be a big transition?  That depends on what you call "big".  In general, this thing is supposed to be traversable.  You need spacetime to be pretty flat on distance scales roughly on order of the size of the object that goes through - otherwise you either turn the item into spaghetti, or you have a problem of the subject's head being in today, while her feet are already in next week.  So, where you expect people to go, spacetime isn't terribly curved, and what curvature there is can't change a whole lot.
> 
> I think most of the traversable solutions have spacetime largely flat along the throat of the wormhole and out into "normal" space, and squeeze to very steep curves towards the edges/sides.  So, you can travel down the middle, but hitting the "wall" would be bad for you.




And how to make them flat enough to traverse was the point of the Morris-Thorne paper.  Key point --- you have to make them big to avoid either tidal forces, time dilation, or both.


----------



## Umbran (Jun 11, 2015)

freyar said:


> Fair enough.  Please note, I've not seen the movie, so I'm just going on hearsay there.  However, the rest of the point is that Morris & Thorne have argued that, if you want to engineer a wormhole to have negligible time dilation and tidal forces (ie, comfortable for human travel), it has to be big.  Like 20x as big as the solar system.




I am completely unsurprised and unfazed by that.  We are talking about taking apart spacetime for our own use, after all.  "Big", in purely human physical terms, is not really an issue by comparison. 



> Side point: Gauss-Bonnet gravity need not be associated with brane-world models.




I know.  I noted it merely because it is a model that some folks may have heard of that was relevant.



> The point is that the Gauss-Bonnet term doesn't change the Einstein equations in 4D; they only do that in 5D and above.  That's interesting for brane-worlds as well as other extra dimensional models.  Also as a side note, I've written a paper using 5D GB gravity myself.




Yes.  To my knowledge, no 5+D model has made a practically testable prediction.  Thus theories that use them are pretty speculative.  Thus, let us not *expect* wormholes that don't use exotic matter. 




> In technical terms, the wormhole solutions written down connect two distinct asymptotic regions.  In plain English, the two ends of the wormhole are connected to two distinct universes, or maybe more precisely two regions of space that don't connect to each other except through the wormhole.




Since you've done more of the reading recently than I now: is that "not connected" in an absolute topological sense?  Or is it "not connected" in the causal, "won't be in each other's light cone even though there's continuous spacetime between points" sense?


----------



## freyar (Jun 11, 2015)

Umbran said:


> I am completely unsurprised and unfazed by that.  We are talking about taking apart spacetime for our own use, after all.  "Big", in purely human physical terms, is not really an issue by comparison.



True enough.  Though natural ones, if they exist, are likely to be small with strong time dilation.  And neither traversable nor stable.  So who knows?  This stuff is in some ways more speculative than the 5D stuff you mention below.



> Yes.  To my knowledge, no 5+D model has made a practically testable prediction.  Thus theories that use them are pretty speculative.  Thus, let us not *expect* wormholes that don't use exotic matter.



 Sure, there were a bunch made in the late '90s that probably could have been detected by now through measurements of the gravitational inverse square law (ie, Newton's law) over short distances.  Measurements have been made since that have put stronger limits on those models.  There are also 5+ D models that could have effects at the LHC.

There are also 5D physics models related to 4D physics through dualities.



> Since you've done more of the reading recently than I now: is that "not connected" in an absolute topological sense?  Or is it "not connected" in the causal, "won't be in each other's light cone even though there's continuous spacetime between points" sense?



Well, I'm not really on top of things like topics I do work on, but it's in the topological sense for any explicit metric I've seen yet.


----------



## freyar (Jun 12, 2015)

freyar said:


> Well, I'm not really on top of things like topics I do work on, but it's in the topological sense for any explicit metric I've seen yet.



I wrote that very quickly before I had to leave work --- to clarify, the two "sides" of the wormhole are topologically disconnected _except_ by the wormhole.  In other words, if the wormhole(s) disappeared, they'd be disconnected (in the topological sense), and travel from one to the other would be impossible even at FTL speeds.  That doesn't mean there can't be a wormhole that connects two points on a single space (that would be topologically connected w/o the wormhole), but I haven't seen that written down in math anywhere.


----------



## Olgar Shiverstone (Jun 12, 2015)

... and I'm back.  What did I miss?


----------



## Mishihari Lord (Jun 12, 2015)

freyar said:


> I wrote that very quickly before I had to leave work --- to clarify, the two "sides" of the wormhole are topologically disconnected _except_ by the wormhole.  In other words, if the wormhole(s) disappeared, they'd be disconnected (in the topological sense), and travel from one to the other would be impossible even at FTL speeds.  That doesn't mean there can't be a wormhole that connects two points on a single space (that would be topologically connected w/o the wormhole), but I haven't seen that written down in math anywhere.




That seems an odd way to approach the problem to me.  Instead of looking at just one improbable thing, wormholes, you're positing two, wormholes and parallel universes.  It would seem simpler to work with just one unproven impossibility/improbability at a time.  But then, I'm not a physicist.  I only needed up to Quantum 2, EM fields, and some specialized stuff for EE.


----------



## freyar (Jun 12, 2015)

Mishihari Lord said:


> That seems an odd way to approach the problem to me.  Instead of looking at just one improbable thing, wormholes, you're positing two, wormholes and parallel universes.  It would seem simpler to work with just one unproven impossibility/improbability at a time.  But then, I'm not a physicist.  I only needed up to Quantum 2, EM fields, and some specialized stuff for EE.




It's a matter of the math, I'd expect, not having worked it out myself.  It's often easier to write down and work with a toy model that's less realistic.  For example, the full spacetime of a real black hole like the ones we have evidence for should include (at early times) a star or some other matter that collapses to create the black hole.  But that's a very messy situation.  It's much easier to write down a solution describing a black hole that just sits there unchanging forever with nothing else around.  Incidentally, black holes like that also have two distinct "universes" that can only access each other IIRC by FTL travel through the black hole.  But that toy mathematical description (the Schwarzschild metric) is also very useful as a close approximation for the metric around any roughly spherical object --- it works very well for the sun, for example.  I'm sure you dealt with the same kind of thing in the physics classes you mention you've taken.


----------



## Umbran (Jun 12, 2015)

freyar said:


> True enough.  Though natural ones, if they exist, are likely to be small with strong time dilation.  And neither traversable nor stable.




If they aren't stable, then natural ones would not last long enough for us to ever observe them, except in terms of an energy burst across teh cosmos when they collapse, so that's a bit moot.



> Well, I'm not really on top of things like topics I do work on, but it's in the topological sense for any explicit metric I've seen yet.




Well, in that case, these aren't really wormholes in the sense most people expect.  This is effectively, "travel to a different universe," not ,"travel a distance in your own universe".  I then becomes impossible to have a long route through normal space, and a short one through a wormhole.  But, I've heard good folks more recent than Thorne speak about such... so I think there's something missing here.

And it that raises a whole lot of questions - like how a distinct and separate space just happens to have physical laws compatible enough with those in our space to support the connection?  And how would this be different from a bottleneck region in our own universe?


----------



## freyar (Jun 12, 2015)

Umbran said:


> If they aren't stable, then natural ones would not last long enough for us to ever observe them, except in terms of an energy burst across teh cosmos when they collapse, so that's a bit moot.



Well, true, but keep in mind that the "artificial" ones we're talking about (like the metric Kip Thorne and collaborators used for visualization in _Interstellar_) aren't stable either.  I do believe there are stable ones, but they're not actually easy to find in the literature.  My guess is they're complicated and require more or weirder "exotic matter" to make.  And we also have to remember that this exotic matter may not exist.  And that Morris & Thorne proved that at least a large class of wormholes are automatically time machines and therefore may be impossible.  It's conjectured after all that quantum gravity obeys a form of temporal cosmic censorship.



> Well, in that case, these aren't really wormholes in the sense most people expect.  This is effectively, "travel to a different universe," not ,"travel a distance in your own universe".  I then becomes impossible to have a long route through normal space, and a short one through a wormhole.  But, I've heard good folks more recent than Thorne speak about such... so I think there's something missing here.




True.  The rigorous but general definition of a wormhole seems to be something like this: if you consider spherical coordinates in flat space, concentric spheres around the origin shrink in surface area as  you approach that origin.  In a wormhole space(time), those concentric spheres first shrink and then expand as you move "toward the origin."  The expanding region is when you've gone "past the origin" and into the "second universe."  But with this definition, there doesn't even have to be a full second universe.  The whole thing could just be a funny-shaped dimple on a single spacetime.

But keep in mind that I'm talking about just the mathematical formulae people write down because they're simple enough to give exactly.  People draw pictures that basically fold over "universe 2" and glue it to "universe 1," which gives you a tunnel between two points on the same space.  That's the more colloquial expectation for a wormhole.  I just don't think it's feasible to write a formula for a spacetime like that.  On the other hand, I don't see a reason that a mathematical description of such a space can't exist --- it'd just be very ugly to write down and not as enlightening as these "toy wormholes."



> And it that raises a whole lot of questions - like how a distinct and separate space just happens to have physical laws compatible enough with those in our space to support the connection?  And how would this be different from a bottleneck region in our own universe?




Well, even the "two-universe" wormholes are really a single universe/spacetime as long as the wormhole is there. It holds the whole spacetime together.  Another simplification of all the wormhole spacetimes I've found written down today is that they are time-independent, meaning they sit there forever unchanging.  As for physical laws being compatible, I suppose that depends what you mean.  But it's a premature question in some ways, since even these toy models rely either on exotic matter that may not be possible (ie, would lead to inconsistency of any theory that contains it) or else very particular speculative higher-dimensional physics.  

I'm not sure quite what you mean by a bottleneck region.  If you can elaborate, I may be able to answer.


----------



## Bullgrit (Jun 12, 2015)

> wormholes have never been observed, of course - they're just a solution to an equation



Can someone explain this? There is some equation about the universe where the answer is, "There must be essentially useless holes between points in space."

Bullgrit


----------



## Umbran (Jun 12, 2015)

Bullgrit said:


> Can someone explain this? There is some equation about the universe where the answer is, "There must be essentially useless holes between points in space."




Not, "there must be.".  More like, "there can be."  These structures in spacetime are allowed by the math.

And, anything not strictly forbidden, we are going to think about endlessly, and base science fiction stories upon.


----------



## Umbran (Jun 12, 2015)

freyar said:


> Well, true, but keep in mind that the "artificial" ones we're talking about (like the metric Kip Thorne and collaborators used for visualization in _Interstellar_) aren't stable either.  I do believe there are stable ones, but they're not actually easy to find in the literature.  My guess is they're complicated and require more or weirder "exotic matter" to make.




My understanding is that most solutions are unstable, unless you apply exotic matter to stabilize them.

There are a class of solutions in 5+D space that may be stable without exotic matter - they may be stabilized by quantum effects. 



> I'm not sure quite what you mean by a bottleneck region.  If you can elaborate, I may be able to answer.




You hit upon the same basic idea with, "The whole thing could just be a funny-shaped dimple on a single spacetime."

I think it may be more clear to say it that it doesn't match the colloquial conception of a wormhole unless it makes the spacetime to be multiply connected (as opposed to simply connected, in a topological sense).  Leave out the singularities like black holes, and if the space with the wormhole is still simply connected, that wormhole is just an oddly shaped dimple, as you put it.  The wormhole may locally have different curvature than the rest of the spacetime, but from a practical standpoint... who cares?  




> As for physical laws being compatible, I suppose that depends what you mean.  But it's a premature question in some ways, since even these toy models rely either on exotic matter that may not be possible (ie, would lead to inconsistency of any theory that contains it) or else very particular speculative higher-dimensional physics.




The question becomes very important when you start considering artificially creating a wormhole.  A natural wormhole, as you've described them, is really just an area with slightly different curvature between two more normallly curved regions - one can imagine those developing naturally as the spacetime originally formed.

But, when you consider an artificial wormhole - if you cannot connect to your own spacetime, then you must connect to another one - and then the question of how that can be possible when that other spacetime cannot be assumed to have similar properties, becomes a bigger question.  We could find that even if we have exotic matter, we cannot create such a thing because there's nothing compatible to connect to!


----------



## Bullgrit (Jun 12, 2015)

> These structures in spacetime are allowed by the math.



Can you explain this? What does "something is allowed by the math" mean?

Bullgrit


----------



## Morrus (Jun 12, 2015)

Bullgrit said:


> Can you explain this? What does "something is allowed by the math" mean?




It means you can do it theoretically on paper. You take all the equations and stuff, and for some values the results give you cool answers. That doesn't mean that an example of it actually exists, though; it just means that you can find a solution to some equations which allow for it (or, often, simply don't prohibit it).  Wormholes are classic examples - there are equations which describe them, but none have ever been observed (and we have no idea how to make one).


----------



## Umbran (Jun 12, 2015)

Bullgrit said:


> Can you explain this? What does "something is allowed by the math" mean?




It means, the math does not forbid it.  The math does not show it to be impossible.  When you run through the math, the results may seem strange, but not entirely nonsensical or self-contradictory.

This is perhaps best compared to faster than light travel.  The math of relativity blows up, or gives you self-contradictory results, if you try to have an object that travels faster than the speed of light.  Things get divided by zero, and you get a big mess.  FTL is forbidden by the laws of physics as we currently understand them.


----------



## Morrus (Jun 12, 2015)

Umbran said:


> This is perhaps best compared to faster than light travel.  The math of relativity blows up, or gives you self-contradictory results, if you try to have an object that travels faster than the speed of light.  Things get divided by zero, and you get a big mess.  FTL is forbidden by the laws of physics as we currently understand them.




Question - isn't that when you try to accelerate something to the speed of light?  Doesn't the maths allow for objects moving permanently faster than light?  Hypothetical particles like tachyons, for example? Or am I misremembering?


----------



## Bullgrit (Jun 12, 2015)

I guess I'm getting hung up on the idea that looking at the math, (which I'm picturing as multitudes of equations, each filling up an entire whiteboard , can give someone the concept of something as extremely specific as a wormhole. It's not like a wormhole is like dark matter (another concept that the math allows [read: requires]) -- it seems that a wormhole is a very specific structure with a very specific function with very specific requirements, compared to dark matter that is basically, something has to be somewhere to make this calculate out.

I mean, I'm picturing someone looking at those whiteboards of equations, saying, "Hmm, what if this variable right here was a 2?" And the whole equation calculating out three feet away to "= tiny spacetime tubes connecting two points".

Bullgrit


----------



## Umbran (Jun 12, 2015)

Morrus said:


> Question - isn't that when you try to accelerate something to the speed of light?  Doesn't the maths allow for objects moving permanently faster than light?  Hypothetical particles like tachyons, for example? Or am I misremembering?




In special relativity, if you have a particle moving faster than light, its mass must be imaginary.  That's not a real, physical result, and thus cannot correspond to a real, physical particle.

There are quantum fields that can have imaginary mass (and are called "tachyonic" for it), but when the field does have that quality, it doesn't correspond to a real physical particle.  It must first undergo what is called 'condensation', at which point it ends up with a real mass, and is going slower than light.  Higgs bosons, which have been big in the news, are an example of a condensate from a tachyonic field.


----------



## Umbran (Jun 12, 2015)

I'm going to address a couple bits here out of order...



Bullgrit said:


> It's not like a wormhole is like dark matter (another concept that the math allows [read: requires])




Actually....  not quite.

Dark matter (and dark energy) is not "required by the math".  The math does not *predict* the existence of dark matter.  Instead, the existence of dark matter is required to explain observation and empirical evidence.  We have observed matter in galaxies and in the Universe moving in ways that it *shouldn't*, that cannot be explained by the matter we see and the math we have.  From this, we *infer* the existence of matter we cannot see (and is thus "dark").  




> I guess I'm getting hung up on the idea that looking at the math, (which I'm picturing as multitudes of equations, each filling up an entire whiteboard , can give someone the concept of something as extremely specific as a wormhole.




Hundreds of pages of squiggles can give you the concept of Middle Earth, and hobbits, magic rings, and wizards.  All pretty darned specific.

Of course if you don't know the language, you can't *read* that book.  

Just as English is a way to represent information and ideas, so is math - math is just a representation that is far more efficient at helping you manipulate certain kinds of ideas.  It isn't easy to do, but once you've learned the language, yes, you can look at the equation, and say, "that represents a thing with these physical properties."  It is kind of awesome.



> -- it seems that a wormhole is a very specific structure with a very specific function with very specific requirements, compared to dark matter that is basically, something has to be somewhere to make this calculate out.




Wormholes do not have a function, any more than hills have a function.  A hill is ground that is shaped a given way by the forces put upon it.  Yes, if I *build* a hill, then I say the hill has a function, but it only has such because I am *using* it for that function.  When I go away, the hill remains, but it has no function.  It is more appropriate to say that I can use the hill to perform a function, rather than the hill has the function innately.  A wormhole is similar.


----------



## tomBitonti (Jun 12, 2015)

Would you put the exotic matter required for wormholes in the same category as tachyonic matter?  That is, a kind of matter which, if it existed, would behave in exceptional ways compared to ordinary matter, but for which there is no physical evidence?

I'm thinking this gets at the difference between the equations which are used, and the actual underlying physics (which is in the realm of what can be tested).

Although, if you look at neutron stars and black holes, those were predicted by looking at particular parts of the domains for equations (what happens when you put a lot of mass together).  Not really a hard question, but it's curious that the implied objects (black holes) have been observed, while worm holes have not.  I'm thinking that is not a hard question, since black holes do not require a new exotic kind of matter.

Thx!

TomB


----------



## Umbran (Jun 12, 2015)

tomBitonti said:


> Would you put the exotic matter required for wormholes in the same category as tachyonic matter?  That is, a kind of matter which, if it existed, would behave in exceptional ways compared to ordinary matter, but for which there is no physical evidence?




Well, there are a few things often referred to as "exotic matter" for which we do have at least some physical evidence.  Neutronium comes to mind - this is a material produced when matter gets so compressed that the protons and electrons get pressed together so much they combine into neutrons, but not so compressed it falls into itself to become a black hole.  

We have neutron stars - objects so compact that there's nothing else (that we know of) that they could be made of!  All our math says that it should exist.  But its properties are... extreme, when compared to the matter of normal human experience.  

With a couple such caveats, yes, for the most part, "exotic matter" is stuff that might exist, but would have some really weird properties that we have never directly seen in physical reality.


----------



## tomBitonti (Jun 12, 2015)

I thought that neutronium was, more or less what is in the nucleus of an atom, with protons removed, and with a lot more of it (really, a lot more) than is in a typical atom.

Neutronium may have exotic properties, but, it doesn't seem to be quite the same as what worm holes are said to require, e.g., matter which is repelled by gravity.  Neutronium is normal matter in an unusual configuration.

Thx!

TomB


----------



## Bullgrit (Jun 12, 2015)

> Hundreds of pages of squiggles can give you the concept of Middle Earth, and hobbits, magic rings, and wizards. All pretty darned specific.



But those squiggles were not placed by nature/god/accident, and the elements (hobbits, etc.) are not theoretical within the world so described. 



> Wormholes do not have a function, any more than hills have a function.



"Function" was a poor word, perhaps. Properties? Functional properties? A hill has functional properties (i.e. water will move from one high spot to a low spot).



> Dark matter (and dark energy) is not "required by the math". The math does not *predict* the existence of dark matter. Instead, the existence of dark matter is required to explain observation and empirical evidence. We have observed matter in galaxies and in the Universe moving in ways that it *shouldn't*, that cannot be explained by the matter we see and the math we have. From this, we *infer* the existence of matter we cannot see (and is thus "dark").



I thought we had math for dark matter. I thought I've seen calculations of the actual "size" of dark matter. "There must be this much dark matter in the universe."

Isn't this the way all the math is determined? We see this phenomenon, we figure out the equations for it. If we see a new phenomenon, or the original shows to be slightly different, so we adjust the equations. From what I'm reading here, it seems that we have these equations based on what we've seen; the equations suggest something that we have no evidence for; so we theorize these things may exist -- rather than change the equations to match what we do have evidence for.

For the record: I'm not arguing. I'm discussing to better understand the concepts.

Bullgrit

Edit: This whole post is not written as well as it probably should be. But I don't have time right now to straighten it out.


----------



## Morrus (Jun 12, 2015)

I just want to say that I love long informative threads like this. More of these, please!


----------



## Umbran (Jun 12, 2015)

tomBitonti said:


> I thought that neutronium was, more or less what is in the nucleus of an atom, with protons removed, and with a lot more of it (really, a lot more) than is in a typical atom.




It is slightly different, in that once you get beyond a few neutrons in an atom, you start seeing bulk properties.



> Neutronium may have exotic properties, but, it doesn't seem to be quite the same as what worm holes are said to require, e.g., matter which is repelled by gravity.  Neutronium is normal matter in an unusual configuration.




Before you say that, remember that neutronium is required to make the more basic forms of time machine!


----------



## Umbran (Jun 12, 2015)

Bullgrit said:


> But those squiggles were not placed by nature/god/accident, and the elements (hobbits, etc.) are not theoretical within the world so described.




Oh?  Read a sonnet that moves you.  One that reveals an emotional truth that sits close to the human heart.  That's what math does, but for the physical universe.

Math was not placed by nature/god.  Math is merely how we express what *was* placed by nature/god.   We just happen to have stumbled upon a *REALLY GOOD* form of expression, for things that are actually pretty simple, though much of it is outside our everyday experience.

This, as compared to the human heart, which is very complex, but solidly within our everyday experience.  Far harder to express it in fullness.



> I thought we had math for dark matter. I thought I've seen calculations of the actual "size" of dark matter. "There must be this much dark matter in the universe."




"There must be this much dark matter in the universe... to explain the observed motion of stars in galaxies," is the full statement



> Isn't this the way all the math is determined?




Nope.  By no means does all math follow observation.



> We see this phenomenon, we figure out the equations for it.




That is a common approach, yes.  But it is not the only one.  It was not how Einstein did Special and General Relativity, for example.  

In the case of relativity, Einstein started with a thought ("What of the speed of light was really constant?") and he worked out what were the logical consequences of that.  Experiment has then verified that his work is an accurate model to pretty darned high precision.  This model even predicts effects that we hadn't seen or considered before he published.  In fact, pretty much all the effects his model predicts that we have been able to measure, have come true!  

Large sections of quantum mechanics also predated observations of the effects they predicted/explained.


----------



## freyar (Jun 13, 2015)

Several great points in this thread to reply to --- I'm going to try to take them in related bunches.



Umbran said:


> My understanding is that most solutions are unstable, unless you apply exotic matter to stabilize them.
> 
> There are a class of solutions in 5+D space that may be stable without exotic matter - they may be stabilized by quantum effects.



The purely 4D wormhole solutions I've perused in the last couple days require exotic matter just to exist in the first place, let alone be stable!  I haven't yet read through the 5D wormhole stuff since I'm in a busy time at work.



> I think it may be more clear to say it that it doesn't match the colloquial conception of a wormhole unless it makes the spacetime to be multiply connected (as opposed to simply connected, in a topological sense).  Leave out the singularities like black holes, and if the space with the wormhole is still simply connected, that wormhole is just an oddly shaped dimple, as you put it.  The wormhole may locally have different curvature than the rest of the spacetime, but from a practical standpoint... who cares?



Yes, I agree with that.  The point of the simply-connected wormhole spacetimes is that they have the local structure a multiply-connected wormhole needs.  So you can understand some basic facts about what we'd colloquially call a wormhole using a simplified model.  Think of Morris-Thorne wormholes as the spherical cows of the wormhole world.



> The question becomes very important when you start considering artificially creating a wormhole.  A natural wormhole, as you've described them, is really just an area with slightly different curvature between two more normallly curved regions - one can imagine those developing naturally as the spacetime originally formed.



Well, if wormholes can exist at all, I don't see why a natural one could lead to multiple connections in our universe.  It's just that, like most real things, the equations are a lot messier than they are for the spherical cows.



> But, when you consider an artificial wormhole - if you cannot connect to your own spacetime, then you must connect to another one - and then the question of how that can be possible when that other spacetime cannot be assumed to have similar properties, becomes a bigger question.  We could find that even if we have exotic matter, we cannot create such a thing because there's nothing compatible to connect to!



  Yeah, obviously we'd like to drill through to alpha Centauri or whereever instead.   But also remember that all the discussion of wormholes in the literature (at least that I've seen) is about wormholes that exist forever unchanging or else analysis of whether a wormhole that's been around forever collapses if something goes through it.  There's nothing at all about the formation of them.  (This is really quite different than the situation for black holes; I'm writing a series of papers about black hole formation myself, for example.)  

Just for emphasis, the formation/creation of a wormhole isn't worked out anywhere that I've seen.  But I can make one statement: wormhole formation would change the topology of space. As Umbran says, it makes the universe multi-connected. In English, before there's as wormhole, there is one type of path from A to B; after the wormhole forms, there are two types of path, either the old type or a path through the wormhole.  This type of change is pretty problematic in general relativity --- it's hard to avoid naked singularities.  There are a few specific types of topology changes that are well-behaved in string theory, though.

Next post: comments on the exotic matter required for wormholes...


----------



## freyar (Jun 13, 2015)

Let me collect some of the comments from the thread first:


tomBitonti said:


> Would you put the exotic matter required for wormholes in the same category as tachyonic matter?  That is, a kind of matter which, if it existed, would behave in exceptional ways compared to ordinary matter, but for which there is no physical evidence?






Umbran said:


> Well, there are a few things often referred to as "exotic matter" for which we do have at least some physical evidence.  Neutronium comes to mind - this is a material produced when matter gets so compressed that the protons and electrons get pressed together so much they combine into neutrons, but not so compressed it falls into itself to become a black hole.
> ...
> With a couple such caveats, yes, for the most part, "exotic matter" is stuff that might exist, but would have some really weird properties that we have never directly seen in physical reality.






tomBitonti said:


> Neutronium may have exotic properties, but, it doesn't seem to be quite the same as what worm holes are said to require, e.g., matter which is repelled by gravity.  Neutronium is normal matter in an unusual configuration.






Umbran said:


> It is slightly different, in that once you get beyond a few neutrons in an atom, you start seeing bulk properties.
> 
> Before you say that, remember that neutronium is required to make the more basic forms of time machine!




In this case, TomB has the right idea.  Neutronium is really just normal matter in extreme conditions.  The "exotic matter" required for wormhole spacetimes very specifically must violate what is known as the Null Energy Condition (NEC); this is actually proven mathematically.  Neutronium doesn't do that, nor does any kind of matter we know about.  However, some quantum effects (such as the Casimir effect) and some objects in string theory (which is hypothetical, of course) do violate the NEC in ways that lead to sensible physics, so it's not possible to rule out wormholes on this basis.  Other modified gravity theories can also mimic violations of the NEC.  *However*, usually if you include violations of the NEC in physics, you get some kind of pathology like instability of the vacuum, causality violation (superluminal propagation), or negative probabilities.  It's not clear that the NEC violation you need for a wormhole is a "safe" kind.  So it may exist or it may not.

Incidentally, tachyonic fields to violate the NEC.  As Umbran says elsewhere in the thread, they represent an instability.  Specifically, a tachyon field at zero value is like a ball sitting on the top of a hill --- if it moves a tiny amount, say due to a quantum fluctuation, it falls off the hill and rolls into a valley.  And when the tachyon settles in the valley, it's no longer tachyonic.  This is indeed like the Higgs field of the Standard Model.

Anyway, the murky status of NEC violation is probably why wormholes are still kind of a niche subject.  I mean, there's a reasonably sizeable literature on them but not a big one, and there seem to be a few authors who have done most of the work on them.

But I don't know anything about neutronium and time machines.


----------



## freyar (Jun 13, 2015)

On how the math works....



Bullgrit said:


> Can you explain this? What does "something is allowed by the math" mean?




Morrus and Umbran gave some great initial answers, so I'm just going to give those a shout-out here rather than quote them (given the length this post will be).

In fact, I think rather than quote much of the above posts, I'm going to condense some of the points/questions and offer my thoughts.  My apologies if I mangle what you're trying to say, but I want to paraphrase and go a bit out of order.  I'll highlight when I start talking about a new point.

One conversation thread was basically Bullgrit asking about the math corresponding to a wormhole and how/why physicists came up with it.  *First off*, we can talk about the "amount" of math needed.  At the basic level, it's easy to write down in one line a formula that, with training, you could look at and puzzle out "oh, that's a tunnel connecting two parts of space."  However, then you want to learn about it.  That's where you can get into reams of paper.  And 


			
				Bullgrit said:
			
		

> I mean, I'm picturing someone looking at those whiteboards of equations, saying, "Hmm, what if this variable right here was a 2?" And the whole equation calculating out three feet away to "= tiny spacetime tubes connecting two points".



is great, because there are times like that for any theoretical physicist.  Well, it's usually some symbol rather than a 2, and usually you're not changing things at whim but trying to figure out new ways to rearrange them, but, yes, I sometimes make a point to work on my chalkboard rather than paper just because the visual effect helps.

*As to "why wormholes,"* my understanding is that people went looking for them.  Let me explain more precisely.  The Einstein equations of general relativity take in sketch form "geometry" = "matter-energy."  Usually, what we do is start with some configuration of matter-energy and ask what the corresponding geometry of spacetime is.  That's not what people have done in the case of wormholes.  People wanted to study a specific geometry, namely a tunnel between two regions of space, so they plugged that into the Einstein equations and figured out what kind of matter-energy is required.  That's why we've been talking about "exotic matter" so much in this thread.  You'd never find a wormhole if you were looking at all the ways normal matter than bend spacetime.  In a way, wormholes were a solution looking for a problem.  I suspect that sci-fi fandom may have played a role motivating people to look at wormhole solutions, but I don't know that history.

*On math as a language of physics* (or maybe all science, if you like), Umbran states very nicely just how powerful math is in describing the world (yes, it is awesome).  My personal view is a bit more along the lines that mathematical principles are something we discover, like we discover the laws of physics, but that's really more like a feeling.

If you want to *compare wormholes to dark matter*, you have two very different situations.  As I said, people really went looking for wormholes.  On the other hand, basically no one believed the first evidence of dark matter for decades until a new set of evidence came along.  So we were really sort of forced to admit that we needed to add something to our models of cosmology.  Interestingly, there's not really new math required --- dark matter behaves according to similar principles as normal matter in the simplest theories.  *But dark matter is predictive*: once we knew that we needed dark matter to explain the rotation of galaxies, we could predict the consequences of that amount of dark matter for the cosmic microwave background (light from the very early universe), and the predictions really match the measurements excellently.  We always like predictions out of physics theories.

*Regarding the history of special/general relativity and quantum mechanics*, Einstein was very much thinking about several unexplained experiments when he discovered special relativity, even though the thought experiments look unrelated.  He had that rare genius to relate that simple thought to problems that were confusing everyone else.  On the other hand, he did discover general relativity just because of questions in his own mind.  There were no experimental prompts for him.  But a successful prediction of general relativity was what made him famous.  As for quantum mechanics, the basics were again discovered in an effort to understand confusing experiments, but, as Umbran says, there were indeed predictions of the math that weren't seen experimentally for a long time.


----------



## freyar (Jun 13, 2015)

Morrus said:


> I just want to say that I love long informative threads like this. More of these, please!




I've often thought it might be fun to do an "Ask a physicist" thread for questions of this nature (other physics people like Umbran would be more than welcome to chip in answers, of course).  Would it be kosher to run something off-topic like that in the AMA forum?  The thread management features would be extremely helpful, I think.


----------



## Jhaelen (Jun 15, 2015)

freyar said:


> Interestingly, there's not really new math required --- dark matter behaves according to similar principles as normal matter in the simplest theories.



Actually, the way I understood it, the existence of dark matter was posited to avoid having to introduce new math. I.e. if there was no dark matter, we'd have to revise our math to match our observations.

Likewise dark energy: if our math is correct, then dark energy must exist, otherwise our observations make no sense. Again the alternative, if I understood it correctly, is that our current math is simply incorrect (when applied to the universe at large).

I agree that wormholes are an entirely different beast: they're not required to explain any observations we've made. They're just something that might exist, according to our math. I had assumed that tachyons were in the same category...


----------



## Morrus (Jun 15, 2015)

freyar said:


> I've often thought it might be fun to do an "Ask a physicist" thread for questions of this nature (other physics people like Umbran would be more than welcome to chip in answers, of course).  Would it be kosher to run something off-topic like that in the AMA forum?  The thread management features would be extremely helpful, I think.




I think that would be fun!


----------



## Umbran (Jun 15, 2015)

freyar said:


> But I don't know anything about neutronium and time machines.




Tipler, 1974, "Rotating Cylinders and the Possibility of Global Causality Violation".  Bascially, if you get a long cylinder of neutronium, and you spin it on its long axis fast enough, and fly around it, you can get closed timelike curves - a time machine.  Nothing less than neutronium has the mass to do frame dragging to produce the effect.

Tipler maintained that you could do this with a cylinder of finite length if it was spinning fast enough.  Hawking held that anything less than an infinite cylinder would violate the weak energy condition, and thus need something with negative energy density to do the trick.

Neither one, of course, includes any quantum effects.


----------



## Umbran (Jun 15, 2015)

freyar said:


> *As to "why wormholes,"* my understanding is that people went looking for them.  Let me explain more precisely.  The Einstein equations of general relativity take in sketch form "geometry" = "matter-energy."  Usually, what we do is start with some configuration of matter-energy and ask what the corresponding geometry of spacetime is.  That's not what people have done in the case of wormholes.  People wanted to study a specific geometry, namely a tunnel between two regions of space, so they plugged that into the Einstein equations and figured out what kind of matter-energy is required.




Kip Thorne came at it this way, yes.  But there's history further back that gave him the thought to look.  

The original concept of wormholes goes back to 1916, I think.  Take the basic idea of an eternally-existing black hole.  Now, take the idea that, aside from the gravitational singularity, that spacetime does not have any "edges" to run into - you should be able to trace the path of any particle infinitely into the past or future.  This leads to weirdness - the "black hole/white hole", which has a black hole connecting to another universe, falls out of this.  It was shown that for this original formulation, the bridge between universes is unstable.  This bridge does not exist for black holes made from collapsing stars - it only exists for black holes that have existed since the beginning of time.  And they weren't *looking* for bridges - they were doing some of the initial considerations of what black holes were like, and what spacetime in and around them would have to be, and happened to stumble upon it.

Then comes Kerr, in 1963, who shows that within a *spinning* black hole (and a couple years later, a spinning black hole with charge) there are even more clear paths that look like you can jump between universes if you enter a black hole.  And this is, if I recall correctly, more stable.  

These are the first "wormholes" - they call for entering a black hole, and coming out in some other universe.

Thorne shifted this to, "Well, can we do this without a black hole? What does that look like?"


----------



## freyar (Jun 15, 2015)

Jhaelen said:


> Actually, the way I understood it, the existence of dark matter was posited to avoid having to introduce new math. I.e. if there was no dark matter, we'd have to revise our math to match our observations.
> 
> Likewise dark energy: if our math is correct, then dark energy must exist, otherwise our observations make no sense. Again the alternative, if I understood it correctly, is that our current math is simply incorrect (when applied to the universe at large).




That's the essence of it.  If you don't like dark matter or dark energy, you are forced to change the laws of gravity.  You then have to ask whether you can get the effects you want even by changing gravity.  In the case of dark energy, it seems possible, but it's also possible that all the modified gravity theories that mock up dark energy are mathematically inconsistent (there is some diversity of opinion on this).  In the case of dark matter, you can pretty easily reproduce its effects in galaxies by modifying Newtonian gravity, but even very few proponents of modified gravity have claimed they can reproduce observations of the early universe (which were predicted by dark matter).


----------



## freyar (Jun 15, 2015)

Morrus said:


> I think that would be fun!




Cool, hopefully I can start something like that soon (assuming work gets a little less busy).


----------



## freyar (Jun 15, 2015)

Umbran said:


> Tipler, 1974, "Rotating Cylinders and the Possibility of Global Causality Violation".  Bascially, if you get a long cylinder of neutronium, and you spin it on its long axis fast enough, and fly around it, you can get closed timelike curves - a time machine.  Nothing less than neutronium has the mass to do frame dragging to produce the effect.
> 
> Tipler maintained that you could do this with a cylinder of finite length if it was spinning fast enough.  Hawking held that anything less than an infinite cylinder would violate the weak energy condition, and thus need something with negative energy density to do the trick.
> 
> Neither one, of course, includes any quantum effects.




Interesting.  Though I thought negative energy densities are generally from quantum effects (like Casimir effects).  In any case, Hawking is generally right about this kind of stuff.  There is also a commonly held view (though certainly not proven) that closed timelike curves (at least ones not shielded by event horizons) shouldn't be allowed in a final theory of gravity.  So you have to wonder how they'd be forbidden in this case.



Umbran said:


> Kip Thorne came at it this way, yes.  But there's history further back that gave him the thought to look.
> 
> The original concept of wormholes goes back to 1916, I think.  Take the basic idea of an eternally-existing black hole.  Now, take the idea that, aside from the gravitational singularity, that spacetime does not have any "edges" to run into - you should be able to trace the path of any particle infinitely into the past or future.  This leads to weirdness - the "black hole/white hole", which has a black hole connecting to another universe, falls out of this.  It was shown that for this original formulation, the bridge between universes is unstable.  This bridge does not exist for black holes made from collapsing stars - it only exists for black holes that have existed since the beginning of time.  And they weren't *looking* for bridges - they were doing some of the initial considerations of what black holes were like, and what spacetime in and around them would have to be, and happened to stumble upon it.
> 
> ...




Oh, yes, wormholes through black holes came up earlier in the thread.  I think the "wormholes" in the rotating/charged black hole case are still unstable, though.  At least that's what my notes from GR class say.


----------



## Umbran (Jun 15, 2015)

freyar said:


> Interesting.  Though I thought negative energy densities are generally from quantum effects (like Casimir effects).




Well for many, the idea of "exotic matter", as in actual *physical* material that you can gather up and manipulate, is a bit far fetched.  Those same folks sometimes don't have as much of a problem with invoking quantum effects (like the Casimir effect) that produce the same results without having any odd physical materials around.  

The point, though, is that Tipler wasn't calling on any such to make his cylinder work.  



> In any case, Hawking is generally right about this kind of stuff.




Yes.  As if we needed Hawking to prove that a light-year long cylinder of neutronium wasn't going to be able to allow anyone to move through time 



> There is also a commonly held view (though certainly not proven) that closed timelike curves (at least ones not shielded by event horizons) shouldn't be allowed in a final theory of gravity.  So you have to wonder how they'd be forbidden in this case.




You can forbid them by noting that you probably can't get enough neutronium in one place to build the thing without it collapsing into a singularity. 

As for the commonly held view - that sounds like assuming the conclusion, if you ask me.  We don't get to determine whether closed timelike curves should be allowed in a final theory.  Either they are possible, or they are not, no matter our desires on the issue.



> Oh, yes, wormholes through black holes came up earlier in the thread.  I think the "wormholes" in the rotating/charged black hole case are still unstable, though.  At least that's what my notes from GR class say.




Which is why I noted them as "less unstable", as opposed to actually unstable.

The Kerr solution is also for a black hole with no beginning or end, not one formed by a collapsing star, in any event.  And it isn't like anything other than a super-massive black hole has an event horizon we can cross without being destroyed.  Though, admittedly, galactic black holes are likely to be rotating, and may well be charged.  So, if someone really wants to check it out, they can just go a few tens of thousands of light years away.  

Not that the rest of the universe will even know about it, as you don't get to come back and tell us if you reached another universe.  The Kerr solution is a one-way trip.


----------



## Bullgrit (Jun 15, 2015)

> the existence of dark matter was posited to avoid having to introduce new math. I.e. if there was no dark matter, we'd have to revise our math to match our observations.



Um... this sounds ... weird. I'm trying to find a way to understand this without people thinking I'm saying "scientists are stupid." This seems to say that if our calculations don't match observation, then we decide our observations are wrong or incomplete. Isn't this the definition of "bad science"? Shouldn't "good science" refigure the math to match observations rather than hold firm to incorrect math (incorrect according to observation)? I fully posit that it's obvious I'm just not getting this idea. 

Isn't this how aether theory was believed for a while.

Bullgrit


----------



## Umbran (Jun 15, 2015)

Bullgrit said:


> Um... this sounds ... weird. I'm trying to find a way to understand this without people thinking I'm saying "scientists are stupid." This seems to say that if our calculations don't match observation, then we decide our observations are wrong or incomplete.  Isn't this the definition of "bad science"? Shouldn't "good science" refigure the math to match observations rather than hold firm to incorrect math (incorrect according to observation)? I fully posit that it's obvious I'm just not getting this idea.




Any halfway decent scientist knows that taking a measurement is really taking a sample.  It is thus subject to potential sampling error.  Scientists need to be honest with themselves about when they might be introducing such errors.

Dark Matter arose as a theory thusly:  We looked at distant galaxies, estimated their masses, and realized that their motion was not consistent with their masses.  By a *long* shot.

The models of motion in question were based on Newtonian mechanics and Einsteinian relativity - a couple of theories whose areas of validity are very well known, tested, and verified.  It is  difficult to arrange a correction to these that would allow for the galactic motion observed that didn't bollix up other times when they have been proven to work quite well.  There are some that do so, but to me they sort of cherry pick how they are applied, and lead to an inelegant solution.

Or, we could just realize that we might not be estimating how much matter was in these galaxies properly.  How could that estimate be off?  Well, for one thing, our estimate only included mass that we could *see* - that glowed brightly enough to be seen over these extreme distances.  

What's more likely?  That Newton and Einstein need to be completely thrown out, or that maybe there's just  mass out there that doesn't glow brightly?

It isn't like that was a lightweight question at the time, by the way.  It was a major point of contention.  It was hashed over a *lot*.  It really only clarified when it was shown that the presence of dark matter implied some things about microwave background radiation... and that measurements were consistent with the presence of this mass, and not consistent with it's absence.  So, the presence of dark matter has been put through the usual wringer, requiring a new theory to make a prediction we can test.


----------



## freyar (Jun 15, 2015)

Umbran said:


> As for the commonly held view - that sounds like assuming the conclusion, if you ask me.  We don't get to determine whether closed timelike curves should be allowed in a final theory.  Either they are possible, or they are not, no matter our desires on the issue.




Absolutely.  It's just that causality, the inability to create time-travel paradoxes like killing your parents before you're conceived, is expected to be a important principle.

I have to run off to a conference but will hopefully answer Bullgrit re: dark matter tonight.


----------



## tomBitonti (Jun 15, 2015)

Edit: Sorry, a bit of a ramble.  I'll try to clean this up tonight.



Bullgrit said:


> Um... this sounds ... weird. I'm trying to find a way to understand this without people thinking I'm saying "scientists are stupid." This seems to say that if our calculations don't match observation, then we decide our observations are wrong or incomplete. Isn't this the definition of "bad science"? Shouldn't "good science" refigure the math to match observations rather than hold firm to incorrect math (incorrect according to observation)? I fully posit that it's obvious I'm just not getting this idea.
> 
> Isn't this how aether theory was believed for a while.
> 
> Bullgrit




In detailed literature, there are collations of explanations for phenomena (what we observe), with an exporation of different explanations, and the necessary adjustments to current known laws for a particular explanation to work.  Those necessary adjustments can lead to predictable results in associated phenomena, which can pretty much wreck a candidate explanation.

I found this sort of collation googling explanations for the red shift a while back.  All sorts of explanations have been explored, and discarded due to solid evidence, leaving, more or less, universal expansion as the current best explanation.

I would think that that exhaustive searches are ongoing for explanations, with folks looking in all sorts of directions.  For example, one might try to explain unexpected measurements of radial velocities of observable matter in galaxies by postulating a modification to the inverse square law of gravity.  That would have a measurable consequence in other areas, which will be looked at, and the scientific community would, over time, make a judgement of whether the explanation was compelling.

At this level of detail, there would end up being an enumeration of explanations, with qualifications attached to each as the quality of each explanation, with a sense of "this is it" applying to one or another explanation only after enough evidence was gathered.

I would guess that all of the following, and more, are or have been considered:

Measurement errors (somehow, we aren't measuring the phenomenon accurate);

Analysis errors (we measured it correctly, but aren't reasoning through the data correctly)

Errors in currently accepted physical theories (some equation which is being used has a necessary modification which we aren't making)

Errors in our understanding of the underlying physical reality (our basic idea of what is there to explain is missing an important detail)

I'm thinking that a modification to a widely held law (say, the inverse square law of gravity) would be looked at pretty hard, since an observably provable modification would be a very big deal, but somewhat on the fringe, since fundamental changes are pretty rare things.

These provides a little information:

https://en.wikipedia.org/wiki/Galaxy_rotation_curve



> Though dark matter is by far the most accepted explanation for the resolution to the galaxy rotation problem, other proposals have been offered with varying degrees of success. Of the possible alternatives, the most notable is Modified Newtonian Dynamics (MOND), which involves modifying the laws of gravity.




And:



> Alternatives to dark matter
> 
> There have been a number of attempts to solve the problem of galaxy rotation curves by modifying gravity without invoking dark matter. One of the most discussed is Modified Newtonian Dynamics (MOND), originally proposed by Mordehai Milgrom in 1983, which modifies the Newtonian force law at low accelerations to enhance the effective gravitational attraction. MOND has had a considerable amount of success in predicting the rotation curves of low-surface-brightness galaxies, the baryonic Tully–Fisher relation,and the velocity dispersions of the small satellite galaxies of the Local Group. These results are surprising in the context of dark matter, which does not predict the same things as MOND without considerable fine-tuning.




I would be very very careful before accepting the above quotes as being mainstream within the the scientific community.  This is, I'm thinking, one of several areas where non-scientific literature does a poor job of representing the general scientific viewpoint, at least so far as presenting the current prevailing thoughts.  A particular exciting alternate theory can garner a lot more attention than it deserves, and because of the extra attention, seem to have a lot more support than it really does.

Thx!

TomB


----------



## Umbran (Jun 15, 2015)

freyar said:


> Absolutely.  It's just that causality, the inability to create time-travel paradoxes like killing your parents before you're conceived, is expected to be a important principle.




Fuddy-duddies and wet blankets, I say!  


I understand the desire, but... well... Einstein thought God didn't play dice, right?  

I am not sure we need the cosmic censorship if the effect can only be seen in properly extreme circumstances.  

Plus, it would give us a neat answer to the Fermi Paradox.  If FTL travel is equivalent to creating a time machine - we don't see alien species because anyone advanced enough to reach us extinguishes themselves in massive temporal conflicts.


----------



## Scott DeWar (Jun 15, 2015)

Umbran said:


> Any halfway decent scientist knows that taking a measurement is really taking a sample.  It is thus subject to potential sampling error.  Scientists need to be honest with themselves about when they might be introducing such errors.




As a point of interest, When taking a measuring voltage in a circuit, a volt meter will take a sample like a resister in  parallel - a sample of the current -  the higher the ohm value of the tester, the smaller the sample.


----------



## Mishihari Lord (Jun 16, 2015)

Bullgrit said:


> Um... this sounds ... weird. I'm trying to find a way to understand this without people thinking I'm saying "scientists are stupid." This seems to say that if our calculations don't match observation, then we decide our observations are wrong or incomplete. Isn't this the definition of "bad science"? Shouldn't "good science" refigure the math to match observations rather than hold firm to incorrect math (incorrect according to observation)? I fully posit that it's obvious I'm just not getting this idea.
> 
> Isn't this how aether theory was believed for a while.
> 
> Bullgrit




That's pretty much my take on it too, except I don't mind if people think I'm saying "scientists are stupid." Well, maybe foolish rather than stupid but that's a fine point.  Making up some magic unobserved thing to explain why your theory is right even though it doesn't match physical evidence is pretty strained.  Occam's razor and all that.

Edit:  Reading further down the thread, Umbran's point about microwave radiation seems supportive but not conclusive.  Even if dark matter turns out to be real, the methodology used was poor.


----------



## Morrus (Jun 16, 2015)

Mishihari Lord said:


> That's pretty much my take on it too, except I don't mind if people think I'm saying "scientists are stupid." Well, maybe foolish rather than stupid but that's a fine point.  Making up some magic unobserved thing to explain why your theory is right even though it doesn't match physical evidence is pretty strained.  Occam's razor and all that.
> 
> Edit:  Reading further down the thread, Umbran's point about microwave radiation seems supportive but not conclusive.  Even if dark matter turns out to be real, the methodology used was poor.




Dark matter *is* the application of Occam's Razor. The simplest explanation is dark matter; "all our years of math about gravity, despite a million things and a century of solid empirical evidence except this one thing which show it to be right, is wrong" is a less simple explanation. 

Sure, it *might* be the latter. Nobody has ever claimed otherwise. But Occam's Razor says it's the former. There's some matter we can't see. It's a far more likely explanation.

And, even so, scientists don't take it as read even remotely.  Nobody is stating absolutes. They are devoting vast amounts of time and resources to detecting dark matter.

Planets have been discovered that way. Motion of other bodies suggests a planet must be there, but we haven't seen it yet. Lo and behold, we find Neptune exists! This is not shoddy science, it's exactly how it's supposed to work. Evidence - hypothesis - theory. We're at the hypothesis stage. But it's a good hypothesis.

It's easy to post on the Internet that scientists are "foolish" and inventing "magic". It's also pretty unfair. Especially coming from those of us - like myself - who wouldn't even know where to begin pointing out the flaws in current theories to support such accusations.

I mean, you declare that the methodology is poor. We're talking hundreds of complex papers filled with intricate maths and observations and carefully honed computer models, from hundreds of scientists who approach the problem in different ways, and come to the same conclusion. 

Plus scientists would be delighted to discover that there's a new law of gravity. That's far more exciting than dark matter! They'd *love* that. Discovering that the universe doesn't work the way we think it does is what they like doing. That would be their equivalent of 1000 Christmasses rolled into one.

Remeber the higgs boson? It was just like this. It was theorized to exist because evidence suggested it must do, or our models of the universe are all wrong. So they made a giant particular collider in CERN and looked for it. And they found it. 

I remember Prof. Brian Cox saying that they best result would be to not find it. Finding it is just confirmation that the maths is right. If it didn't exist - now THAT would be exciting, because it would mean a whole new area of physics would open up. Our theories would be all wrong; we'd have so much to discover. Sadly, they found it. The boring result! The universe worked the way the maths said it would.


----------



## Umbran (Jun 16, 2015)

Mishihari Lord said:


> Even if dark matter turns out to be real, the methodology used was poor.




With respect - I suspect you speak from ignorance on methodology.  I gave a quick precis, so that laymen could get a basic idea of where ideas came from.  Do not, for a moment, take that as a full accounting of the intellectual rigor of the actual scientific discussion involved.

And, no, there was nothing in the methodology that was poor.   When doing scientific investigation, if your empirical measurements don't match what you expect, proper methodology includes trying to find *all* possible culprits.  Closing your mind to alternatives is poor methodology.  Assuming that you caught all the data, and that the mathematical model is incorrect, is a form of arrogance - "I couldn't *possibly* have missed anything!"  

So, no, there was no issue with the methodology.  There may have been a problem with how you were taught about the "scientific method" - it is common to teach kids that science (specifically physics) goes from equation -> experiment -> updated equation.  This is demonstrative, but an oversimplification of valid scientific process.


----------



## Umbran (Jun 16, 2015)

Morrus said:


> Plus scientists would be delighted to discover that there's a new law of gravity. That's far more exciting than dark matter! They'd *love* that. Discovering that the universe doesn't work the way we think it does is what they like doing. That would be their equivalent of 1000 Christmasses rolled into one.




Really.  Imagine having your name in history as the person who proved Einstein was wrong!  That'd give a theoretician *shivers of glee!*


----------



## Mishihari Lord (Jun 16, 2015)

Umbran said:


> With respect - I suspect you speak from ignorance on methodology.  I gave a quick precis, so that laymen could get a basic idea of where ideas came from.  Do not, for a moment, take that as a full accounting of the intellectual rigor of the actual scientific discussion involved.




I'll admit I don't have a complete idea of everything done in the field - that would take more time than I have available - but I'm not exactly a layman either.  I have been a professional researcher in the past and I'm pretty familiar with how things go, at least in my domains of knowledge (control theory, inertial navigation, optics, acoustics).  After chatting with a mathematician friend last year about dark matter, I was interested enough to spend an afternoon reading up on what's been done.  What I read reminded me of personal experiences where social dynamics among the people involved drove the direction of research as much as the data does.  As you said, it makes sense to look into every possibility, but in the stuff I read, at least, it looked like the possibility that the gravity model needed to be modified had almost been ignored, with 99%+ of research effort looking into dark matter rather than other possibilities.  Even now the alternatives to dark matter are considered very much fringe science.  Whatever the truth turns out to be, I don't think that giving almost all research effort to looking for something that had never in any way been observed rather than giving some effort to investigating the possibility that the theories were incorrect was a good idea.


----------



## freyar (Jun 16, 2015)

Umbran said:


> Plus, it would give us a neat answer to the Fermi Paradox.  If FTL travel is equivalent to creating a time machine - we don't see alien species because anyone advanced enough to reach us extinguishes themselves in massive temporal conflicts.




Haha, that would be fun!  Incidentally, I just saw a lecture this evening by Miguel Alcubierre, discoverer of the warp drive spacetime, that talked about wormholes and warp drives at a public level.  Very apropos.  Anyway, he's in favor of chronology protection, for whatever that's worth.


----------



## freyar (Jun 16, 2015)

How is it that a wormhole thread comes around to dark matter so much?  Anyway, Morrus and Umbran have already made nice posts, but this will be my take.



Mishihari Lord said:


> I'll admit I don't have a complete idea of everything done in the field - that would take more time than I have available - but I'm not exactly a layman either.  I have been a professional researcher in the past and I'm pretty familiar with how things go, at least in my domains of knowledge (control theory, inertial navigation, optics, acoustics).  After chatting with a mathematician friend last year about dark matter, I was interested enough to spend an afternoon reading up on what's been done.  What I read reminded me of personal experiences where social dynamics among the people involved drove the direction of research as much as the data does.  As you said, it makes sense to look into every possibility, but in the stuff I read, at least, it looked like the possibility that the gravity model needed to be modified had almost been ignored, with 99%+ of research effort looking into dark matter rather than other possibilities.  Even now the alternatives to dark matter are considered very much fringe science.  Whatever the truth turns out to be, I don't think that giving almost all research effort to looking for something that had never in any way been observed rather than giving some effort to investigating the possibility that the theories were incorrect was a good idea.




With all due respect, I don't think you can get a fair picture of what's been done in an afternoon even with the research background you have.  And I don't think your mathematician friend has one either, unless he/she is actually an astrophysicist or particle physicist of the right type.  Here is my disclaimer: I am a university professor of physics, and half of my research program is particle physics related to dark matter.  I get research grants to carry out that work; those grants *do not* pay my salary, but they do pay some of my students (on the grand scale of things, I am pretty small potatoes, but whatever).  That said, dark matter was something I picked up after about a decade of work on other subjects, and it took me quite a while to become fully conversant with the details of the evidence and current work. There is a lot going on!  My point is just that there is a lot of detailed analysis that people really do check over independently.  This is science done the same way all good science is done.

But, anyway, here's the story: 
Dark matter was first discovered in the 1930s.  No one believed in it at the time.  People were probably not convinced by the measurements (errors were large at the time, and in fact the numbers were off compared to current measurements). 
It took 35 years for the idea to come up again due to (better) measurements in somewhat more controlled systems (orbits of stars in galaxies as opposed to more random motions of galaxies in clusters).  At that point, it was fair to say that the measurements could be explained by a new particle that does not interact with light, dark matter, or a modified gravity theory, which I will call MOND after the most prominent.  
Now we have other measurements besides stars in galaxies.  We have:
1) Galaxies in clusters.  Motion of galaxies is well explained by a similar amount of dark matter as in galaxies (in proportion to normal matter).  MOND that explains galaxy rotations can explain clusters only if you also postulate some additional matter that does not interact with light --- dark matter again.  Maybe there is an epicycle that fixes this problem; I see differing claims.
2) Gravitational lensing of light passing galaxies and clusters of galaxies.  Again, this is well explained by the expected amount of dark matter and not by MOND.  A famous example is the bullet cluster, which is really two clusters colliding.  The prediction of dark matter theory matched the observations.  MOND predicted something else, though I will say its adherents quickly came up with a tweak  to match the observations pretty quickly after the fact.
3) The overall structure of the universe matches very well with simulations that include the expected amount of dark matter.  There are multiple types of measurements to support this.  Something related is the following.
4) The prediction of dark matter theory matches in extreme detail observations of the cosmic microwave background, which is relic light that gives us a good picture of the universe when it was very nearly uniform.  Because of that uniformity, physics was in many ways simpler then than now, so its relatively easy to make predictions for the CMB given some theory.  And those measurements just can't work without a different source of gravity than normal matter.  Even some high profile proponents of MOND theories have admitted that modifying gravity without dark matter just can't explain these observations.  I should note that CMB observations of sufficient detail were not done until 2000 and later, long after dark matter was first really accepted.  

So, here we have quite a few observations made over decades of history.  There are two sets of theories to explain them.  One, dark matter, was calibrated by the first accurate measurement and subsequently did very well predicting the results of later observations. The other, MOND or modified gravity, was calibrated very well to the first measurement and didn't do very well at all with subsequent ones.  Yet people still study it (I see papers on it reasonably often) because it does well on the galaxy level.  But I hope you can see in a limited way why there is so much work done on dark matter.  It is the Occam's razor theory.

Then there is the issue of whether it's crazy to invent a new type of particle and look for it.  Here's my response.  To have the chemistry we know, we only really need 4 types of particles: electrons, protons, neutrons, and photons.  But instead protons and neutrons are made up of a couple of types of quarks held together by gluons.  That's really all you need.  But we have also discovered neutrinos, which are totally different and weird in their own way.  And muons, which are just like electrons but about 200x heavier. And taus, which are still just like electrons but even heavier.  And 4 more kinds of quarks.  None of these really had to be there.  The equations work better with two additional force carriers for some nuclear interactions, and we found them.  Same thing for the Higgs boson.  But 2/3 or more of the "matter" particles are just there, and we don't know what principle says they should be there.  So is it that strange to say there might be one more type of particle?  Especially since there are hints in the equations and experiments that physics might just work better with some more particles of specific types that include some that could reasonably be dark matter?  

Well, I hope that's enough for you to read for one night.


----------



## Mishihari Lord (Jun 16, 2015)

freyar said:


> How is it that a wormhole thread comes around to dark matter so much?  Anyway, Morrus and Umbran have already made nice posts, but this will be my take.
> 
> --- snip good stuff ---




Thanks for the overview - I wasn't aware of some of the older stuff you mentioned.  With that in mind I will concede that the focus of research on the dark matter theory is not as unreasonable as I had thought.  The danger of self-directed literature search is that it's tough to get even a good overview if you don't know where all to look in the first place.  The postulating of dark matter still seems different to me than the particles you mentioned.  IRC they either fell out of the math or were directly observed in some manner rather than invented to explain a theory's shortcomings, but that's just quibbling at this point.


----------



## freyar (Jun 16, 2015)

Mishihari Lord said:


> Thanks for the overview - I wasn't aware of some of the older stuff you mentioned.  With that in mind I will concede that the focus of research on the dark matter theory is not as unreasonable as I had thought.  The danger of self-directed literature search is that it's tough to get even a good overview if you don't know where all to look in the first place.  The postulating of dark matter still seems different to me than the particles you mentioned.  IRC they either fell out of the math or were directly observed in some manner rather than invented to explain a theory's shortcomings, but that's just quibbling at this point.




Hey, thanks for listening.  My point about the other particles is that there are a lot of them without much "point" in some sense, so it's not a stretch to add more, really.  The other thing I should have mentioned but forgot (because I was way past my bedtime) was that people worked really hard to explain dark matter as normal matter that we just didn't see.  There was a huge research program at one point to look for brown dwarfs (small stars that couldn't quite start nuclear burning and never really became stars).  There aren't enough.  And the CMB data really directly tells us that whatever dark matter is, it can't be normal matter (by comparison to the math describing the behavior of normal stuff in the early universe).


----------



## Bullgrit (Jun 16, 2015)

Coincidentally, I've just started listening to an Einstein biography on CD.

Bullgrit


----------



## Umbran (Jun 16, 2015)

freyar said:


> I just saw a lecture this evening by Miguel Alcubierre, discoverer of the warp drive spacetime...




*ENVY!*


----------



## Umbran (Jun 16, 2015)

freyar said:


> How is it that a wormhole thread comes around to dark matter so much?




I think it is because:

1) Physicists like dramatic words, and to the general public they begin to bleed into each other.  "Dark Matter", "Exotic matter", and so on.

2) Wormholes lead to Relativity leads to cosmology leads to dark matter.


----------



## Morrus (Jun 16, 2015)

Umbran said:


> I think it is because:
> 
> 1) Physicists like dramatic words, and to the general public they begin to bleed into each other.  "Dark Matter", "Exotic matter", and so on.
> 
> 2) Wormholes lead to Relativity leads to cosmology leads to dark matter.




I think it's because the very phrases dark matter and dark energy sound like magic. And then you get tons of folks who use a basic layman's description they found somewhere, mistake that for the actual science, and declare that they know better than the folks who actually did the equations. 

Then again, on another board, I frequently see people proclaim Einstein was wrong because they found a weak analogy somewhere and applied what they considered logic to it (the analogy, not the equations) and arrived at a different conclusion.


----------



## Umbran (Jun 16, 2015)

Morrus said:


> Then again, on another board, I frequently see people proclaim Einstein was wrong because they found a weak analogy somewhere and applied what they considered logic to it (the analogy, not the equations) and arrived at a different conclusion.




True.  Though hardly a physics-specific phenomenon.  My wife is a veterinarian, and you should hear some of the arguments people give her, to her face, about proper and effective medical practice for animals.

Someone recently asked her if they could clean their dog's ear infection with Listerine.  Because, apparently, it kills germs, so it should be fine, right?


----------



## Bullgrit (Jun 16, 2015)

> I think it's because the very phrases dark matter and dark energy sound like magic. And then you get tons of folks who use a basic layman's description they found somewhere, mistake that for the actual science, and declare that they know better than the folks who actually did the equations.
> 
> Then again, on another board, I frequently see people proclaim Einstein was wrong because they found a weak analogy somewhere and applied what they considered logic to it (the analogy, not the equations) and arrived at a different conclusion.



To be fair, sometimes people receive questions as challenging an idea's legitimacy rather than as merely requesting clarification on the idea. This is why I try very hard to explain that when I'm asking science questions here, I'm not poking holes and saying, "Aha! Explain _that_ eggheads!" I assume science is right and I'm just not understanding something when I poke and say, "Um, hey, what's up with that (eggheads)?"

For instance, elsewhere, I asked "What's wrong with the American immigration system?" I meant the question to get an explanation of what is wrong with the system (because I've had no interaction with it), but nearly everyone took it as "I challenge you to tell me there is something wrong with this system (because it is perfect)."

Bullgrit


----------



## Umbran (Jun 16, 2015)

Bullgrit said:


> For instance, elsewhere, I asked "What's wrong with the American immigration system?" I meant the question to get an explanation of what is wrong with the system (because I've had no interaction with it), but nearly everyone took it as "I challenge you to tell me there is something wrong with this system (because it is perfect)."




Well, it also happens that fairly often, challenges are made in the form of questions.  In text format, the tonal nuances are lost, and you have to go a few extra steps to make the connotation clear.


----------



## Bullgrit (Jun 16, 2015)

> Well, it also happens that fairly often, challenges are made in the form of questions. In text format, the tonal nuances are lost, and you have to go a few extra steps to make the connotation clear.



Yep, understood. But who is at fault for the misunderstanding? The person who asks the question or the person who assumes the question means something beyond the words? I mean, if you text someone, "Is it raining at home?" Are you inquiring about the weather, or are you hinting that they need to get up off the couch and finally go cut the grass before it starts raining? The tonal nuances and body language would tell you whether the question had deeper meaning, but without those, it's the fault of the listener to infer more meaning to neutral/non-contextual text.

I'm just saying that people should sometimes give a person the benefit of the doubt when there is no tonal context. Questioning science isn't the same as doubting science -- heck, it *is* science to question science.

Bullgrit


----------



## Mishihari Lord (Jun 16, 2015)

Scientific "fact" should be poked at.  Just because someone does some work or claims they're an expert doesn't make them omniscient.  Frex, we've been taught for the last 30? 40? years that eating eggs is unhealthy because of the cholesterol.  Guess what, just kidding!  The latest study shows that's not true at all.  There's an idea that should have been poked at a lot earlier.  That's what you get for blind faith in the experts.

When you get an insider's view of how scientific facts are made it's different than what the public thinks.  Sure there are facts and experiments.  There's also politics, personalities, rivalries, social dynamics, obfuscations, and sometimes (rarely) downright lies.  Leading views and direction of research are sometimes determined more by who has the biggest reputation than who has the best data.  The process is pretty good overall, and we do generally come up with results we can trust, but it's tough to tell when it's done, and blind faith in the results is never a good idea.


----------



## Umbran (Jun 16, 2015)

Bullgrit said:


> Yep, understood. But who is at fault for the misunderstanding?




Assigning fault is overrated.  Remember that it happens, adjust when you find it has, and move on.


----------



## Morrus (Jun 16, 2015)

Mishihari Lord said:


> Scientific "fact" should be poked at.




Absolutely.  But not *scientists*!  They're not politicians, or PR people.  Anything you hear from them is really secondary to the hard work they're busy doing.  And if they fail to communicate it well - that's unfortunate, but that's not what they're busy trying to do.  Take that up with your teachers and college lecturers, maybe - that is _their_ job. 

So ask questions; but be nice to the scientists and don't call them names like "stupid" because you didn't understand something. That's just mean, and says more about the speaker than the subject.  They're just normal people working hard. That's reasonable, don't you think?


----------



## Umbran (Jun 16, 2015)

Mishihari Lord said:


> Scientific "fact" should be poked at.




Yes. However, I don't think that is what is happening here. The way to poke at scientific fact is with scientific experiment.  Casting shade on the efforts of hundreds from the comfort of relative anonymity and you own home? That doesn't serve.


----------



## Mishihari Lord (Jun 17, 2015)

Morrus said:


> Absolutely.  But not *scientists*!  They're not politicians, or PR people.  Anything you hear from them is really secondary to the hard work they're busy doing.  And if they fail to communicate it well - that's unfortunate, but that's not what they're busy trying to do.  Take that up with your teachers and college lecturers, maybe - that is _their_ job.
> 
> So ask questions; but be nice to the scientists and don't call them names like "stupid" because you didn't understand something. That's just mean, and says more about the speaker than the subject.  They're just normal people working hard. That's reasonable, don't you think?




That first bit's not true at all.  A successful scientist has to be a PR person, politician, and marketer to win funding and other resources for their work.  Convincing people of the value of their work is part of their job, and if they can't do it they don't get far.  Politics in academia can be vicious, and in the corporate world it can be bad enough.  Decisions do get driven by social dynamics rather than data, and when they do, it's reasonable to call it "stupid," although you can soft-pedal it with other terms if you wish.  Freyar pointed me to some info that showed that the emphasis on dark matter is reasonable than I had thought, but based on my previous readings the research emphasis looked a lot more like something driven by social dynamics than by data.  Your "trust the experts" stance is not always helpful because sometimes the experts act like a herd of lemmings.  Umbran's "you must not understand science" approach is not helpful because it doesn't convince, and just because someone doesn't agree with you doesn't mean they don't understand.  Frex in my case, I've participated in research so it was a bit insulting.  Freyar's approach of engaging on the subject matter is the one that's useful.


----------



## Mishihari Lord (Jun 17, 2015)

Umbran said:


> Yes. However, I don't think that is what is happening here. The way to poke at scientific fact is with scientific experiment.  Casting shade on the efforts of hundreds from the comfort of relative anonymity and you own home? That doesn't serve.




Experiments don't happen unless someone looks at what's there and says "hey, that doesn't look right" first.


----------



## Morrus (Jun 17, 2015)

Mishihari Lord said:


> That first bit's not true at all.  A successful scientist has to be a PR person, politician, and marketer to win funding and other resources for their work.  Convincing people of the value of their work is part of their job, and if they can't do it they don't get far.  Politics in academia can be vicious, and in the corporate world it can be bad enough.  Decisions do get driven by social dynamics rather than data, and when they do, it's reasonable to call it "stupid,"




This is an unpleasant viewpoint that I can't engage with. If you feel you're entitled to call working scientists stupid because they didn't spend enough time explaining what they were doing to you, I really feel we have no common frame of reference.



> Your "trust the experts" stance is not always helpful because sometimes the experts act like a herd of lemmings.




I didn't say that. You're misrepresenting my position. I said don't call them rude names just because you don't understand something.



> Umbran's "you must not understand science" approach is not helpful because it doesn't convince,




But their job isn't to convince you of anything. The universe doesn't ask for your belief, or even your understanding!  You jumped into the thread and called scientists "stupid".   Umbran and Freyar are kindly spending their own time to explain stuff to you as a courtesy, because they're nice guys and chose not to take offence.



> and just because someone doesn't agree with you doesn't mean they don't understand.




Nobody said that, either.  What they said was that *when* you don't understand something (as in this case) calling people who do understand it stupid is rather rude.


----------



## Bullgrit (Jun 17, 2015)

Mishihari Lord said:
			
		

> Your "trust the experts" stance is not always helpful because sometimes the experts act like a herd of lemmings. Umbran's "you must not understand science" approach is not helpful because it doesn't convince, and just because someone doesn't agree with you doesn't mean they don't understand.



I'm not seeing this at all. Even though I'm the main one asking about the science, I don't see any "trust the experts" or "you must not understand science" in anyone's responses. 

Bullgrit


----------



## Umbran (Jun 17, 2015)

Mishihari Lord said:


> Experiments don't happen unless someone looks at what's there and says "hey, that doesn't look right" first.




If anyone on EN World other than freyar, myself, or a couple of others are going to engage in any such experiment... I will be pleasantly surprised.

Until then, though, I'm unimpressed by your methodology.


----------



## freyar (Jun 17, 2015)

Mishihari Lord said:


> That first bit's not true at all.  A successful scientist has to be a PR person, politician, and marketer to win funding and other resources for their work.  Convincing people of the value of their work is part of their job, and if they can't do it they don't get far.  Politics in academia can be vicious, and in the corporate world it can be bad enough.  Decisions do get driven by social dynamics rather than data, and when they do, it's reasonable to call it "stupid," although you can soft-pedal it with other terms if you wish.  Freyar pointed me to some info that showed that the emphasis on dark matter is reasonable than I had thought, but based on my previous readings the research emphasis looked a lot more like something driven by social dynamics than by data.  Your "trust the experts" stance is not always helpful because sometimes the experts act like a herd of lemmings.  Umbran's "you must not understand science" approach is not helpful because it doesn't convince, and just because someone doesn't agree with you doesn't mean they don't understand.  Frex in my case, I've participated in research so it was a bit insulting.  Freyar's approach of engaging on the subject matter is the one that's useful.




Scientists are marketers in a sense (some more so than others).  I just got back from a presentation by a Canadian MP (who is a PhD physicist), who made a pretty clear case that scientists are way behind most other groups in terms of marketing to the public, developing a clear constituency, and influencing policy.  This is an important issue, because it influences how much money goes into science overall and to some big earmarked projects in particular.  Outreach, or explaining things to the public, is something that I feel is also part of the job in a sense, since the public is generally who pays for science.

However, nearly all individual projects are funded based on peer review, both anonymous referees and expert committees.  These are not generally composed of people from the same subfield as the applicant, which reduces the chance of "group think."  Now, "trendiness" does sometimes enter into hiring decisions in academia, which is problematic, though generally the pendulum swings back after some time leading to an eventual balance.  It's also fair to say that there are good reasons for this in the cases I've observed --- research areas are particularly productive, a big experiment is coming on line, etc.

I'd also say that I haven't seen group think lead to major errors over a long period of time.  Scientists, like other people, are pretty competitive.  We always want to be the first to come up with the right idea, so it means you try to shoot holes in other people's ideas.  Ones that don't work eventually get knocked out.  And when there are multiple good ideas, people are pretty honest about it.  Now, this competitiveness also means that some people hold onto ideas that appear unworkable, but (1) that's easy to recognize and (2) it's part of the system that continually tests all the ideas.  Science is adversarial in some ways, even though it is also a cooperative enterprise.

It's also worth distinguishing physics research from medical research in a couple of ways.  First, the body is a really complex system, so a lot of research looks at correlations more than causation.  It's necessary, and it's the way we advance knowledge, but it can lead to apparent reversals.  But what I think is also important is that medical research gets reported widely very quickly, so sometimes I think the public gets a perception that the science is settled by one study when really a lot more work is needed.  Of course, this is not my area, so that's just an outsider's impression.  Physics doesn't usually get that kind of scrutiny.


----------



## freyar (Jun 17, 2015)

Umbran said:


> *ENVY!*




One of the very nice perks of my job is getting to meet a lot of very cool and interesting people.  I'll try not to name drop, but as even just a fairly successful physicist, I've met and in some cases gotten to know quite a few famous and extremely smart and interesting scientists.  Recently, the top scientists are experiencing this themselves by increasingly meeting celebrities (mainly actors from what I gather).  For example, my PhD supervisor (who is one of the top theoretical physicists) posted a selfie he took with Anna Kendrick at an Oscar party to his facebook timeline this year.  I'm also pretty sure Morgan Freeman narrated the video his sister put together for his 60th birthday.  It's actually a little surreal in some ways.


----------



## freyar (Jun 17, 2015)

Umbran said:


> If anyone on EN World other than freyar, myself, or a couple of others are going to engage in any such experiment... I will be pleasantly surprised.




I'm mildly offended -- I am a theorist, and it is my inalienable right to break experiments by looking at them funny!


----------



## Umbran (Jun 17, 2015)

Mishihari Lord said:


> Umbran's "you must not understand science" approach is not helpful because it doesn't convince, and just because someone doesn't agree with you doesn't mean they don't understand.  Frex in my case, I've participated in research so it was a bit insulting.




Right.  So, let me get this straight....

I questioned whether you were well informed about how this research was conducted, and I'm being insulting.

But this line started when you questioned the professional skill and approach of tens to hundreds of professionals.  That wasn't insulting? 

I am not sure how your critique of my approach is particularly valid, unless you also apply it to yourself.


----------



## Umbran (Jun 17, 2015)

freyar said:


> Scientists are marketers in a sense (some more so than others).  I just got back from a presentation by a Canadian MP (who is a PhD physicist), who made a pretty clear case that scientists are way behind most other groups in terms of marketing to the public, developing a clear constituency, and influencing policy.  This is an important issue, because it influences how much money goes into science overall and to some big earmarked projects in particular.  Outreach, or explaining things to the public, is something that I feel is also part of the job in a sense, since the public is generally who pays for science.




All true.  But...

(there is always a but)

You (the general you, not freyar, here) shouldn't by any means expect *every* practitioner to be engaged in public outreach.  The skill and talent set for actually doing the work of science is a pretty specific one.  It is not the same skillset as is used for marketing, or even education.  There are tons of spectacular researchers out there who should never be allowed in front of a lecture hall full of undergraduates!

We are slowly building out the ranks of scientists who also have the charisma and communication skills to do proper public outreach - folks like Phil Plait, and other science-literacy writers, bloggers, and the like.  Neil deGrasse Tyson took Star Talk to TV for a good reason.

Communications are always tailored to the expected audience.  As Morrus noted, I'm speaking here just as a guy who knows about a topic, speaking to an audience already known to be mostly interested in what I have to say on the matter, and positively engaged.  This is entirely casual, and I'm not really engaged in "convincing" anyone that the science in question is valuable - for the most part, I've been given sufficient evidence to think the audience already thinks so.  If you need to be convinced that physicists know how to do their jobs, that's really a different discussion than we are having here.  

Yes, science is done by humans, and humans have flaws and foibles, and certainly social trends impact science.  But, we have processes and mechanisms that combat this - this is what peer review and reproducible results are about.  In the long run, science filters out the flaws and foibles.  It can take a little time, but it does so consistently.  The computer you are currently using owes its existence to that fact.


----------



## dragoner (May 7, 2018)

Bullgrit said:


> A long flashy tunnel or an instantaneous popover?




I agree with Kip Thorne and Morrus that it would be most likely an instantaneous movement from one location to the next. The rub lies in that if the entry and exits have different velocities, one could travel into the future. I have a near future sci-fi campaign where the primary way of interstellar transit is by wormhole, where they create a Einstein-Rosen Bridge (wormhole) using a Thorne-Ellis Event Generator, travel is instantaneous from the occupants subjective viewpoint while it is 1d6+1 days objectively.


----------



## MarkB (May 7, 2018)

dragoner said:


> I agree with Kip Thorne and Morrus that it would be most likely an instantaneous movement from one location to the next. The rub lies in that if the entry and exits have different velocities, one could travel into the future. I have a near future sci-fi campaign where the primary way of interstellar transit is by wormhole, where they create a Einstein-Rosen Bridge (wormhole) using a Thorne-Ellis Event Generator, travel is instantaneous from the occupants subjective viewpoint while it is 1d6+1 days objectively.




For some really weird continuity effects, you could make it 1d6-2.


----------



## dragoner (May 7, 2018)

MarkB said:


> For some really weird continuity effects, you could make it 1d6-2.




Like on Futurama: "OK Mister: I'm my own Grandpa!" 

Going back in time would be weird, you would arrive before you left, though the interval is short enough with 1d6-2 that it would be hard to cross your own path. ISTR Thorne or Ellis said that going backwards in time wasn't possible, just for the record; but in a game, there is no limit.


----------



## Janx (May 8, 2018)

I miss these interesting discusssions on EN World.


----------



## Umbran (May 12, 2018)

dragoner said:


> I agree with Kip Thorne and Morrus that it would be most likely an instantaneous movement from one location to the next.




You know, this thread is years old, so I cannot remember if this got said, and I'm not going back to read the whole thing for just this point...

A wormhole is a shortcut through a curved space.  To go through normal space from points A to B, you travel some distance X.  Go between points A and B through a wormhole, and you go some distance Y.

Now, you have three cases.  Y=0 is the "instantaneous travel" option.  The wormhole is a portal, and there is zero distance between its ends.

But zero is a very specific number.  All in all, you'd kind of expect Y to have some value, but one that specific?  That would have to be a specific result of the math, to come out that elegantly.

The more realistic idea is that Y has some non-zero value.  If X>Y, we travel some distance through the wormhole, and it is a shortcut.  But that doesn't mean it is actually short, on human scales.  It is ~4 light years to Alpha Centauri.  A trip of only 2 light years to get there would be a shortcut, but still a long distance on human scales.  

There's also the possibility that Y > X, that the wormhole trip is *longer* than the trip through normal space.  It would be kinda dumb to spend all the effort warping spacetime and taking a longer trip, rather than a shorter one, but I think the math allows it.  We just aren't interested i this, so we disregard it.



> The rub lies in that if the entry and exits have different velocities, one could travel into the future. I have a near future sci-fi campaign where the primary way of interstellar transit is by wormhole, where they create a Einstein-Rosen Bridge (wormhole) using a Thorne-Ellis Event Generator, travel is instantaneous from the occupants subjective viewpoint while it is 1d6+1 days objectively.




Um, the whole point is that there is no "objective" time.  That's why it is the Theory of *Relativity*.  If there's objective time, wormholes are not possible!

The best you can get is, "it is 1d6+1 days with respect to some specific location in normal space".  I would imagine that best to be the point of departure, so if you take a round trip out and back, the total time you are gone is at least 2d6+2 days.


----------



## dragoner (May 12, 2018)

This is an old thread, agreed, it just appeared as a recommended discussion at the bottom of the page in another discussion, and I found it to be interesting.

Time in this instance, I mean inside and outside the wormhole, and the acceleration, or gravitational effect on the mouth causes time dilation. So point of departure would be objective.

For path distance, a non-zero number could still be a few meters, and for a longer path in a traversable wormhole, it might not be realistic to make it too much longer, because that could create a schwarzschild wormhole with a event horizon and ultimately a black hole, by warping spacetime even more.


----------



## tomBitonti (May 14, 2018)

For simulated images, check out this:

https://arxiv.org/abs/1502.03809
https://arxiv.org/pdf/1502.03809.pdf

Lots of math, but you can ignore that and just look at the simulated pictures.  (But the math is good, too!)

I particularly like the simulated images on pages 10 and 11 of the PDF.

Thx!
TomB


----------



## tomBitonti (May 15, 2018)

Umbran said:


> A wormhole is a shortcut through a curved space.  To go through normal space from points A to B, you travel some distance X.  Go between points A and B through a wormhole, and you go some distance Y.
> 
> Now, you have three cases.  Y=0 is the "instantaneous travel" option.  The wormhole is a portal, and there is zero distance between its ends.
> 
> ...




Additional text omitted.

Wouldn't it be easier to work directly with the wormhole geometry?  That's what the linked PDF does, with, effectively, a wormhole length as one of the wormhole parameters.  Also, in their analysis, time dilation effects are virtually nil, so the length can be used directly to measure transit times.

There is an interesting example of one of the cases: Greg Egan, in "Diaspora" (Millennium, 1997), postulates that elementary particles are wormholes.  But Egan's wormholes have Y=X, so not useful for crossing our universe.  (But useful for entering a higher level dimension, since the worm holes have shape in higher dimensions and can be used to transport information and create material in the higher dimension space.)

From the wikipedia article (https://en.wikipedia.org/wiki/Diaspora_(novel)):

"An appended glossary explains many of the specialist terms in the novel. Egan invents several new theories of physics, beginning with Kozuch Theory, the dominant physics paradigm for nearly nine hundred years before the beginning of the novel. Kozuch Theory treats elementary particles as semi-point-like wormholes, whose properties can be explained entirely in terms of their geometries in six dimensions. Certain assumptions common to Egan's works inform the plot."

Thx!
TomB


----------



## freyar (May 16, 2018)

Wow, blast from the past!

tomB, Umbran really is using the wormhole geometry, but he's just talking about it in somewhat less technical terms than the paper you linked.  Also, you're right, the length of the wormhole is a parameter, but (assuming a wormhole does exist) that parameter should be determined by whatever matter creates the wormhole in the first place.


----------

