# Speed of Light question



## Harmon (Oct 20, 2005)

Something has been bugging me for a while.

If a Black Hole draws everyting into it, nothing can escape, not even light, and a beam of light is traveling towards the black hole at the SoL (d'uh) then does the Black Hole speed the light up beyond the SoL?

If it does speed light up beyond the speed of light then doesn't that mean that there is something that travels faster then the speed of light- that being anything traveling into a black hole?


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## Thotas (Oct 21, 2005)

No.  Primo Numero Uno fact of the universe we live in: light has one speed.  It's a consequence of "Time" and "Space" being in reality one single, absolute entity called "space-time", which warps itself as needed to preserve that speed no matter what viewpoint you use to measure said speed.


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## jgbrowning (Oct 21, 2005)

Light is a constant in a vacuum. Its speed varies in other mediums. Slower in air, and even slower in water... etc.. I believe it was even slowed down to 20 or so meters per second in some experiments.

joe b.


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## der_kluge (Oct 21, 2005)

I think black holes warp space and time in their vicinity, so yes, I believe that the speed of light is accelerated.


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## RangerWickett (Oct 21, 2005)

No, der kluge, that's not right.

A black hole does not pull things into it. Rather, its gravity causes the space around it to warp so that what an outside observer might perceive as a curve is actually a straight line. So, a beam of light passing within distance X of a black hole will take a straight path into the black hole, where X is dependent upon the actual gravity of the black hole.

Or at least, that's how I understand it.


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## Harmon (Oct 21, 2005)

*More to the question*

We know that light bends when it comes near a great mass- say a distant star's light can have its position change to our perception if that light passes by a black hole or a star between here and there.

Would it be to much to think that light can be bent, could it not be slowed by the same force that bends it?  Could that same force not alter its speed to go faster?  Say the light speeds towards the massive star then slows as it passes then tries to pull away from that same star.  Could that be true?

Could it be true that light is just the fastest think we can perceive?

I recall hearing once that a scientist stated flat that man would never surpass the speed of sound, the laws of physics would not allow.  It was on one of those history shows about breaking the speed of sound.  Of course we broke the speed of sound many times over, but it proves to me that our understanding of physics is pretty much at kindergarten level while the universe is running at beyond Doctoral level.

Just a thought that has been bothering my sleep.


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## Edena_of_Neith (Oct 21, 2005)

I am sorry, RangerWickett.


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## Edena_of_Neith (Oct 21, 2005)

I have heard that Time slows down as you move closer to a Black Hole.
  At the Point (or whatever they call the actual implosion) Time comes to a standstill.


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## fuindordm (Oct 21, 2005)

Light always travels at the speed _c_.  Strong gravity will deflect it, but not change its speed.

Conservation of energy still applies, however.  As light heads towards a massive object, it blueshifts--the wavelength gets shorter, and the energy in the light increases (just as the kinetic energy of a falling
object increases).  As light heads away from a massive object, it redshifts and loses energy.

The issue of time is stickier.

From the perspective of someone sitting at a safe distance and watching, nothing unusual happens to an abject falling into a black hole.  It crosses the event horizon, and 'plouf' goes away.

From the perspective of someone falling into a black hole, time does slow down, to the point that you never actually reach the surface of the black hole.  The closer you get, the slower it gets, to the limit that time doesn't move at all at the actual surface.  So it's also fair to say that nothing actually ever gets inside a black hole.  Some physicists like this because in theory, all the stuff that's ever fallen into a black hole is still there squished all over its surface, so energy and information aren't getting destroyed and violating conservation laws.

Ben


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## physicscarp (Oct 21, 2005)

fuindordm said:
			
		

> The issue of time is stickier.
> 
> From the perspective of someone sitting at a safe distance and watching, nothing unusual happens to an abject falling into a black hole.  It crosses the event horizon, and 'plouf' goes away.
> 
> ...




Isn't this the other way around?


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## Hand of Evil (Oct 21, 2005)

The one that gets me is speed of gravity


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## mps42 (Oct 21, 2005)

Here's the way I understand it (which could be wrong).

 Think of space-time as a huge sheet or blanket. There are things of different sizes and masses on this blanket. Each object creates a dent in the blanket, the more massive the object, the bigger the dent. That's gravity.
 A "Black hole" is created when a white dwarf star (which is a super dense object) collapses in on itself to the point where there is an incredible amount of mass in almost no space. A virtual singularity.
 Now, light passes along the surface of space-time but, for our discussion, is't really a "part" of it. Einstein theorized that light, being almost pure energy, traveled at a constant velocty, written as "c". That being the case, it can be effected by gravity (the dents in the fabric of space-time) but cannot be sped up by those dents. If that is true, then a "black hole" is bending light to the point where it keeps going around in circles and never gets out again. Light doesn't "stop". If it did, it couldn't be light, it would be something else. There are ways to detect black holes, by radio waves and radiation and such which means that we can currently either detect something that IS escaping the gravity hole or, in my opinion more likely, we can find them (black holes) by observing the movements of other objects.
 Now, as for relativity, that's almost a completely different animal...

 Dangit, now I gotta read "A brief history of time" again...


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## fuindordm (Oct 21, 2005)

Black holes can come from a variety of different sources.  AFAIK, they are usually created in supernova; a white dwarf is also a supernova remnant, but from a star that wasn't massive enough to make a black hole. There are also galactic black holes, which have as much mass as hundreds of thousands to millions of stars, and live in the core of many galaxies.  These are thought to be the source of quasars.

When we observe black holes, we detect radiation emerging from its immediate environment: radio waves from electrons going into a death spiral down its magnetic field, X-rays from the accretion disc of superheated gas flowing into the black hole, and so forth.

Light that gets captured by a black hole really gets captured, although it is possible to get trajectories that spiral hundreds or thousands of times before they reach the hole.

Ben


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## Umbran (Oct 21, 2005)

Harmon said:
			
		

> Would it be to much to think that light can be bent, could it not be slowed by the same force that bends it?  Could that same force not alter its speed to go faster?  Say the light speeds towards the massive star then slows as it passes then tries to pull away from that same star.  Could that be true?




Nope, it can't  

Above, you fall into the trap of thinking about light as a normal physical object.  If you toss a ball near a massive object, it'll speed up and slow down as you describe, but light is not a rubber ball.

What happens to the ball is that it gains energy while falling towards the massive body, and loses energy when falling away.  The speeding up and slowing down is a result of that energy gain and loss.

Light will also gain and lose energy, like the ball.  But, having no mass, the speed of the light cannot vary.  So, instead, the light _changes wavelength_.  It shifts towards the blue while falling inwards, and shifts to the red when falling away.

Fuindorm: I'm sorry, but you've got the time thing backwards.  Remember - time is relative. To a local observer, his own clock always seems to run normally.  It's everyobdy else's clocks that change.

Take two clocks.  Keep one yourself.  Hand another to your friend, and drop him into the black hole (well, don't use a _good_ friend for this).

You look at your clock, and it seems to be running normally.  You take a telescope and look at your friend's clock, and it will seem to be running more and more slowly.  The light reflecting off the clock face gets more and more red - from your perspective it's got fewer and fewer oscillations per second, a lower frequency.  His clock ticks more and more slowly, and eventually, just as he reaches the Event Horizon, the ticks are infinitely far apart, and you never see the last one.  From your point of view, he never actually falls into the hole.  He can't, because time would have to pass for him to cross the threshold, his clock would have to advance, but he's frozen with the next tick off at forever...

From his own point of view, though, his clock keeps ticking just fine.  But if he looks up at your clock, it seems to be speeding up!  Ticks come faster and faster.  The light falling in towards him has more and more oscillations per second, so it looks more and more blue.  He just falls into, and through, the Event Horizon normally.  From his point of view, it's the rest of the Universe that's speeding by.


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## Kemrain (Oct 21, 2005)

http://homokaasu.org/gasgames/game.gas?21

It's not about light, but it involves Gravity heavily!

- Kemrain the Enjoying the Discussion.


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## Harmon (Oct 21, 2005)

Thank you, Umbran for the time it took you to write that.  

I actually knew all of that, modern understanding of the speed of light seems a little narrow minded to me and thus the question.

Reasons why I disagree with the theory about the speed of light and black holes pull is that the rules of our understanding change to an unknown bases when our physics is applied to a black hole.  However when you consider that our perception is what limits our understanding it kinda opens up some new possibilities.

So- say there is something that is faster then the speed of light, something we can't see.  How would we detect it it travels/moves faster then anything we can percieve (using our eyes, using advanced machines, etc- light is still the fastest "thing")

So if we open our minds to the possibility that there is something faster then light then we understand that light does not have a constant.

Our knowledge of the speed of light is based on the gravity around us, if we understood that light travels at different speeds in different areas of gravity then maybe we'd be able to understand the physics of a black hole a little better.

<shrug> thank you for your time and trying to help me get past this theory.  I guess I am just not willing to accept that we understand everything absolutely.  Like I said- thank you for your help.


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## fuindordm (Oct 21, 2005)

Thanks, Umbran.  You're right, of course, and I'm ashamed of myself.  I even wrote a light-propagation code for black hole accretion disks a couple of years ago for my previous research project, and I still have to think hard every time the question comes up.

Ben


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## fuindordm (Oct 21, 2005)

Harmon said:
			
		

> So- say there is something that is faster then the speed of light, something we can't see.  How would we detect it it travels/moves faster then anything we can percieve (using our eyes, using advanced machines, etc- light is still the fastest "thing")
> 
> So if we open our minds to the possibility that there is something faster then light then we understand that light does not have a constant.
> 
> Our knowledge of the speed of light is based on the gravity around us, if we understood that light travels at different speeds in different areas of gravity then maybe we'd be able to understand the physics of a black hole a little better.




Well, people have theorized about tachyons (particles of negative mass that travel faster than light) and warp fields (configurations of gravitational fields that allow faster-than-light travel while still obeying the rules of relativity locally) in scientific journals.  

I don't follow your question, though.  Whether or not we can detect something has little to do with how fast it goes; it has everything to do with how it interacts with other particles.  You're welcome to postulate a particle that moves faster than light and participates only in the gravitational interaction, which would be incredibly hard to detect unless the universe was filled to brimming with them.  (Or maybe we have--who knows, it could be the 'dark energy' that we're hearing so much about these days.)   But the existence of such a thing wouldn't change our understanding of light.

There's a lot of stuff out there in the theoretical journals that *could* exist as far as we know, and some of it's pretty wacky.

But I would bet all the limbs of my body that we do understand the nature of LIGHT completely.  The theories governing the behavior of light have been subjected to experiments of incredible precision.  Our understanding of gravity may change, but that won't affect the basic features of light--constant speed, redshifting and blueshifting, etc.

Cheers,
Ben


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## Michael Morris (Oct 22, 2005)

Note on black holes and stars.  Only really, really, really big stars produce black holes on their deaths. Most (like our sun) will expand into red giants (sufficient to make the sun absorb all the planets closer to it than Jupiter) and then after another billion years or so it will fly apart leaving a white dwarf.

If a star is bigger than the sun - say - 100x the mass - it will collapse into a neutron star.  Here the collapse is so violent that all the atomic nuclei in the star loose their charge and become neutrons (hence the name).

Ten times bigger still are the stars which, in theory, spawn black holes. Here Stephen Hawkings (I believe) theorized that all black holes have a single mass - any thing that collides with them (and any excess mass at their creation) is slung off as high energy xrays.

Nothing - not even light, penetrates the "hole"  It's slung off the exact opposite side.

Incidently, a black hole's gravity doesn't somehow grow due to it's new status.  It's gravitational pull is the same as the star it came from and the event horizon is, usually, near where the surface of the star was in life.

(Disclaimer - I am not a scientist.  Astronomy is one of my favorite hobbies but don't quote me as an expert).


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## Tarrasque Wrangler (Oct 22, 2005)

Edena_of_Neith said:
			
		

> I have heard that Time slows down as you move closer to a Black Hole.




Possibly.  I know that time slows to a crawl if you have to spend a weekend in Fontana, CA.


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## jgbrowning (Oct 22, 2005)

mps42 said:
			
		

> Light doesn't "stop". If it did, it couldn't be light, it would be something else.




http://www.msnbc.com/news/242698.asp



joe b.


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## Umbran (Oct 22, 2005)

Harmon said:
			
		

> Reasons why I disagree with the theory about the speed of light and black holes pull is that the rules of our understanding change to an unknown bases when our physics is applied to a black hole.




Ah, but you see, until you get inside the event horizon, a black hole is not fundamentally different from any other massive object.  A black hole bends light in exactly the same way a star of the same mass does.  You are correct that inside, all bets are off.  But then, the stuff inside cannot communicate with the rest of the universe anyway, so what happens inside hardly matters. 



> However when you consider that our perception is what limits our understanding it kinda opens up some new possibilities.




Not really - because if you can't percieve the thing, or the thing's effects, then it isn't really part of your universe.  And if you can percieve the thing, or it's effects, you can know about it.



> So- say there is something that is faster then the speed of light, something we can't see.  How would we detect it it travels/moves faster then anything we can percieve (using our eyes, using advanced machines, etc- light is still the fastest "thing")




We'd detect it the same way we detect _everything_, by watching it interact with other things.  We measure the speed of light by knowing when it left point A, and detecting when it interacts with something at point B.  It moves faster than anything else in the Universe, so it isn't like we see it coming to point B, or watch it as it goes along.  We wait until it gets there, and figure out how long the trip took after the fact. This hypothetical thing that moves faster than light would be no different.  

Believe me, it isn't as if people have not been searching for evidence of things that move faster than light.  They've been searching since Einstein posited that C was a constant for all observers.  And they haven't found anything yet.



> Our knowledge of the speed of light is based on the gravity around us, if we understood that light travels at different speeds in different areas of gravity then maybe we'd be able to understand the physics of a black hole a little better.




If what you said were true, that light increased in speed as it fell inward to a massive body, then we should be able to measure the difference.  Experiments to test this have been done.  The thing you describe is not observed to occur.  Instead, something else occurs - instead of speeding up, the light shifts frequency.  

The frequency shifts just enough to cover the energy gain or loss that we expect to occur.  So, if the speed were to change as well, where would the extra energy be coming from or going to?



> <shrug> thank you for your time and trying to help me get past this theory.  I guess I am just not willing to accept that we understand everything absolutely.




Nobody but you has said anything about our understanding being absolute.  But new theories do have to be consistent with the observations we already have.  And those observations are not consistent with your hypothesis.


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## Nellisir (Oct 22, 2005)

Spoony Bard said:
			
		

> Nothing - not even light, penetrates the "hole"  It's slung off the exact opposite side.
> 
> Incidently, a black hole's gravity doesn't somehow grow due to it's new status.  It's gravitational pull is the same as the star it came from and the event horizon is, usually, near where the surface of the star was in life.




Neither of these sounds quite right. If #1 is true, then a certain number of "black holes" would, to us, look like massive streams of x-rays bursting out of nowhere.

If #2 is true, then what happens when two black holes meet?  And where did that monster black hole at the center of the Milky Way (that's the latest thinking, I think) come from?

I haven't done much science reading though, so I might be out of date.
Cheers
Nell.


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## Umbran (Oct 22, 2005)

Spoony Bard said:
			
		

> If a star is bigger than the sun - say - 100x the mass - it will collapse into a neutron star.  Here the collapse is so violent that all the atomic nuclei in the star loose their charge and become neutrons (hence the name).




Small corrections - this has nothing to do with the violence of the collapse.  The collapse could be nice and slow and quiet and gentle, and you'd still end up with a neutron star.  

In addition, it isn't that the atomic nuclei lose their charge.  What happens is the pull of gravity is so great that it overcomes the forces that keep the electrons out of the nucleus of the atom.  The electrons are inexorably forced into the nucleus, close enough to the protons that they fuse and become neutrons.



> Ten times bigger still are the stars which, in theory, spawn black holes. Here Stephen Hawkings (I believe) theorized that all black holes have a single mass - any thing that collides with them (and any excess mass at their creation) is slung off as high energy xrays.




Another correction - this is not true.  We have black hole candidates of many masses.  From supermassive things that live in the cores of galaxies, down to more run of the mill remnants of supernovae.


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## Michael Morris (Oct 22, 2005)

Ok..

Anyway, If I understand correctly black holes are also a big hole in our physics model. They are example of things which are both (potentially) insanely massive and small. The science of really big things (relativity) and really small things (Quantum Mechanics) start to fray and fall apart when they both have to apply at the same time.  Figure out what's going on in a black hole and win the Nobel prize, right?


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## trancejeremy (Oct 22, 2005)

Spoony Bard said:
			
		

> Incidently, a black hole's gravity doesn't somehow grow due to it's new status.  It's gravitational pull is the same as the star it came from and the event horizon is, usually, near where the surface of the star was in life.
> 
> (Disclaimer - I am not a scientist.  Astronomy is one of my favorite hobbies but don't quote me as an expert).




I am not a scientist, but I was a space science major in college. That is not quite true  The event horizon (the part where light cannot escape from) is much, much, much smaller than the star that collapsed.

Just do a search on "Schwarzschild radius" (If I spelled his name right, it's been a while) and you can see the equation.  It's pretty simple.

If our sun were to somehow magically turn into a black hole and keep the same mass, it would only be a few kilometers big. The event horizon. The actual mass is concentrated into a single point, though. 

(But stars have to be 5 times the mass of the sun to turn into one via a supernova.)

Anyway, black holes actually do make sense according to relativity and quantum mechanics. They do cause problems with Newtonian physics, but observating black holes has helped to prove a lot of things predicted by relativity.

It's just that relativity and quantum physics don't make much sense. Well, they do, but it's not common sense, since it's not something we observe on an everyday basis in life. But mathematically they do.


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## Thotas (Oct 22, 2005)

Yes, Spoony; figure out what's inside the event horizon, get the Nobel and a huge place in history.  But as Umbran points out, we've got things on this side very thoroughly investigated.  That whole speed of light thing has been analyzed to whatever degree has been technically possible since Einstein, and the results always come out the same.  The speed of light in a vacuum is the speed of light in a vacuum equals c which is a constant as per what Einstein called his invariance theory.  

As far as the force that bends light being able to slow it down, that's gravity.  It might seem obvious to some that this is what happens with a black hole, right?  Can't escape 'cause it's slowed down too much, right?  No again.  Gravity accelerates things, but acceleration isn't always a change in over all speed; change in direction is acceleration, also.  A photon trapped in a black hole is now orbitting the singularity ... at the speed of guess what.

In fact, if you really want to get technical about it, light isn't the only thing that always moves at the speed light.  That other thing would be ... everything.  Again, "time and space" as separate entities is an inaccurate assumption that leads to a whole mess of inaccurate deductions.  Through space-time, everything has a space-component-velocity and a time-component-velocity.  A beam of light has a much higher space-velocity than most things we encounter day to day, but all of those things have a much higher time-velocity than the light.  But for any object/particle/signal, if you add the two components together, you get that same universal speed all the time.


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## der_kluge (Oct 22, 2005)

www.pbs.org has a lot of good articles on black holes. You can do a search for "black hole" to find all kinds of stuff.

Once upon a time I found episodes of, I though NOVA, regarding black holes and physics topics like quantum physics, but I can't find them now. It might not have been NOVA, but I thought it was.


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## glass (Oct 26, 2005)

Spoony Bard said:
			
		

> The science of really big things (relativity) and really small things (Quantum Mechanics) start to fray and fall apart when they both have to apply at the same time.  Figure out what's going on in a black hole and win the Nobel prize, right?



I thought M Theory was supposed to have smoothed over the disconnect between Relativity and Quantum Mechanics?


also non-physicist glass.


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## glass (Oct 26, 2005)

Thotas said:
			
		

> In fact, if you really want to get technical about it, light isn't the only thing that always moves at the speed light.  That other thing would be ... everything.  Again, "time and space" as separate entities is an inaccurate assumption that leads to a whole mess of inaccurate deductions.  Through space-time, everything has a space-component-velocity and a time-component-velocity.  A beam of light has a much higher space-velocity than most things we encounter day to day, but all of those things have a much higher time-velocity than the light.  But for any object/particle/signal, if you add the two components together, you get that same universal speed all the time.



That would mean that light has no time component velocity right? So from a photons point of view, it exists for zero time? Freaky.  


glass.


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## Pinotage (Oct 26, 2005)

glass said:
			
		

> I thought M Theory was supposed to have smoothed over the disconnect between Relativity and Quantum Mechanics?
> 
> 
> also non-physicist glass.




In theory, mostly. Unfortunately, very few of the concepts in M-theory have actually been proven. While the theory postulates many things, such as the existence of gravitron particles and its supposed properties etc., until somebody actually proved it, it's little more than a theory. I believe the new Hadron collider will hopefully generate enough energy to detect various new particles, such as the heavy super particles, that may give support to M-theory (a form of string theory where the world is not only made up of vibrating strings, but also membranes, called branes).

It is also, unfortunately, a horribly complicated theory, and not easy to use for predicting things, but yes, it does smooth over some of the issues between quantum mechanics and relativity.

Pinotage


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## Pinotage (Oct 26, 2005)

glass said:
			
		

> That would mean that light has no time component velocity right? So from a photons point of view, it exists for zero time? Freaky.
> 
> 
> glass.




Relatively speaking, yes. A person travelling at the speed of light, or a photon, would be considered to have a zero time component relative to an observer. The person travelling at that speed, however, would experience time normally.

Pinotage


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## Nellisir (Oct 26, 2005)

Pinotage said:
			
		

> I believe the new Hadron collider will hopefully generate enough energy to detect various new particles, such as the heavy super particles, that may give support to M-theory (a form of string theory where the world is not only made up of vibrating strings, but also membranes, called branes).




Mmmmm, branes.

 
Nell.


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## fuindordm (Oct 26, 2005)

Nellisir said:
			
		

> Mmmmm, branes.
> 
> 
> Nell.





 Watch out for those zombie physicists!

The term was originally N-brane, meaning an N-dimensional generalization of a membrane, meaning an N-dimensional surface in the 11-dimensional space of string theory.

_The Elegant Universe_, by Green, is a very enjoyable book on string theory that is accessible to people of all backgrounds.   Not only are the predictions of the theory still
untestable in the lab, humanity's math skills aren't yet really advanced enough to treat it
fully.

The LHC might be able to test some predictions of string theory if we're lucky, but just identifying the Higgs particle would be enough to make all the physicists of the work squeal and clap their hands like little girls.  I'm not holding my breath for more from them.

Ben


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## Umbran (Oct 26, 2005)

glass said:
			
		

> I thought M Theory was supposed to have smoothed over the disconnect between Relativity and Quantum Mechanics?




None of the theories that try to connect gravity and quantum mechanics have yet to make one very important milestone - verifiable predictions.

In order to be considered a viable candidate, a theory needs to do more than "smooth over" a disconnect.  It needs to make testable predictions that other theories don't make.  The various brane, string, and holographic theories still don't do this, even though we've been working on the problem since the 1970s.


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## Desdichado (Oct 26, 2005)

Umbran said:
			
		

> Above, you fall into the trap of thinking about light as a normal physical object.  If you toss a ball near a massive object, it'll speed up and slow down as you describe, but light is not a rubber ball.



Exactly.  Photons have no mass, so they are not affected as massed(?-is that the word?) particles would be by gravity.


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## Desdichado (Oct 26, 2005)

der_kluge said:
			
		

> www.pbs.org has a lot of good articles on black holes. You can do a search for "black hole" to find all kinds of stuff.
> 
> Once upon a time I found episodes of, I though NOVA, regarding black holes and physics topics like quantum physics, but I can't find them now. It might not have been NOVA, but I thought it was.



If it's still accurate.  How old is it?  There's been a lot of changes in our theoretical picture of the universe over the last few years, starting with M-Theory, which has already been mentioned here.


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## Harmon (Oct 26, 2005)

JD- photons aren't effected by mass?

Is that true?


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## Desdichado (Oct 26, 2005)

No, they don't _have_ mass, so they're not affected the same way as particles with mass.  Notably, that they don't slow down when subjected to gravitational attraction.  They can however still be diverted by the gravity well of large mass.


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## Harmon (Oct 26, 2005)

Joshua Dyal said:
			
		

> No, they don't _have_ mass, so they're not affected the same way as particles with mass.  Notably, that they don't slow down when subjected to gravitational attraction.  They can however still be diverted by the gravity well of large mass.




Okay, my misunderstanding.  The other way would have had some intentesting implications


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## glass (Oct 27, 2005)

Joshua Dyal said:
			
		

> No, they don't _have_ mass, so they're not affected the same way as particles with mass.  Notably, that they don't slow down when subjected to gravitational attraction.  They can however still be diverted by the gravity well of large mass.



I thought light did have mass, on account of it having energy. Like moving objects gain mass, but with the mass-velocity starting at 0,0, rather than starting from the mass due to an object's matter.

Have I been barking up the wrong tree all this time?


still confused glass.


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## Pinotage (Oct 27, 2005)

glass said:
			
		

> I thought light did have mass, on account of it having energy. Like moving objects gain mass, but with the mass-velocity starting at 0,0, rather than starting from the mass due to an object's matter.
> 
> Have I been barking up the wrong tree all this time?
> 
> ...




Here's an article that might help:

Does light have mass? 

Pinotage


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## glass (Oct 27, 2005)

Pinotage said:
			
		

> Here's an article that might help:
> 
> Does light have mass?
> 
> Pinotage



Thanks, that makes sense (sorta). So, I was just a bit behind the times.


glass.


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## Raven Crowking (Oct 27, 2005)

Harmon said:
			
		

> We know that light bends when it comes near a great mass- say a distant star's light can have its position change to our perception if that light passes by a black hole or a star between here and there.
> 
> Would it be to much to think that light can be bent, could it not be slowed by the same force that bends it?  Could that same force not alter its speed to go faster?  Say the light speeds towards the massive star then slows as it passes then tries to pull away from that same star.  Could that be true?
> 
> ...





This is a misperception of what relativity actually says.  The light is not bent by mass.  Space-time is bent by the mass (compressed in its vicinity, if you like), and the light is following a straight line.

Re:  Speed of light in a vacuum; when light travels through different media, it is actually travelling farther.  The speed of light remains constant in all media; the media just makes it seem to slow down to the outside observer.

(more later)


RC


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## Umbran (Oct 27, 2005)

Raven Crowking said:
			
		

> This is a misperception of what relativity actually says.  The light is not bent by mass.  Space-time is bent by the mass (compressed in its vicinity, if you like), and the light is following a straight line.




To be even more accurate, this is true for all objects, not just light.  All objects travel along straight lines in spacetime.  Near massive objects, spacetime is curved.  

The disconnect is that our brains are wired to assume spacetime is flat.  We see bent trajectories in a flat space, when we are actually looking at straight trajectories in flat space.  To a large degree, it is only a difference in perspective.



> Re:  Speed of light in a vacuum; when light travels through different media, it is actually travelling farther.




That's one way to say it, but note that it is not travelling farther due to curvature of spacetime...


RC[/QUOTE]


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## Raven Crowking (Oct 27, 2005)

Umbran said:
			
		

> That's one way to say it, but note that it is not travelling farther due to curvature of spacetime...





....Depending upon your interpretation of quantum mechanics.  In some interpretations, matter itself is essentially space-time curvature scrunched up into little knots.


RC


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## Umbran (Oct 27, 2005)

Raven Crowking said:
			
		

> ....Depending upon your interpretation of quantum mechanics.  In some interpretations, matter itself is essentially space-time curvature scrunched up into little knots.




Well, I was about to come back and edit my post there - it isn't curvature in spacetime due to gravitational mass.  

I am a physicist, and I don't trust any model that doesn't make unique testable predictions.  The models you mention here are among that class.


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## fuindordm (Oct 27, 2005)

Umbran said:
			
		

> Well, I was about to come back and edit my post there - it isn't curvature in spacetime due to gravitational mass.
> 
> I am a physicist, and I don't trust any model that doesn't make unique testable predictions.  The models you mention here are among that class.




Well, they may be testable one day.  And it's possible that improved cosmological data (for example, from the Planck satellite mission) may provide indirect constraints.  The early universe (and by extension the current universe) is the only naturally occuring laboratory for very high-energy interactions.

The key thing to realize about general relativity (GR) is that it's a differential theory.

In the same way that calculus analyzes curves by assuming they can be approximated by straight lines locally (the derivative = dy/dx is an example of this), GR is a theory that stitches together differential patches of flat space-time (described by special relativity) into a continuum.   So GR can't accurately describe an environment where space-time might be cuspy (inside the event horizon of a black hole) or 'frothy' (on string theory scales, where particles are being created and destroyed by quantum fluctuations). There's no clear way of extending the theory to cover such situations, which is why most unified theories are particle/field theories (modeling the effects of gravity with gravitons rather than curved space-time). 

Ben


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## Desdichado (Oct 27, 2005)

This discussion is really fun!  But sadly, over my head.    But this is pretty heady, fascinating stuff.


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## Raven Crowking (Oct 27, 2005)

Umbran said:
			
		

> Well, I was about to come back and edit my post there - it isn't curvature in spacetime due to gravitational mass.
> 
> I am a physicist, and I don't trust any model that doesn't make unique testable predictions.  The models you mention here are among that class.





Actually, the models I mention here (coupled with relativity) led me to believe that, as one left an area of concentrated mass, the expansion of the universe would add apparent speed to one's motion.  Of course, I am not a physicist (except in the amature theoretical sense), so I was simply doing armchair speculation and "common-sense" analysis of the theories and data that I was reading, coupled with a few insights gained from conversations while taking astrophysics at Santa Monica Community College.  I suppose it would be fair to say that I came up with my own interpretation of the data, based upon elegance more than anything else.  When I first read of the discovery of quintessence, I was quite pleased.  I just wish I was mathematician enough to determine whether or not it fell within the range my interpretation would allow for.


RC


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## Umbran (Oct 27, 2005)

fuindordm said:
			
		

> Well, they may be testable one day.




*nod*.  All I mean to say is that until that time, it's not a reliable source of descriptions of what is happening.  Science education is in bad enough shape as it is, we shouldn't go using untested theories as education tools 




> There's no clear way of extending the theory to cover such situations, which is why most unified theories are particle/field theories (modeling the effects of gravity with gravitons rather than curved space-time).




Yep.  And this is also why it is nearly impossible to compute anything useful in them.  Differential theories are, mathematically, far more tractable.


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## Umbran (Oct 27, 2005)

Raven Crowking said:
			
		

> Actually, the models I mention here (coupled with relativity) led me to believe that, as one left an area of concentrated mass, the expansion of the universe would add apparent speed to one's motion.




Yes, I've seen models that describe it that way.  But it is important to note that the normal gravitational mass of the matter is not sufficient to describe the slowing/bending. If that were the case, the bending of light by lenses would be merely a function of the physical density of the material, which is demonstrably not the case.

I find such theories to be excessive and inelegant, honestly.  It's using a bazooka to kill a fly.  The change of the speed of light in a medium can be more easily handled using standard QM/solid state physics - the EM fields of the propagating light interact with the EM fields that exist within the material, and that changes the effective speed one can move through the area.


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## Harmon (Oct 27, 2005)

*After a bit of research I am still confused*

I find it pretty strange, a guy with my level of education, with my level of reading comprehension, asking questions about the Speed of Light (SoL).


http://archive.ncsa.uiuc.edu/Cyberia/NumRel/EinsteinTest.html

“The first prediction put to test was the apparent bending of light as it passes near a massive body. This effect was conclusively observed during the solar eclipse of 1919, when the Sun was silhouetted against the Hyades star cluster, for which the positions were well known. 

“Sir Arthur Eddington stationed himself on an island off the western coast of Africa and sent another group of British scientists to Brazil. Their measurements of several of the stars in the cluster showed that the light from these stars was indeed bent as it grazed the Sun, by the exact amount of Einstein's predictions. Einstein became a celebrity overnight when the results were announced. 

“The apparent displacement of light results from the warping of space in the vicinity of the massive object through which light travels. The light never changes course, but merely follows the curvature of space. Astronomers now refer to this displacement o f light as gravitational lensing. 

“But the Sun's gravity is relatively weak compared with what's out there in the depths of space. In the dramatic example of gravitational lensing below, the light from a quasar (a young, distant galaxy that emits prodigious amounts of radio energy) 8 billi on light years away is bent round by the gravity of a closer galaxy that's "only" 400 million light years distant from Earth.”


Here is another site- this one has a drawing about light being bent by gravity, am I miss understanding what you guys are saying about light and its bend-ability or am I miss understanding these sites?

http://www.uh.edu/engines/epi1765.htm


On the following site it kinda hits on something I was thinking about the exacts of the SoL.  Heavier and lighter gravity areas in the universe would alter the speed of light.  Am I reading this right?

http://www.grantchronicles.com/astro09.htm

“In the next parallel universe our laws of gravity and light would not work due faster velocities for the gravity and light particles. So a missing piece of the puzzle is that the concentration of Dark Matter not only determines the speed of light in our part of the universe, but the behavior of returning gravity particles.”


I really appreciate you guys taking the time to check out this thread.  Personally I don’t think I understand this subject very well, but the logic of what I think and what I feel about it just seem so… well logical.

If I have not said it in a while- thank you for your time on this, its really killer that you would try to help me out.  Thanks.


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## the Jester (Oct 27, 2005)

Harmon said:
			
		

> I find it pretty strange, a guy with my level of education, with my level of reading comprehension, asking questions about the Speed of Light (SoL).




It's actually much more complex- and interesting- than you would think. 

Re: light bending via gravity vs. traveling in a straight line- the light hasn't actually bent, the space it is traveling through has.  The light continues in a straight line, but space deforms around it, so the straight line leads to a bent-looking course.


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## Slife (Oct 28, 2005)

So, if light can't escape from a black hole, and nothing can go faster than light, how would gravitons escape?


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## Harmon (Oct 28, 2005)

Slife said:
			
		

> So, if light can't escape from a black hole, and nothing can go faster than light, how would gravitons escape?




One of the things that started me thinking about "faster then light" was that very question.  Has to do with what is attractable and what is not- I will however step to the side and ask that someone more savy then I, speak up on the subject.


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## mojo1701 (Oct 28, 2005)

Slife said:
			
		

> So, if light can't escape from a black hole, and nothing can go faster than light, how would gravitons escape?




They don't escape, they are emitted.


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## Umbran (Oct 28, 2005)

Harmon said:
			
		

> I find it pretty strange, a guy with my level of education, with my level of reading comprehension, asking questions about the Speed of Light (SoL).




What's strange?  Curiosity comes at all levels of education.



> On the following site it kinda hits on something I was thinking about the exacts of the SoL.  Heavier and lighter gravity areas in the universe would alter the speed of light.  Am I reading this right?
> 
> http://www.grantchronicles.com/astro09.htm




Whether you're reading it right or not is irrelevant.  That website needs to be taken 'round back of the barn and shot, to put it out of its misery.  That content is so poorly organized and presented that it is difficult to tell if they're technically correct.  




> I really appreciate you guys taking the time to check out this thread.  Personally I don’t think I understand this subject very well, but the logic of what I think and what I feel about it just seem so… well logical.




Don't sweat it.  Very few people actually understand the subject very well.  

The problem is that what most folks refer to as logic doesn't necessarily apply.  Everyday logic is based upon assumptions found in the everyday world, but out everyday world is only a small subset of the Universe.  What really applies here is mathematical logic, that starts with only a few, well-stated assumptions.


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## fuindordm (Oct 28, 2005)

Harmon said:
			
		

> “Sir Arthur Eddington stationed himself on an island off the western coast of Africa and sent another group of British scientists to Brazil. Their measurements of several of the stars in the cluster showed that the light from these stars was indeed bent as it grazed the Sun, by the exact amount of Einstein's predictions. Einstein became a celebrity overnight when the results were announced.




Note that the bending of light by gravity is also predicted by Newtonian gravitational theory.  Einstein's prediction was that the degree of deflection would be exactly double the about predicted by Newton, and he was right!  This was a pretty difficult measurement to make at the time, but a lot of his contemporaries thought his theory was unnecessarily complicated.



> “The apparent displacement of light results from the warping of space in the vicinity of the massive object through which light travels. The light never changes course, but merely follows the curvature of space. Astronomers now refer to this displacement o f light as gravitational lensing.




A phenomenon used extensively today to verify the existence of dark matter and map its distribution in the universe. Here's a dramatic example, where the images of very distant galaxies are stretched out into long arcs by the gravity of the galaxy cluster in the foreground.

http://antwrp.gsfc.nasa.gov/apod/ap011007.html



> On the following site it kinda hits on something I was thinking about the exacts of the SoL.  Heavier and lighter gravity areas in the universe would alter the speed of light.  Am I reading this right?
> 
> http://www.grantchronicles.com/astro09.htm




This site is written by a crank.  Their writing style is very distinctive.  Lots of people are interested in the answers to the grand questions, and borrow the trappings of physics and mathematics without bothering to learn enough of the subject to be able to analyze their own ideas with any rigor.  Instead, they assert the truth of their statements as self-evident or mystical revelation, and throw in a bit of math to make it seem as if they have done the work.

When I started my first postdoc, I received an unsolicited paper from a crank who had a new theory on the origins of the universe, something about a fundamental wave spawning other waves of ever-increasing complexity.  None of it was comprehensible, but the paper attracted my attention because of the last line of his abstract:

"...and in addition, we rationalize pi."

Intrigued, I turned to the appendix.  He started off by taking pi=22/7, wrote out the decimal version to something like twenty places, then did some arithmetic to prove that the result was a rational number.

It was good for a laugh, at least. 

Ben


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## fuindordm (Oct 28, 2005)

Slife said:
			
		

> So, if light can't escape from a black hole, and nothing can go faster than light, how would gravitons escape?




That's a very good question, and you're right that it poses a problem for particle theories of gravity.  

IIRC, the string theories that deal with this generally have 11 spatial dimensions, and gravitons are special because they can propagate along all of these--the other particles are confined to the three spatial dimensions of our usual experience.  This has the advantage of explaining to some degree why gravity is so weak relative to the other fundamental forces--the gravitons are spread thinner than the other particles.

So that way you can end up with black holes that trap all particles except gravitons.

But I really know very little about string theory, and what I know is gleaned from popular science books like _The Elegant Universe_ and others.  I could well be wrong.

Ben


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## Umbran (Oct 28, 2005)

fuindordm said:
			
		

> So that way you can end up with black holes that trap all particles except gravitons.




That doesn't work - if the gravitons travel in those other dimensions, gravity does.  And that's what'd keeping things from escaping.  There is no "back door" out of a black hole into our normal space.

There's a simpler answer - the statement "nothing can escape a black hole" is true, but insufficiently detailed to give us what we need.  In full, the statement is "nothing can escape _from within the event horizon_ of a black hole.

In effect, all you need is that the gravitons be emitted from the outside of the surface, rather than from the interior, and all is well.


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## Cowpanzamie (Oct 28, 2005)

....


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## Cowpanzamie (Oct 28, 2005)

You're all nuts.  In the movie _The Black Hole_ the spaceship travels RIGHT THROUGH a black hole! How come none of you have mentioned that, if you're all so smart?


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## Chainsaw Mage (Oct 28, 2005)

Cowpanzamie said:
			
		

> You're all nuts.  In the movie _The Black Hole_ the spaceship travels RIGHT THROUGH a black hole! How come none of you have mentioned that, if you're all so smart?




Don't forget Star Trek.  The Enterprise can travel at *several times* the speed of light.  Utter nonsense to say that nothing can go faster than the speed of light.  IF any of you doubt it, I'll be glad to lend you my old Wrath of Khan tape.


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## Vindicator (Oct 28, 2005)

mojo1701 said:
			
		

> They don't escape, they are emitted.




Nope.  They don't escape, nor are they emitted.  They depart.


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## mojo1701 (Oct 28, 2005)

Chainsaw Mage said:
			
		

> Don't forget Star Trek.  The Enterprise can travel at *several times* the speed of light.  Utter nonsense to say that nothing can go faster than the speed of light.  IF any of you doubt it, I'll be glad to lend you my old Wrath of Khan tape.




Pfft. In a different medium.


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## Raven Crowking (Oct 28, 2005)

Umbran said:
			
		

> That doesn't work - if the gravitons travel in those other dimensions, gravity does.  And that's what'd keeping things from escaping.  There is no "back door" out of a black hole into our normal space.
> 
> There's a simpler answer - the statement "nothing can escape a black hole" is true, but insufficiently detailed to give us what we need.  In full, the statement is "nothing can escape _from within the event horizon_ of a black hole.
> 
> In effect, all you need is that the gravitons be emitted from the outside of the surface, rather than from the interior, and all is well.





Of course, that still isn't sufficient, because a rotating black hole has two "event horizons" -- one of which can be escaped, and one of which cannot.


RC


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## tomBitonti (Oct 29, 2005)

*Photons inside of a black hole ...*

>> As far as the force that bends light being able to slow it down, that's gravity. It might
>> seem obvious to some that this is what happens with a black hole, right? Can't escape
>>  'cause it's slowed down too much, right? No again. Gravity accelerates things, but
>> acceleration isn't always a change in over all speed; change in direction is acceleration,
>> also. 

*** A photon trapped in a black hole is now orbitting the singularity ... at the
*** speed of guess what.

My understanding is that there are no stable orbits below the point at which the
escape velocity is 1/2 c.  I'd have to go back and read up more to explain why.

Note that anything that passes the event horizon has no hope but to meet the
central singularity.  The usual light cone is bent so much that all available paths
point inward.

A photon in a vacuum always travels at c, even inside of a black hole.  Note that,
aside from tidal effects, one can't tell that they have passed the event horizon.
Some equations go haywire at the event horizon, but that's because of the choice
of coordinate systems, not because of real (local) physical effects at the horizon.
The only place that is mathematically intractable is the central singularity.  My
impression is that folks generally can't describe the physics of the central
singularity.


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## Umbran (Oct 30, 2005)

tomBitonti said:
			
		

> A photon in a vacuum always travels at c, even inside of a black hole.  Note that, aside from tidal effects, one can't tell that they have passed the event horizon.




Similarly, note that the tidal effects at the event horizon may not be large.  

Effectively - black holes are what they are because they are a deep gravity well.  They only cause notable tidal effects if the sides of the well are steep at the horizon - but the more massive the hole, the less steep the sides.  So, while a stellar-mass black hole may rip you to shreds before you reach the horizon, you may be able to waltz across the horizon of a supermassive hole without noticing.

Of course, everyone who enters reaches the singularity eventually, and get turned into spaghetti before they reach it...


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## Sebastian Francis (Oct 30, 2005)

Raven Crowking said:
			
		

> Of course, that still isn't sufficient, because a rotating black hole has two "event horizons" -- one of which can be escaped, and one of which cannot.
> 
> 
> RC




Actually, that isn't accurate.  I saw the movie EVENT HORIZON, and the spaceship is clearly sucked into the dimensional vortex at the film's end.  No escape.


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## Harmon (Oct 30, 2005)

Okay, so all the matter that enters a black hole incleases the size of the BH(?), which increases its strength(?)- now, my new question is- what happens to that energy/the stuff that fell into the black hole, what happens to it?

Do all black holes eventually fall together (billions, perhaps trillions of years from now), making one supper massive black hole that has nothing left to "eat".  What happens now?  (Not that anyone would care, that the black hole is alone in the multiverse.)  Does the BH explode throwing all its energy out into a new universe?  Does it just sit there sucking up the nothingness of an empty void?


A follow up question-
What about the idea that eventually a black hole will have so much crap flowing around it, that nothing or very little can get into the BH because everything is "plugging up the drain"; nothing can get closer because everthing is bumping into each other.  

Sorry, guys when I can't sleep my mind wonders.


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## Umbran (Oct 31, 2005)

Harmon said:
			
		

> now, my new question is- what happens to that energy/the stuff that fell into the black hole, what happens to it?




It falls in towards the singularity at the center.  There are no stable orbits inside the event horizon.  It may take a while, but eventually, everything reaches the singularity.

Whatever may have happened at the event horizon, the tides near the singularity are enough to shred anything.  And physical object gets shreded down to it's consituant particles before it actually reaches the singularity.

What happens at the singularity?  Nobody knows.  This is the place where everything we know breaks down.  We have no reliable prediction.

There are some edge cases - I believe it's for spinning black holes - there are paths within the hole that don't lead to the singularity, but instead lead into other "spaces" - this is where the old story about going into a black hole and coming out in another universe comes from.  I'd have to look at my notes, but I don't recall these spaces can be reached by anything in "free fall".  



> Do all black holes eventually fall together (billions, perhaps trillions of years from now), making one supper massive black hole that has nothing left to "eat".  What happens now? [/qutoe]
> 
> Well, remember that even if every scrap of matter in the Universe has been eaten, the space itself isn't sitting aroudn doing nothing.  Right now, our Universe is still expanding.  If it continues, the holes may not ever coalese into one megahole, because they'll be too far apart.  On the other end of the scale, if the Universe comes to a "Big Crunch", what happens to the black holes becomes moot, because the universe squeezes down to a singularity itself.
> 
> ...


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## Harmon (Nov 1, 2005)

Mr Hawking said that BHs can "evaporate" - whow!  Hold up, if those particals kinda drift back out of the BH, bleeding off wouldn't the BH suck them back in?  

Is it my understanding that black holes don't rotate?  Did I understand that right?  It was my understanding that most things rotate in an unstable orbit or when something effects them in a not so predictable way (say our moon, which doesn't rotate on its axis but does wround the Earth, and Pluto and its moon rotate around a central point- they always face each other).  Shouldn't a BH rotate because of the unpredictable matter that is coming at it?

Something else- the area between the singularity and where we can still see light, that area is called the Event Horizon (right?), so what happens to the light that enters that area.  If what I think is true its where the light speeds to past a speed that we can perceive, past the speed of light (which according to what I understand (which admittedly isn't much)) is not possible by all that I have read here.

Sorry, I feel like I am asking questions you have already gone over and I am just not getting through my thick skull.  Again, another thank you for your time and patience.


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## Umbran (Nov 1, 2005)

Harmon said:
			
		

> Mr Hawking said that BHs can "evaporate" - whow!  Hold up, if those particals kinda drift back out of the BH, bleeding off wouldn't the BH suck them back in?




It isn't "drifiting out" of the hole.

In QM, a particle-antiparticle pair can be created out of nothingness for a short time.  Generally, they fly apart and come back together in the smallest fraction of a second, annihilate each other, and are gone, and the Universe is none the wiser.

When this happens just outside the black hole, you can have the problem that occasionally, they'll fly apart - and one of them will go across the Event Horizon, and the other won't.  It doesn't fall into the hole, and it doesn't annihilate with it's partner.  It just flies off into space, as if teh hole had emitted it.  



> Is it my understanding that black holes don't rotate?  Did I understand that right? It was my understanding that most things rotate in an unstable orbit or when something effects them in a not so predictable way (say our moon, which doesn't rotate on its axis but does wround the Earth, and Pluto and its moon rotate around a central point- they always face each other).  Shouldn't a BH rotate because of the unpredictable matter that is coming at it?




Do not confuse "spin" with "orbit".  A top spins aroudn it's axis.  The Earth spins on its axis to produce day and night.  The Earth also orbits around the Sun.  Rotation and orbiting are two separate things.

Our moon does spin.  The moon is what we call "tidally locked" - it has a period of spin exactly one monthlong, so that it always has one side facing us.

A black hole may rotate because the star from which it formed rotated.  Our Universe likes to conserve angular momentum, so spinning things tend to keep spinning unless somethign slows them down.  There's nothing in the process of becoming a black hole that would eliminate spin - so, if the star was spinning before it went supernova, the resulting black hole will also spin.  Spin has little to do with "unpredictability".  



> Something else- the area between the singularity and where we can still see light, that area is called the Event Horizon (right?)




You can imagine a black hole as a balloon, with a dot down in it's center.  The "singularity" is a mathematical point at the very center of the black hole.  The "event horizon" is the surface of a sphere around the singularity.  From within the sphere, no light can ever hope to escape to the rest of the Universe - so that sphere will appear black.  We can see nothing within the hole at all.



> so what happens to the light that enters that area.  If what I think is true its where the light speeds to past a speed that we can perceive, past the speed of light (which according to what I understand (which admittedly isn't much)) is not possible by all that I have read here.




Like everything else, light that finds itself within the event horizon can never escape again.  It has only one way to go - down to the singularity.  It can take its time getting there, but eventually, it must go to the center.

Now, to be honest, nobody can truly say what goes on inside the event horizon.  Because no signal can ever get out of the hole, we can't get a picture of what happens inside.  But there's no reason to think that the light speeds up as if falls inwards.  The basic laws of physics still hold for most of the way down to the signularity.  Only very, very close to the mathematical point do we lose grasp of what goes on.

About light - one can consider it this way:
If a thing has mass, it can have energy associated with it's motion, called kinetic energy.  The kineic energy of an object is 1/2 * M * V^2 (one half times it's mass times the square of it's velocity).  An object with mass can gain or lose energy by speeding up or slowing down.

Light has no mass.  I has a kinetic energy of zero, always.  It cannot gain energy by speeding up, nor lose energy by slowing down.  If it has to change energy, it has to do it some other way (usually by changing frequency).

Now, here's the rub - if light could speed up or slow down, you wouldn't have black holes.  The base assumptions that lead to the existance of black holes include the speed of light being a constant.


----------



## ssampier (Nov 1, 2005)

*blinks*

...

*scratches head*

....

Anyone else lost as I am?

Any primers for us non-scientist/physics majors? Keep in mind my grasp of physics can be summed up in Newtonian physics. Anything more and my head explodes.




			
				Sebastian Francis said:
			
		

> Actually, that isn't accurate.  I saw the movie EVENT HORIZON, and the spaceship is clearly sucked into the dimensional vortex at the film's end.  No escape.




Of course, they ended up in a different dimension (HELL, if you're wondering). I think the movie was neat until that was revealed.


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## Umbran (Nov 1, 2005)

ssampier said:
			
		

> Any primers for us non-scientist/physics majors? Keep in mind my grasp of physics can be summed up in Newtonian physics. Anything more and my head explodes.




And these posts certainly wouldn't be my choice for trying to teach this stuff, either.  Remember that entire college courses and books can be done on this topic.  Addressing questions in a haphazard way, without any visual aids, is certianly going to be disjointed.

You want a primer that's good for folks without the background?  Stephen Hawking's _A Brief History of Time_ covers the basics.  I think it has all of one equation in it.  Theory developed after that book was written is coverred in _The Universe in a Nutshell_.


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## physicscarp (Nov 1, 2005)

In addition to _A Brief History of Time_, which is the book that drove me to be a physicist, I'd recommend _The Elegant Universe_ by Brian Greene.  The first five chapters of the book provide, IMO, the best layperson description of special/general relativity and quantum mechanics.  His prose is easy to read, and he makes sure to describe both the common misconceptions that might be held by the reader, as well as the actual nature of the theories.  

Moving beyond those first chapters takes you into new territory (string theory), but it still remains interesting.


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## Harmon (Nov 1, 2005)

Umbran said:
			
		

> Stephen Hawking's _A Brief History of Time_ ..... _The Universe in a Nutshell_.




Read the first, have to check out the second.  Its pretty heady stuff (probably why I can't get it through my head), but its always been something that has interested me.

Umbran is doing a fair job in a really bad medium.  Its hard to explain this stuff and for your questions to be understood.


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## fuindordm (Nov 1, 2005)

Harmon said:
			
		

> Do all black holes eventually fall together (billions, perhaps trillions of years from now), making one supper massive black hole that has nothing left to "eat".  What happens now?  (Not that anyone would care, that the black hole is alone in the multiverse.)  Does the BH explode throwing all its energy out into a new universe?  Does it just sit there sucking up the nothingness of an empty void?




That depends on what the universe is doing.  The biggest black holes are in the center of galaxies, and its conceivable that over trillions of years such a hole could swallow the whole galaxy. (Probably longer; more likely it would swallow the core, then every billion years or so interactions between the stars in the outer regions would send a star drifting close enough to the center to get eaten as well.)

Meanwhile, however, the universe is expanding and the galaxies are getting farther apart.  According to the current state of affairs, the expansion of the universe is accelerating, and rather than one big black hole we'll eventually end up with a universe consisting of nothing but black holes, but these getting farther and farther apart forever.

Some older models of the universe predicted a collapse, in which case all the black holes would get closer and closer and absorb each other.  These models weren't supported by observations, though.

Ben


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## glass (Nov 2, 2005)

Harmon said:
			
		

> Umbran is doing a fair job in a really bad medium.



Seconded.


glass.


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## Kemrain (Nov 2, 2005)

fuindordm said:
			
		

> Some older models of the universe predicted a collapse, in which case all the black holes would get closer and closer and absorb each other.  These models weren't supported by observations, though.



The 'Big Crunch' theory. I remember when that was popular. Those were the days.

Nope, physicists are predicting now, much like fuindordm says, that the universe will eventually be nothing but black holes moving away from eachother forever. The universe will 'end' with a quiet, frigid shudder, not a cataclysmic bang. For some reason, this seems more depressing to people than the Big Crunch theory. Strange.

I'm wondering why people aren't taking this to it's next extreme, though. Wouldn't those black holes (I wish we'd stop calling them holes, it would make things so much less confusing to call them Darkstars) emit most of their mass as Hawking radiation eventually? That would leave them as masses too small to be black holes, and fill the universe with hawking radiation, wouldn't it?

Umbran and fuindordm, I think you're doing a fine job explaining these things. I understand them MUCH better now than I had before, and your consice explanations are the culprit. Don't think you're doing a bad job, because this highschool grad with no formal physics training at all has gained a much deeper understanding of the nature of gravity and black holes because of you. Thanks a lot, this stuff is wicked interesting.

What I'm interested in at the moment is this crazy String Theory stuff. Care to give that one a deeper try?

- Kemrain the Psysicophyle.


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## fuindordm (Nov 2, 2005)

Kemrain said:
			
		

> I'm wondering why people aren't taking this to it's next extreme, though. Wouldn't those black holes (I wish we'd stop calling them holes, it would make things so much less confusing to call them Darkstars) emit most of their mass as Hawking radiation eventually? That would leave them as masses too small to be black holes, and fill the universe with hawking radiation, wouldn't it?
> 
> What I'm interested in at the moment is this crazy String Theory stuff. Care to give that one a deeper try?
> 
> - Kemrain the Psysicophyle.




They have: check out this book by a professor from my alma mater:

The Five Ages of the Universe: Inside the Physics of Eternity by Fred Adams

As for string theory, I'm already out of my depth.  The Green book is a great one.

And thanks for the kind words.

Ben


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## Umbran (Nov 3, 2005)

Kemrain said:
			
		

> The 'Big Crunch' theory. I remember when that was popular. Those were the days.




Well, the "Big Crunch" and the "Heat Death" universes are not really separate theories.  They are merely different possibilities from the same model.  If there's enough mass in the Universe, it'll crunch.  If there isn't enough mass, it never stops expanding, and eventually every bit of potential energy in the Universe gets ground down into waste heat, and nothing can ever happen again..  The Big Crunch is out of favor because we dcan't find evidence of enough mass for it.



> For some reason, this seems more depressing to people than the Big Crunch theory. Strange.




Not at all strange - because buried in the Big Crunch is the possibility of the Oscillating universe: another Bang that follows the Crunch.  That gives at least the possibility that something might come after, and the possibility of something is usually less depressing than the surety of nothing.



> I'm wondering why people aren't taking this to it's next extreme, though. Wouldn't those black holes (I wish we'd stop calling them holes, it would make things so much less confusing to call them Darkstars) emit most of their mass as Hawking radiation eventually? That would leave them as masses too small to be black holes, and fill the universe with hawking radiation, wouldn't it?




"Holes" really is a better description, to my eye.  You can drop a thing into a hole, and expect to never get it back again.  It isn't just a star that is dark.

Yes, the extreme you mention is recognized - eventually, all the black holes would emit their mass as Hawking radiation.  And all the particles that could decay will eventually do so.  And you're left with a Universe of elementary particles spread infinitely thin, all at just that minute smidge above absolute zero temperature required by the uncertainty principle.



> What I'm interested in at the moment is this crazy String Theory stuff. Care to give that one a deeper try?




Well, in many ways, there isn't much to tell.

There are a number of different String Theories.  They all have in common the idea that the universe isn't made up of point particles, so much as little knots of "string".  And the different ways they are twisted up and vibrate tell you about the particles the strings are.

The there's three things about string theories:

1)They are, mathematically speaking, horrendously complicated.  In some ways they have elegant design, but actually squeezing real-world results out of them for all but the simplest things is quite a chore.  

2)None of them have made any testable predictions yet. Either the things they do predict are outside the energy range we can easily reach, or they just don't predict anything different at all.  Perhaps, with time, we will get solid tests of some of them.  Until then, though, they're more methemtical curiosities.

3)Rather quickly out of quantum mechanics we got useful things - like modern electronics, and nuclear power.  The energy ranges required to see String Theory effects are quite high, far above what's seen in the normal home.  It may be a while before we see any impact of these in our everyday lives, if ever.


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## ssampier (Nov 3, 2005)

Umbran said:
			
		

> And these posts certainly wouldn't be my choice for trying to teach this stuff, either.  Remember that entire college courses and books can be done on this topic.  Addressing questions in a haphazard way, without any visual aids, is certianly going to be disjointed.
> 
> You want a primer that's good for folks without the background?  Stephen Hawking's _A Brief History of Time_ covers the basics.  I think it has all of one equation in it.  Theory developed after that book was written is coverred in _The Universe in a Nutshell_.




Thanks. I don't feel so dumb. Hard science has never been my thing.


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## fuindordm (Nov 3, 2005)

Umbran said:
			
		

> Well, the "Big Crunch" and the "Heat Death" universes are not really separate theories.  They are merely different possibilities from the same model.  If there's enough mass in the Universe, it'll crunch.  If there isn't enough mass, it never stops expanding, and eventually every bit of potential energy in the Universe gets ground down into waste heat, and nothing can ever happen again..  The Big Crunch is out of favor because we dcan't find evidence of enough mass for it.




To elaborate a bit:

The current cosmological theory is derived from General Relativity and the assumption that on large scales the universe can be treated as a perfect fluid of some variety, or as a mixture of perfect fluids.  We're talking about scales larger than the average distance between galaxies here, so normal matter (most of which is in galaxies) is treated as a collisionless, pressureless gas (because galaxies almost never come near enough to each other to interact).  The eventual fate of the universe, as Umbran points out, depends only on what it's made out of--the theory describing its evolution is the same in any case.

At early times, most of the mass/energy density in the universe was in the form of radiation.
At later times, most of the mass/energy density was in the form of neutral matter.

Twenty years ago, the only question was how much matter there was in the universe--since not all of it is in luminous stars, it's harder to add up than the radiation.  People have been trying to measure the amount of dark matter for a long time, generally coming up with anywhere from 30% to 90% of the amount needed to stop the expansion of the universe.  No one has ever come up with enough matter to cause a Big Crunch, and only the die-hard theorists have come up with enough matter to slow the expansion to a halt.  (A universe with exactly enough matter and energy to just barely stop expanding is slightly easier to explain, as its density is a stable number--other models of the universe have an energy density that evolves with time, and a certain amount of fine-tuning is required in the initial conditions to get a universe old enough to create stars.)  So the Big Crunch was never really that viable as a theory.

Aside from matter and energy, there's a third medium that can drive the evolution of the universe.  This is 'vacuum energy', the energy density you find even in completely empty space due to the creation and annihilation of particle pairs according to the uncertainty principle.  

In the framework of General Relativity, this can be described with a single number: the cosmological constant.  In the framework of quantum field theory, it's much harder to quantify and no one yet has come up with a calculation of vacuum energy density that arrives at a reasonable number.  According to theory, the vacuum energy should either be zero or some enormously high value that would make the universe expand into nothingness in an unreasonably short time.  So for a long time the cosmological constant was assumed to be zero--why complicate the theory?  Recent observations of the cosmic microwave background, the distribution of galaxies in the universe, and distant supernova, however, have shown pretty conclusively that the cosmological constant is not zero.

Since vacuum energy density remains constant as volume increases, it exerts a different kind of pressure on space-time than radiation or matter.  The existence of vacuum energy (also called dark energy or quintessence) would eventually drive the universe into a phase of exponential expansion, so the empty universe is coming much sooner than we think.  But it's not all bad--this expansion is only of the space between gravitationally bound systems.  By the time it happens, our galaxy should be safely tucked away in the Virgo cluster of galaxies, still giving us several thousand galaxies to play in.

But again, it's really all one theory.  It's just a matter of plugging in different amounts of matter, radiation, and cosmological constant, and seeing how the mixed fluid evolves according to general relativity.

Ben


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## Umbran (Nov 3, 2005)

fuindordm said:
			
		

> Since vacuum energy density remains constant as volume increases...




Or, so we hope.  It is important to note that for most "constants", the fact that it is constant (in either space or time) is usually only a guess.  There's the occasional theory that plays with the idea that they aren't...

That may seem a bit at odds with the discussion of the speed of light being constant.  But that's one of the better known and measured constants - the consequences of having it be non-constant have been well thought through.  As I understand it, there's really no evidence for it varying in space, and only a little wiggle room for it varying in time.  

But, it is hard enough to just _measure_ the cosmological constant, so it is nigh impossible to prove that it really is constant.


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## fuindordm (Nov 4, 2005)

Umbran said:
			
		

> Or, so we hope.  It is important to note that for most "constants", the fact that it is constant (in either space or time) is usually only a guess.  There's the occasional theory that plays with the idea that they aren't...
> 
> That may seem a bit at odds with the discussion of the speed of light being constant.  But that's one of the better known and measured constants - the consequences of having it be non-constant have been well thought through.  As I understand it, there's really no evidence for it varying in space, and only a little wiggle room for it varying in time.
> 
> But, it is hard enough to just _measure_ the cosmological constant, so it is nigh impossible to prove that it really is constant.




Well, the cosmological constant, if it exists, does have to be constant in time, otherwise it wouldn't fit into the equation of general relativity correctly.  This doesn't prevent us from having a vacuum energy density that evolves with time, however.   A vacuum energy can behave like a cosmological constant, but it can also be more complicated--it goes elsewhere in the equation.

But I was really talking about an energy density that was constant in space.  If the 'dark energy' turns out to be due to the creation and annihilation of particles in vacuum, then this is a process that should be going on in the same way everywhere that there isn't matter or significant curvature of space-time.  So if you compare two boxes of vacuum, one with twice the volume of the other, the bigger box will contain twice as much vacuum energy as the smaller.

When you treat vacuum energy as a perfect fluid, this leads to a very different equation of state than what you get from matter or radiation, whose density always decreases with increasing volume.

Ben


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## Harmon (Nov 6, 2005)

Flipping channels on Wednesday or Thursday came across the middle of a show on Discovery, one of the guys they were interviewing said something like- "there isn't enough (matter/material/energy) in the universe to justify its creation."

I kinda scratched my head, and thought about black holes for a moment- "if what ever amount that isn't here has already been taken into black holes then would that mean that he's really got no clue about the amount of (matter/material/energy) in the universe?"

What about time?  I mean we are looking across a minunum of 5+ yrs to the nearest star to get information, and most of the cool universe stuff is decades, hundreds, and thousands of years older then that, so how can we make an educated understanding of the universe based on reactions and forces that are so much out of date.

Umbran & Fuindordm- thanks for picking this up.  I love to read and watch this sort of converstaion, makes me feel more smarter then me am


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## the Jester (Nov 6, 2005)

Though we can't _see_ what's in a black hole, we can measure its mass indirectly (through its effects on the stuff around it).


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## Alzrius (Nov 6, 2005)

Correct me if I'm wrong, but I thought the dark matter theory had recently been debunked?


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## Umbran (Nov 6, 2005)

Alzrius said:
			
		

> Correct me if I'm wrong, but I thought the dark matter theory had recently been debunked?




Well, that depends.  There isn't just one "dark matter theory".

In the most basic sense "dark matter" is merely that matter which is not glowing brightly enough for us to see it with our devices at interstellar distances.  By this definition, the Earth itself is "dark matter".  The question is simply if there's enough of it or not.

Some attempts have been made to posit some exotic materials among the mundane dark matter, to effectively up the overall mass out there.  None of those have stuck particularly well.



			
				Harmon said:
			
		

> I kinda scratched my head, and thought about black holes for a moment- "if what ever amount that isn't here has already been taken into black holes then would that mean that he's really got no clue about the amount of (matter/material/energy) in the universe?"




It isn't really "no clue".  You can get estimates on masses based upon how objects move. While dark, the holes exert gravitational effects on bodies that aren't dark.  Which isnt' to say that we're 100% certain, but extremes can be eliminated.



> What about time? I mean we are looking across a minunum of 5+ yrs to the nearest star to get information, and most of the cool universe stuff is decades, hundreds, and thousands of years older then that, so how can we make an educated understanding of the universe based on reactions and forces that are so much out of date.




Well, what does it matter that the data is old?  

If I assume the past works as the present, and I look at that old data, and that data  is consident with my current rules, then I can take that as evidence that my current model also held at previous times.

If I assume the past works like the present, and I look at the old data, and that data is not consistent with my current rules, I can take that as evidence that the rules have changed, and I can start looking at how the rules have changed.  

Either way, science wins.  

Plus, understanding is not only about the here and now:

Consider, for a moment, that you knew nothing at all about cats.  Never seen one, never heard of one.  And I put you in a room with a 5 year old cat.  You could watch it's behavior and learn a few things about it, but some things would remain a mystery.  The whole thing about jumpiong in your lap, purring, kneading, and wanting to be stroked would be a bit baffling.

If, however, you look back in that cat's life, back to when it was a kitten, and at how it interacted with it's mother, you'd see where the purring and kneading and stroking come from, and in that context they make much more sense.

Which just goes to show that you need to know about the past in order to fully grok the present.  So not only is the age of that data not a barrier to better understanding.  It's rather a requirement.  If all we saw was how the universe was today, at this instant, we would be able to say much less about how it functions.


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## fuindordm (Nov 7, 2005)

Alzrius said:
			
		

> Correct me if I'm wrong, but I thought the dark matter theory had recently been debunked?




Well, I work in the field and I haven't heard about that.

It's true that we don't know what it is, although we've been chipping away at the problem: it's reasonably certain now that neutrinos have a little mass, which will contribute a few percent of the missing mass.  Recent telescope surveys of our galaxy have turned up a larger population of brown dwarf stars (things more massive than Jupiter, but less massive than the sun, which burn very dimly and for a long long time) than was expected, and it's likely that such objects also contribute a few percent.  That still leaves a big chunk of 'cold dark matter' unaccounted for, which is likely to be an exotic stable particle that we haven't discovered yet because it can only be created at energies higher than what we can produce in the lab.  There is no shortage of candidates for such a particle, believe me!

There are also lots of people out there that believe no such particle exists, and that a modification of our theory of gravity is needed to explain the motion of galaxies.  But no one is saying that galaxies are moving as they should given our current understanding of gravity and the tally of visible matter.

In galaxy clusters, for example, the galaxies are zipping about at hundreds of kilometers a second.  When we add up all the mass in galaxies and hot gas (visible in the X-ray band), we find that these speeds are well above the escape velocity of the clusters.  Conclusion: if there were no dark matter, the cluster would fly apart and disperse in short order.  Since we see galaxy clusters everywhere and over a wide range of distances, this doesn't make sense.  What does make sense is to conclude that there is about three times as more mass in the cluster than what we can see.  If we do this then the galaxies and hot gas obey perfectly the laws of gravity as we understand them.  The hot gas in particular is very useful, because it fills the entire volume of the cluster and its temperature and luminosity decrease as you get farther from the center.  One can actually map out the gravity well as a function of radius this way.

What has been recently debunked is the idea that the total amount of matter in the universe (dark or otherwise) is just enough to stop the expansion of the universe, which was the prevailing theory (for reasons of elegance) for a long time.  But this just means that we need less dark matter than was previously thought--the current observations have cut the amount of matter needed to 30% of this magic number.

Cheers,
Ben


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## fuindordm (Nov 14, 2005)

*Black Holes*

Here's a figure I did a couple of years ago for my research.  It shows photon paths emanating from a point not far from the center of a rotating black hole.  The outer circle is the ergosphere of the black hole; within this radius everything is constrained to revolve in the same sense as the rotation, but may still escape.  The inner circle is the event horizon, and the dotted circle is the inner radius of an accretion disk (a mass of hot gas swirling down into the black hole).  The colors don't mean anything, they just help you tell one photon from another.

Ben


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## mps42 (Nov 14, 2005)

*Speed of light (again)*

I just thought I'd mention that I talked with a very intelligent person not too long ago who is working on the Speed of Light question. In his circles, at least, it is not generally assumed that the speed of light varies baed on the color of light in question.
 The example he gave me (because of my limited science understanding) is that a blue ray of light will travel a given distance .00025% faster than a ray of white light.
 So, the speed of light may NOT be constant, which would mean we would have to re-think both General and Special Relativity...


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## fuindordm (Nov 14, 2005)

mps42 said:
			
		

> I just thought I'd mention that I talked with a very intelligent person not too long ago who is working on the Speed of Light question. In his circles, at least, it is not generally assumed that the speed of light varies baed on the color of light in question.
> The example he gave me (because of my limited science understanding) is that a blue ray of light will travel a given distance .00025% faster than a ray of white light.
> So, the speed of light may NOT be constant, which would mean we would have to re-think both General and Special Relativity...




I don't quite understand this example, but...

The speed of photons in vacuum is constant.  However, a ray or pulse of white light consists of a spectrum of photons, and if it travels through a medium such as air or water then photons of different colors may travel at slightly different speeds (always less than c), and the peak of the pulse might therefore travel at a speed slightly different than c.

I'm not on the cutting edge of photonics.  I am aware that some research groups have claimed to transmit pulses of light at speeds faster than c under highly specialized conditions, but it's not clear whether these speeds are real (in the sense of energy, momentum, or information are being transmitted faster than c) or illusory (in the sense that the separation between two points, such as two wave peaks, can grow faster than light even though no individual particle or photon is traveling faster).  As far as I am aware, research in this field is still ongoing.

Ben


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## Harmon (Nov 15, 2005)

One thing I have noticed about posting here- when you write something that you think is a clear question the reply does not always fit the question, or as one of my old friends use to say- perception seems to be everything.


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