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Author Topic: What is the coldest temperature in the Universe?  (Read 16842 times)

Offline LeeE

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What is the coldest temperature in the Universe?
« Reply #25 on: 11/05/2008 12:22:03 »
No, I hadn't - thanks for that:)

 

Offline DoctorBeaver

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What is the coldest temperature in the Universe?
« Reply #26 on: 12/05/2008 08:35:32 »
Having reconsidered, I'm not sure localisation can provide a mechanism for it. But I think SR1 & SR2 can.

Thinking about it, wouldn't a theory involving Planck scale (or even larger) rolled-up dimensions provide a means for gravity to escape?

Also, something is tickling the back of my little beaver brain concerning quantum entanglement of gravitons. If 1 of an entangled pair of gravitons were inside the BH and the other outside...  ???
 

Offline LeeE

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What is the coldest temperature in the Universe?
« Reply #27 on: 12/05/2008 22:09:48 »
The idea of 'rolled up' dimensions seems odd to me - I can't see how the 'shape' of a dimension is going to be directly perceptable from within it.  That doesn't mean that the contents of a dimension need to be infinite though, and because it'll only be the presence of something within the dimension that has any effect, the dimension could be considered to be finite.  Hmm...  you'd also need another dimension to do the rolling in.

Quantum Entanglement of gravitons is an interesting idea.  Unlike everything else we're aware of (hmm... just remembered that BH's can have a charge - anyone know if a BH's charge acts as though it's from a single particle?), it would seem that gravitons must be preserved in a BH, otherwise there would be no gravity, so entanglement across the event horizon seems possible.  But I think the question, instead of being 'How does a graviton escape from a BH? then becomes 'How does a graviton survive in a BH?  I think that essentially, they are the same question - if something can maintain it's integrity within a BH, then escaping from probably isn't impossible either.
 

Offline DoctorBeaver

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What is the coldest temperature in the Universe?
« Reply #28 on: 12/05/2008 23:32:42 »
An extra dimension can be curled up from every point in a 3D universe. It would exist outside of those dimensions, not within them.

The shape may or may not be perceptible from within. The way gravity spreads out could give a clue. It would spread differently in a simple, curled up dimension than in a Calabi-Yau manifold.

Gravitons surviving within a BH has implications for the entropy of the BH. Maybe they do have hair after all!  :D
 

Offline LeeE

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What is the coldest temperature in the Universe?
« Reply #29 on: 13/05/2008 14:38:36 »
There could be inumerable additional dimensions but I think that the important factor is whether they interact with our ones or not, and to have an effect upon our dimensions they must have some degree of existance within them.

Yes, I think that the way that gravity spreads does give a clue - it has a very simple shape and I think that is suggestive, but the trouble is that I think it's more suggestive of less complex, rather than more complex dimensions, or alternatively, an exchange of existing dimensions.  For example, in relativity we see that time trades off against speed so that the sum of the squares remains a constant, and when one factor is 1, the other is 0, so it's conceivable that when the value of a dimension drops to zero it could be indistinguishable from any other dimension.  Or it could be said to have ceased to have any existance altogether:)  But from our point of view, that dimension must still be accounted for.

I think the issue of BH entropy is still very much up in the (h)air;)
 

Offline syhprum

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« Reply #30 on: 13/05/2008 16:12:49 »
I entered Randall-Sundrum into Google and although I have a vague idea what most physics are about it might as well have been written in Chinese for all I could understand.
 

Offline DoctorBeaver

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« Reply #31 on: 13/05/2008 22:40:19 »
There could be inumerable additional dimensions but I think that the important factor is whether they interact with our ones or not, and to have an effect upon our dimensions they must have some degree of existance within them.


The dimensions themselves would not have to interact directly with our known dimensions to have an effect on them. According to RS, gravity could be diluted by extra dimensions, which would explain its weakness relative to the other forces.

Localisation, also, would have that effect. If our known dimensions are confined to a 3D brane and gravitons originate on another brane (the G-brane) and are free to move in the bulk, but their probability wave peaks close to the G-brane and falls away sharply, the further from the G-brane you get, the less likely you would be to find one. So by the time you get to our brane their probability wave, and hence their effect, is much less than in the localilty of the G-brane.

In either case, although the extra dimensions are not interacting directly with our 3 perceivable dimensions, their existence, and their size & shape, can possibly be inferred indirectly.

It is possible that the new LHC at CERN will detect the K-K partners of gravitons, and that would indicate that extra dimensions exist. The race would then be on to interpret the data and expand on current theories of extra dimensions.
 

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« Reply #32 on: 13/05/2008 22:54:48 »
I entered Randall-Sundrum into Google and although I have a vague idea what most physics are about it might as well have been written in Chinese for all I could understand.

I know what you mean. It took me quite a while, a lot of mental contortions, a fair bit of brain-ache and 2 re-reads of Lisa Randall's book to get some kind of comprehension of what it's all about.
 

Offline DoctorBeaver

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« Reply #33 on: 13/05/2008 23:03:53 »
syhprum - have you read this forum conversation? http://www.physicsforums.com/showthread.php?t=119294
 

Offline LeeE

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« Reply #34 on: 14/05/2008 18:23:40 »
I can't get around the idea that for one thing to affect another, there must be something in common between the two.  No matter how close something may come, unless something is shared or transferred, it's still a 'miss'.

There's no way I'd claim to clearly understand all the RS stuff, but as I interpret it, you could imagine it by dropping a few dimensions and thinking of a cube where the volume of the cube is the 5D bulk and two opposing faces are the 4-branes - hmm... perhaps a better representation might be an open convex lens shape - yeah - that's better - just two faces to deal with :)

Anyway, in this representation, we'd think of the two faces and the volume as separate entities, which strictly speaking, they are.  However, as they're also parts of a single entity, it's easy to imagine how changes in one part may affect the other parts, especially if the changes occur in the volume.  For example, making a scratch on one surface wouldn't change the other surface but if a crack was made through the volume it would affect both surfaces.  In this respect, the surfaces respond to the volume, so the 4-branes responding to events in the bulk makes sense if they're all parts of a single super-space.

Also, I can't help but think of the way that the probability decreases with distance from the strong brane as a gradient, and then we're back to what causes the gradient, swapping gravity for the probability curve:)  Heh:) - perhaps there's a gradient because the the two 4-branes are different sizes, hmm... I wonder if the strong brane could be reduced to a point... <starts thinking about cone shaped universes/>

Like I said though, with only a partial understanding of RS space I could quite easily be missing something important.

It'll be _very_ interesting to see what comes out of the LHC.  Whatever the results, it should clear some things up, one way of another.
 

Offline DoctorBeaver

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« Reply #35 on: 14/05/2008 22:00:49 »
Quote
I can't help but think of the way that the probability decreases with distance from the strong brane as a gradient, and then we're back to what causes the gradient,

The whole basis of RS is that the extra dimension is warped.

Our brane (RS refer to this as the Weakbrane as it works at the weak scale energy) and the Gravitybrane (at the Planck scale energy) warp the bulk dimension (5th dimension) separating them. This warping causes the strength of gravity to vary at different points within that dimension.

Quote
I can't get around the idea that for one thing to affect another, there must be something in common between the two.

The "something in common" is the graviton.

Your analogy of the lens is fine up to a point. As I mentioned, the 5th dimension is warped, and that cannot be represented by the lens.
 

Offline LeeE

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« Reply #36 on: 15/05/2008 17:24:12 »
I guess that, in a way, we only get the reciprocal effect of the graviton, so although it can be vanishingly small it'll have non-zero presence in our brane - that seems fair enough.  I guess too, that the surfaces could dictate the volume, i.e. branes dictate the bulk, and if they're intrinsically different from each other, there'll need to be an intrinsic gradient of some sort between them, in the bulk.

Interesting:)
 

Offline DoctorBeaver

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« Reply #37 on: 16/05/2008 06:47:52 »
i.e. branes dictate the bulk, and if they're intrinsically different from each other, there'll need to be an intrinsic gradient of some sort between them, in the bulk.

Interesting:)


I'm not sure you can say that branes dictate the bulk. However, their energy dictates the warping of the bulk.

There wouldn't be a gradient in the bulk. The warping of the bulk is the gradient.
 

Offline LeeE

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« Reply #38 on: 16/05/2008 14:56:19 »
Actually, I think we're in agreement over the principles but perhaps not the choice of words to describe it:)

If the statement: 'the energy of the branes dictates warping of the bulk' is true, is it correct to say that the energy is a property of the branes, or should it be said that the branes are a property of the energy?

And perhaps I should have said gradient _of_ the bulk instead of _in_ the bulk.

Heh:)  I do think we're actually saying the same thing.

Something that occured to me last night, while I walking somewhere, is that if one of the branes was contracted to a point, it's shape would be a simple 5D cone and the change in the 4D cross-section might do nicely for the warp.  A truncated cone would work too, without the need to contract to zero and if you need a non-linear warp you could use hyperboloids.

 

Offline DoctorBeaver

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« Reply #39 on: 18/05/2008 09:58:08 »

Something that occured to me last night, while I walking somewhere, is that if one of the branes was contracted to a point, it's shape would be a simple 5D cone and the change in the 4D cross-section might do nicely for the warp.  A truncated cone would work too, without the need to contract to zero and if you need a non-linear warp you could use hyperboloids.


I believe that's what RS means. As the dimension becomes smaller, gravity becomes stronger; and the larger it gets, the weaker gravity becomes. That means that the strength of gravity in our perceivable universe is merely a function of the separation between the branes. Were that separation any different, then the force of gravity here would also be different.

It has interesting ramifications for unification of forces.
« Last Edit: 18/05/2008 10:01:42 by DoctorBeaver »
 

Offline LeeE

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What is the coldest temperature in the Universe?
« Reply #40 on: 18/05/2008 14:18:34 »
Yup, it's an interesting model.

 

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What is the coldest temperature in the Universe?
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