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### Author Topic: How is expanding spacetime described in string theory?  (Read 15790 times)

#### Physics Dilettante

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##### How is expanding spacetime described in string theory?
« on: 30/11/2008 03:35:14 »
I'm having trouble understanding what it means when the expansion of the universe is explained in terms of expanding spacetime.

One way to cure my confusion might be to approach the problem in terms of string theory which, as I understand it, supposes spacetime is quantized. In this framework, then, does the expansion of space result from:

a. The quanta are moving further apart. But, if so, what's in between them?

b. The quanta are packed together perfectly and nothing is between them -- they're getting bigger. But, if so, what's inside them, and how is it getting bigger? Seems to me this answer leads to a nested-dolls problem.

c. The quanta are packed perfectly, nothing is between them, and they aren't getting bigger. (Perhaps their size is dictated by a fundamental constant.) Instead, new quanta are "bubbling" into existence from nowhere, shouldering the existing quanta apart and thereby creating more space. This isn't forbidden because no matter or energy is being created. But, if this is happening, the reverse process should work too, so what's causing the asymmetry?

d. Something else I haven't thought of.

#### Soul Surfer

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##### How is expanding spacetime described in string theory?
« Reply #1 on: 02/12/2008 19:45:52 »
Thres quite a lot of other people would like the answer to that question but as far as I know ther is no definitive answer.  I tend to visualise it as if space consisted of a lot of little transparent blobs and every so often they get to gether and make another blob. however there is no solid evidence for this.

#### DoctorBeaver

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##### How is expanding spacetime described in string theory?
« Reply #2 on: 04/12/2008 14:15:46 »
I concur with SoulSurfer about no-one really knowing the answer. I've seen several theories, but none of them are particularly cohesive nor currently testable.

But you've got me interested so I shall trawl through various reference books/sites and see if I can come up with anything.

#### Physics Dilettante

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##### How is expanding spacetime described in string theory?
« Reply #3 on: 04/12/2008 15:10:40 »
In case there's any confusion on this point, let me be clear that I mean only to ask about the explanation(s), if any, that are being utilized in string theories, and not "what's really going on?". The uncertainty principal (assuming it's correct) dictates that we'll never be able to test hypotheses concerning anything smaller than the Planck length, so I presume string theorists refrain from addressing anything that might be smaller.

#### DoctorBeaver

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##### How is expanding spacetime described in string theory?
« Reply #4 on: 04/12/2008 15:19:23 »
Don't expect any answers from string theory. 20 years on and still nothing testable has been described. The problem seems to be that there are an almost infinite number of possibilities in string theory so most of its problems are unsolvable with our present level of knowledge.

#### DoctorBeaver

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##### How is expanding spacetime described in string theory?
« Reply #5 on: 04/12/2008 15:21:00 »
so I presume string theorists refrain from addressing anything that might be smaller [than the Planck length].

I thought the whole idea of string theory was to address sub-Planck level physics.

#### dentstudent

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##### How is expanding spacetime described in string theory?
« Reply #6 on: 04/12/2008 15:22:48 »

I thought the whole idea of string theory was to address sub-Planck level physics.

more commonly known as "under-the-floor" physics

#### DoctorBeaver

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##### How is expanding spacetime described in string theory?
« Reply #7 on: 04/12/2008 15:28:22 »

I thought the whole idea of string theory was to address sub-Planck level physics.

more commonly known as "under-the-floor" physics

Don't you ever get board with those silly puns?

#### dentstudent

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##### How is expanding spacetime described in string theory?
« Reply #8 on: 04/12/2008 15:33:19 »
Listen, when you're surrounded by German humour all day, you take it where you can...

#### DoctorBeaver

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##### How is expanding spacetime described in string theory?
« Reply #9 on: 04/12/2008 15:54:37 »
Point taken

#### Physics Dilettante

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##### How is expanding spacetime described in string theory?
« Reply #10 on: 05/12/2008 01:43:12 »
Don't expect any answers from string theory. 20 years on and still nothing testable has been described.
You mean "nothing testable that differs from the predictions of general relativity and quantum" -- right?

#### DoctorBeaver

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##### How is expanding spacetime described in string theory?
« Reply #11 on: 05/12/2008 02:43:29 »
No. I mean nothing testable. The structure of string theory is such that it can be made to produce an almost limitless number of possible types of universe. When a theory says that anything is possible, how can you test it?

#### Soul Surfer

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##### How is expanding spacetime described in string theory?
« Reply #12 on: 05/12/2008 23:43:26 »
To come at the question from a slightly different angle.  String theory is at source an attempt to describe quantum theory in terms of simple multidimensional elements that are not points but strings that move in space and as such does not really attack the problems of spacetime and quantum gravity.  so one would not expect that to deal properly with the expansion of space

#### Physics Dilettante

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##### How is expanding spacetime described in string theory?
« Reply #13 on: 07/12/2008 13:46:17 »
Clearly, I've seriously misunderstood string theory, then. I've had the idea that it's intended to unify gravity with the other forces and that it approaches the problem by treating gravity, the other forces, matter, energy, and spacetime as quantum phenomena.

#### DoctorBeaver

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##### How is expanding spacetime described in string theory?
« Reply #14 on: 07/12/2008 14:19:38 »
Clearly, I've seriously misunderstood string theory, then. I've had the idea that it's intended to unify gravity with the other forces and that it approaches the problem by treating gravity, the other forces, matter, energy, and spacetime as quantum phenomena.

That's what I thought too

#### Bikerman

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##### How is expanding spacetime described in string theory?
« Reply #15 on: 07/12/2008 15:17:15 »
Clearly, I've seriously misunderstood string theory, then. I've had the idea that it's intended to unify gravity with the other forces and that it approaches the problem by treating gravity, the other forces, matter, energy, and spacetime as quantum phenomena.
I thought the unification of gravity was a bonus. Surely the hypothesis starts from describing the hadrons as strings instead of point particles? I think part of the confusion is that the meaning of 'string theory' has changed over the last decade or so. Most people now use the term to mean supersymmetric string (M) theory, rather than the original.

#### DoctorBeaver

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##### How is expanding spacetime described in string theory?
« Reply #16 on: 07/12/2008 15:36:16 »
Hadronic string theory was discarded years ago, wasn't it? I thought modern string theory is an attempt to reconcile QT with GR. That must involve gravity.

#### Bikerman

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##### How is expanding spacetime described in string theory?
« Reply #17 on: 07/12/2008 15:39:42 »
Hadronic string theory was discarded years ago, wasn't it? I thought modern string theory is an attempt to reconcile QT with GR. That must involve gravity.
Yes, I think you are correct, I was simply pointing out that the origins of string theory were not necessarily in trying for a GUT, though that is certainly the direction that more recent developments have taken.

(Personally I'm pretty sceptical...)

#### Soul Surfer

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##### How is expanding spacetime described in string theory?
« Reply #18 on: 09/12/2008 09:11:23 »
I agree with all the comments following my last posting.  The problem is that trying a top down solution of the problem (modern M theory)has produced so many possible solutions that working through them all to decide which ones are the most likely and then working out how to test them is impossible.  We therefore need a radically different approach.

My currently preferred new approach is a two pronged attack based firstly on trying to identify processes that are self sustaining and recycling within all theories and Secondly on going as far as possible with analysing the behaviour of bodies where quantum gravity may come into play using the best classical and quantum mechanica techniques available.  ie the detailed macro and microscopic processes involved in the collapse of a rotating black hole inside it's event horizon not just the end product as described in the Kerr black hole.  This is likely to give hints about how things could progress.

You can see more details of this in my Evolutionary Cosmology thread in the new theories section.

#### DoctorBeaver

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##### How is expanding spacetime described in string theory?
« Reply #19 on: 09/12/2008 11:34:28 »
Quote from: SoulSurfer
Quote
ie the detailed macro and microscopic processes involved in the collapse of a rotating black hole inside it's event horizon

But surely that is 1 of the areas that string theory (and Loop Quantum Gravity) is hoping to shed some light on. GR breaks down inside an event horizon so we need a deeper understanding of gravity.

We can't do any experiments on the inside of an event horizon directly, or even take measurements. All string theorists can hope for is that they will be able to predict what we see in the real world and then conjecturally apply that to the inside of event horizons. We will never have empiric proof of the physics of the interior of a black hole.
« Last Edit: 09/12/2008 11:39:12 by DoctorBeaver »

#### Soul Surfer

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##### How is expanding spacetime described in string theory?
« Reply #20 on: 09/12/2008 23:53:17 »
String theory starts from a bottom up approach with many possible solutions.

When a rotating black hole collapses inside it' event horizon nothing much happens initally because the normal laws of physics still apply. So you can use computational fluid dynamics, relativistic orbit theory,  quantum and high energy particle interaction theory to describe the process of collapse in some detail until conditions get too extreme. This collapse would take a significant time and I have described the sort of things that I think will happen elsewherein these pages.  I am seriously looking for someone else to do the modelling and confirm my ideas or show me where I went wrong and describe more accurately what would happen

#### Physics Dilettante

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##### How is expanding spacetime described in string theory?
« Reply #21 on: 10/12/2008 05:33:05 »
To summarize where I believe we are WRT to my question:

1. Contemporary string theory is as I described it in my 7-Dec post, so it's reasonable to suppose it might have something to say about how spacetime expansion works.

2. It does, contrary to an earlier assumption of mine, invoke things that are smaller than the Planck length. (Not clear to me yet whether these things are the strings or other things.)

3. An implication of #2 is (I think) that spacetime quanta would have to be smaller than the Planck length.

#### DoctorBeaver

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##### How is expanding spacetime described in string theory?
« Reply #22 on: 10/12/2008 10:05:25 »
Quote from: "SoulSurfer
Quote
So you can use computational fluid dynamics, relativistic orbit theory,  quantum and high energy particle interaction theory to describe the process of collapse in some detail until conditions get too extreme

Precisely. We can describe the process up to a certain point. It's what happens beyond that point, what we don't know, that ST is trying to address.
« Last Edit: 10/12/2008 10:53:01 by DoctorBeaver »

#### Soul Surfer

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##### How is expanding spacetime described in string theory?
« Reply #23 on: 11/12/2008 00:31:22 »
Again Doctor Beaver you are missing the point.  To my knowledge other people have not tried hard to look at the initial process of gravitational collapse inside the event horizion because from a basic quick analysis it will end up with structures that we can't deal with without quantum gravity.

This is a bit like saying our universe cannot be interesting because the second law of thermodynamics states that it all ends in the heat death and maximal entropy.  End states of universes may not be interesting its the way we get there that is interesting because that makes us!

My argument is that this process of collapse will take some time and show some interesting and enlightening things before we reach the stage we cannot model using well understood processes. Also that if you then look at the structures you get from the point if view of the particles in them (not from outside) the results will be very revealing.

Basically you will get particles orbiting together with very high energies unaware of any grvitational field or gravitational gradient because they are in freee fall but the energies of the particles are all very similar so the particles are essentially cold with respect to each other. Because of relatavistic and gravitational distortion space has contracted and become one dimensional and time like while "time" has expanded to become multidimensional and space like.  As far as the particles are concerned this would look very much like a big bang with space expanding very uniformly and evenly without the need for mystic inflation.

#### DoctorBeaver

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##### How is expanding spacetime described in string theory?
« Reply #24 on: 11/12/2008 11:40:19 »
Ah, I see what you mean now - I think; that, maybe, there are other processes and events we aren't looking for because we're assuming that what happens can't be modelled other than by quantum gravity? There comes a point where GR suddenly throws up its hands in despair and we're waiting for QG to come to the rescue. But you're saying that by looking at the process differently, analysing it in alternative ways, we may find something else happening instead? Please correct me if I'm still misunderstanding you.

I've read about this space/time swapping of "roles" insofar as whichever direction you try to go inside an event horizon you will always arrive at the singularity as it would be in your future rather than at a point in space (I think I've got that right).

#### The Naked Scientists Forum

##### How is expanding spacetime described in string theory?
« Reply #24 on: 11/12/2008 11:40:19 »