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Alternatives to QFT

by waterfall
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waterfall
#127
Feb13-12, 07:31 PM
P: 381
Quote Quote by atyy View Post
That's just the starting point of the theory. As Smolin wrote "it seems that any acceptable quantum theory of gravity, whatever its ultimate formulation, is likely to reduce to a perturbative string theory in the appropriate limit."

In fact, string theory's AdS/CFT duality is the first theory to have a pretty convincing proposal of a non-perturbative, almost fully background independent theory of quantum gravity for some universes. The only background in that theory is at the boundary of the space, the bulk is just as background independent as classical GR with a negative cosmological constant.

Also, string theorists are working on generalizations. One example is Heckman and Verlinde's twistor matrix proposal: "Part of the issue is that in situations where maximal theoretical control is available, space-time is treated as a classical background, rather than as an emergent concept. Related to this, the understanding of holography on space-times of positive curvature remains elusive. In this paper we propose and develop a new dual matrix formulation of 4D field theory, in which the space-time and field theory degrees of freedom simultaneously emerge from a large N double scaling limit."
So next time Lee Smolin proclaimed to laymen that superstrings were not background independent. We would tell him "That's just the starting point of the theory. They have a dual in AdS/CFT which is background independent". Good.

Craig Hogan is building the Holo-meter as this month Sci-Am detailed. What's funny is that if it produces non-null. It confirms the discreteness of spacetime and supporting digital universe. But what does it support, the discreteness of spacetime due to LQG or the digital feature due to the Ads/CFT?

So String Theory can only be truly background independent if the universe supports the holographic principle? Yet I think the holographic principle is not widely supported and even on the speculative side. So it means there are some String Theorists who still think nature doesn't have to be background independent and GR is just some side effect of the theory?
atyy
#128
Feb13-12, 07:41 PM
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Quote Quote by waterfall View Post
So next time Lee Smolin proclaimed to laymen that superstrings were not background independent. We would tell him "That's just the starting point of the theory. They have a dual in AdS/CFT which is background independent". Good.

Craig Hogan is building the Holo-meter as this month Sci-Am detailed. What's funny is that if it produces non-null. It confirms the discreteness of spacetime and supporting digital universe. But what does it support, the discreteness of spacetime due to LQG or the digital feature due to the Ads/CFT?

So String Theory can only be truly background independent if the universe supports the holographic principle? Yet I think the holographic principle is not widely supported and even on the speculative side. So it means there are some String Theorists who still think nature doesn't have to be background independent and GR is just some side effect of the theory?
I think that even though they already have AdS/CFT, most string theorists are still looking for other non-perturbative background independent formulations of string theory. This is because the cosmological constant in AdS/CFT is negative, whereas that of our universe is positive. They are studying AdS/CFT or gauge/gravity duality to try and see if it can be generalized. For example, Heemskerk, Marolf and Polchinski write "Gauge/gravity duality presently describes only spacetimes with special boundary conditions, and the duality dictionary describes in direct way only observations made at the boundary. It is important to understand its lessons for more general observations and more general spacetimes."
waterfall
#129
Feb13-12, 08:59 PM
P: 381
Quote Quote by atyy View Post
I think that even though they already have AdS/CFT, most string theorists are still looking for other non-perturbative background independent formulations of string theory. This is because the cosmological constant in AdS/CFT is negative, whereas that of our universe is positive. They are studying AdS/CFT or gauge/gravity duality to try and see if it can be generalized. For example, Heemskerk, Marolf and Polchinski write "Gauge/gravity duality presently describes only spacetimes with special boundary conditions, and the duality dictionary describes in direct way only observations made at the boundary. It is important to understand its lessons for more general observations and more general spacetimes."
What are these other non-perturbative background independent formulations of string theory that doesn't involve AdS/CFT? It's strange that billions of dollars have been invested in String theory and many graduates spent all 5 years of their post-graduate time in it when it is fundamentally not background independent (so don't even support GR at its core). Or maybe the funding and studies only occured after Ads/CFT was discovered and so giving them hopes or the motivation? This is the reason why I asked if background independent is a law of nature that must be followed. If it is. And string theory was not compatible with it 20 years ago. What gave the initial go ahead for billion dollars funding for something that doesn't have promise? Maybe they got impressed by Witten?
martinbn
#130
Feb14-12, 03:20 AM
P: 354
Quote Quote by waterfall View Post
What are these other non-perturbative background independent formulations of string theory that doesn't involve AdS/CFT? It's strange that billions of dollars have been invested in String theory and many graduates spent all 5 years of their post-graduate time in it when it is fundamentally not background independent (so don't even support GR at its core).
What do you mean billions of dollar? And why shouldn't people spend their time on background dependent theories? Almost all of QFT is on a fixed Minkowski background, and many physicists spend their professional lives doing QFT, and it has been very successful.
waterfall
#131
Feb14-12, 04:46 AM
P: 381
Quote Quote by martinbn View Post
What do you mean billions of dollar? And why shouldn't people spend their time on background dependent theories? Almost all of QFT is on a fixed Minkowski background, and many physicists spend their professional lives doing QFT, and it has been very successful.
Smolin claimed those. Maybe he just wanted to start a fad. He looks like a guru and can command followers. But reflecting on all this. Isn't it the background independence in GR is only about mass/energy/momentum causing spacetime curvature. It doesn't say the mass, etc. made up spacetime. In LQG, spin networks make up spacetime. In String theories. Say there are a hundred different vacuo with different spacetimes. If you throw the strings from our universe into any one of those other universes with different backgrounds. It creates the spacetime analogous to our universe, so strings seem to be independent of background. We can give the following summary:

GR = mass/stress/energy causing spacetime curvature
LQG = spin networks/foam make up spacetime
Strings = Strings modes create spacetime irregardless of the backgrounds

Therefore background independence means differently in each case. And maybe we must not prefer one over the other. About QFT. Maybe it just ignores the mass/stress/energy effect on spacetime because it's negligible anyways.
waterfall
#132
Feb14-12, 07:15 AM
P: 381
Quote Quote by atyy View Post
BTW, although massless spin 2 can be equivalent to Einstein gravity in spacetimes that can be covered by harmonic coordinates (or similar), I don't think the reverse is true that the existence of a spin 2 field is sufficient to produce Einstein gravity.

Zhang and Hu, A Four Dimensional Generalization of the Quantum Hall Effect
Elvang and Polchinski, The Quantum Hall Effect on R^4

Bekaert et al, How higher-spin gravity surpasses the spin two barrier
This is just to clarify. You agreed Atyy that "In string theory, part of spacetime emerges as the excitations of strings." How does this differs to the above idea of massless spin 2 producing the curvature? Do you include strings as massless spin 2 thing? You agreed spacetime could emerge as the excitating of strings but not the curvature? Why not?

Also you seem to be saying that perturbative string theory can do that. How does this differs to non-perturbative string theory (is this about AdS/CFT?)?
atyy
#133
Feb14-12, 08:23 AM
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Quote Quote by waterfall View Post
This is just to clarify. You agreed Atyy that "In string theory, part of spacetime emerges as the excitations of strings." How does this differs to the above idea of massless spin 2 producing the curvature? Do you include strings as massless spin 2 thing? You agreed spacetime could emerge as the excitating of strings but not the curvature? Why not?

Also you seem to be saying that perturbative string theory can do that. How does this differs to non-perturbative string theory (is this about AdS/CFT?)?
In perturbative string theory, massless spin 2 = spacetime curvature (deviation from flat spacetime) emerges as an excitation of the string. So this is the same idea as gravitons producing spacetime curvature. However, gravitons are not fundamental since they are just one excitation type of the string, and the string is more fundamental.

In AdS/CFT, even strings are not fundamental, and instead emerge holographically from the boundary theory.
waterfall
#134
Feb14-12, 08:36 AM
P: 381
Quote Quote by atyy View Post
In perturbative string theory, massless spin 2 = spacetime curvature (deviation from flat spacetime) emerges as an excitation of the string. So this is the same idea as gravitons producing spacetime curvature. However, gravitons are not fundamental since they are just one excitation type of the string, and the string is more fundamental.

In AdS/CFT, even strings are not fundamental, and instead emerge holographically from the boundary theory.

Earlier when I mentioned about the idea of flat spacetime + gravitons = curve spacetime. Marcus emphasized it was not standard in string theory. Now you are saying it's standard. Or maybe if we add strings in the context. Then it's standard in string theory. When no strings and just the idea of flat spacetime + gravitons = curve spacetime , then not standard. Is this it? Please elaborate as this got me confused for 5 years already. Thanks.
atyy
#135
Feb14-12, 08:47 AM
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Quote Quote by waterfall View Post
Earlier when I mentioned about the idea of flat spacetime + gravitons = curve spacetime. Marcus emphasized it was not standard in string theory. Now you are saying it's standard. Or maybe if we add strings in the context. Then it's standard in string theory. When no strings and just the idea of flat spacetime + gravitons = curve spacetime , then not standard. Is this it? Please elaborate as this got me confused for 5 years already. Thanks.
It's standard. All the different quantum gravity approaches have gravitons at low energy. The differences are in whether at high energy they still exist in a similar form or whether something completely different like strings are needed.
waterfall
#136
Feb14-12, 09:15 AM
P: 381
Quote Quote by atyy View Post
It's standard. All the different quantum gravity approaches have gravitons at low energy. The differences are in whether at high energy they still exist in a similar form or whether something completely different like strings are needed.
Are you talking in terms of the gravitational field having gravitons at quanta at low energy that is standard? I'm talking about this flat spacetime thing with addition of gravitons that produced curved spacetime. Marcus mentioned in thread #115 this:

"I think some relevant comment is contained in the posts that follow #4. By starting with a flat background you rule out big bang and black hole stuff. Also rule out one of the more common spatially finite versions of standard cosmology. As I recall someone in the thread was pointing that out. Basically it is inconvenient, one could say crippling, to start out that way but you can recover a sector of the geometric theory, at least locally.

I'd say no QG approach has to explicitly deal with this special flat model because it empirically indistinguishable where it applies. (and since it doesn't cover all the cases it would be a bother---so people normally use the full theory.) but mathematically interesting certainly."

Marcus seems to disagree. If it's standard, why didn't he agree? Now I'm confused.
marcus
#137
Feb14-12, 09:24 AM
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Quote Quote by atyy View Post
In perturbative string theory, massless spin 2 = spacetime curvature (deviation from flat spacetime) emerges as an excitation of the string. So this is the same idea as gravitons producing spacetime curvature. However, gravitons are not fundamental since they are just one excitation type of the string, and the string is more fundamental.

In AdS/CFT, even strings are not fundamental, and instead emerge holographically from the boundary theory.
This sounds right to me. The basic idea of "perturbative" is to make an approximation by fixing a flat or other simple background and studying small "perturbations". It provides excellent means of calculation.

But it has recognized limitations as a way to think about reality. The "flat space+small curvature perturbations" picture is not taken as fundamental.

In non-string QG there was a bunch of papers about gravitons, doing calculations. In Loop the graviton papers started coming in around 2007, certain things had to be checked so people did that.
atyy
#138
Feb14-12, 09:32 AM
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Quote Quote by waterfall View Post
Are you talking in terms of the gravitational field having gravitons at quanta at low energy that is standard? I'm talking about this flat spacetime thing with addition of gravitons that produced curved spacetime. Marcus mentioned in thread #115 this:

"I think some relevant comment is contained in the posts that follow #4. By starting with a flat background you rule out big bang and black hole stuff. Also rule out one of the more common spatially finite versions of standard cosmology. As I recall someone in the thread was pointing that out. Basically it is inconvenient, one could say crippling, to start out that way but you can recover a sector of the geometric theory, at least locally.

I'd say no QG approach has to explicitly deal with this special flat model because it empirically indistinguishable where it applies. (and since it doesn't cover all the cases it would be a bother---so people normally use the full theory.) but mathematically interesting certainly."

Marcus seems to disagree. If it's standard, why didn't he agree? Now I'm confused.
I think you misunderstood him. He was just saying that in contrast to strings which built up perturbatively then got to AdS/CFT, loop quantum gravity started out from non-perturbative assumptions (ie. if it were string theory, it'd be like trying to find AdS/CFT without first knowing about strings, which is in principle possible, although it didn't happen that way). Both AdS/CFT and LQG are conjectured to produce gravitons at low energies.
marcus
#139
Feb14-12, 09:34 AM
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Quote Quote by atyy View Post
It's standard. All the different quantum gravity approaches have gravitons at low energy. The differences are in whether at high energy they still exist in a similar form or whether something completely different like strings are needed.
I agree. A perturbative approach is useful (even essential) for calculation at low energy. Loop, for instance "has gravitons" when one is explicitly studying low density, nearly flat, geometries. Low energy=low curvature, so that kind of approximation works.

I don't take issue with that. It's not a good way to picture reality when you are thinking about big bang cosmology. I take issue with someone calling the flat picture the real "territory" and the GR picture a mere "map".
waterfall
#140
Feb14-12, 09:42 AM
P: 381
Quote Quote by atyy View Post
I think you misunderstood him. He was just saying that in contrast to strings which built up perturbatively then got to AdS/CFT, loop quantum gravity started out from non-perturbative assumptions (ie. if it were string theory, it'd be like trying to find AdS/CFT without first knowing about strings, which is in principle possible, although it didn't happen that way). Both AdS/CFT and LQG are conjectured to produce gravitons at low energies.
No. He was referring to the idea of flat space + spin 2 graviton = curved spacetime. He wrote this in message #106.

"Everything the whole universe, would be happening in some fixed eternal Euclidean space, and everything includes BH collapse. Your theory would then have to explain how a "graviton" gets from the heart of a black hole out past the horizon to exert a "pull" on somebody orbiting the BH. And all the stuff about how the clock on the mountain top runs faster than the one in the valley. I guess because the "gravitons" slow clocks down."

Marcus didn't agree with it. But you Atyy agreed that we could be living in a flat spacetime and gravitons giving us GR.

Marcus. I think Atyy is saying the flat picture is the real "territory" and the GR picture a mere "map".
waterfall
#141
Feb14-12, 09:47 AM
P: 381
Wait. You mean in the Wheeler Gravitation book the writers were referring to low energies only? I thought it includes high energies which if true means the strings were moving in flat background and these graviton modes giving all the curvature. Hence. The flat picture is the real "territory" and the GR picture a mere "map". Maybe Hobba just misunderstood this from Carlip and the Wheeler Gravitation.
atyy
#142
Feb14-12, 10:05 AM
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Quote Quote by waterfall View Post
Wait. You mean in the Wheeler Gravitation book the writers were referring to low energies only? I thought it includes high energies which if true means the strings were moving in flat background and these graviton modes giving all the curvature. Hence. The flat picture is the real "territory" and the GR picture a mere "map". Maybe Hobba just misunderstood this from Carlip and the Wheeler Gravitation.
Wheeler was referring to classical gravity. In classical gravity there are two pictures. The first is gravity as spacetime curvature, the second is as spin-2 on flat spacetime. The second picture is equivalent to the first picture if spacetime can be covered by harmonic coordinates.

Now what about the quantum versions? The first classical picture has no known quantization. The second classical picture has a quantum version, but the quantum version only works below the Planck scale.

String theory tries to complete the quantum version of the second picture above the Planck scale by introducing new objects called strings. Loops tries to complete the quantum version of the second picture by quantizing the first classical picture.
waterfall
#143
Feb14-12, 10:06 AM
P: 381
Quote Quote by marcus View Post
This sounds right to me. The basic idea of "perturbative" is to make an approximation by fixing a flat or other simple background and studying small "perturbations". It provides excellent means of calculation.

But it has recognized limitations as a way to think about reality. The "flat space+small curvature perturbations" picture is not taken as fundamental.

In non-string QG there was a bunch of papers about gravitons, doing calculations. In Loop the graviton papers started coming in around 2007, certain things had to be checked so people did that.
I think my mistake was thinking it was fundamental when I learnt it 5 years ago from sci.physics from Hobba. This was because Brian Greene and other laymen book didn't talk about it. So when Hobba was referring to the following for example.. he was talking only of low energies (which I thought include high energies):

"Gravitons interact with all matter-energy. They interact in such a way as to make rulers and clocks behave as if space-time had curvature. It is a semantic issue of zero scientific value if space-time is thus curved or just appears curved. At this stage their is no way to experimentally distinguish between the two views."

Good to understood now it is only low energies. Thanks.
(Bhobba, who is a participant here, please comment if you don't agree).
marcus
#144
Feb14-12, 10:24 AM
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Quote Quote by waterfall View Post
Maybe Hobba just misunderstood this from Carlip and the Wheeler Gravitation.
Yes, I think that's a good guess. Wheeler's book is a huge thick tome about non-perturbative GR which has a section or two about the perturbative treatment. I've met Carlip when he was here giving a talk about several kinds of non-perturbative QG. He works mainly with that (not with "gravitons"). He has his PhD students working on things like CDT, Loop, Shape Dynamics. A Carlip grad student just finished his PhD on Loop last year, I forget the guy's name.

As far as I know CDT and Shape do not have any graviton papers as yet. It is not the main concern, at some point you want to see if you can handle the low energy nearly flat case and reproduce certain results. Loop has done this now to some extent, but those others not.

I don't know if Hobba misunderstood or whether he knew better but was just goofing off.
Attention-getting? I can't say, because I've only a cursory glimpse. The whole thing with Hobba struck me as having a kind of geriatric flavor. Harking back to papers from the 1970s. Weinberg's *Gravitation and Cosmology* book from 1972 etc. Or something Carlip said at some point in the past.
There was a temporary suspicion among particle theorists back then that you actually did not have to take GR seriously and maybe you could do everything with a fixed flat space.

But you might want to look at Weinberg's NEW book (2008). You can browse the ToC and Index on Amazon. It is called *Cosmology*. You will not find much if anything about the perturbative representation of GR. Very little if any mention of "gravitons".

http://www.amazon.com/Cosmology-Stev.../dp/0198526822
The Physics Today review said it would be a great help to "particle physicists tooling up for cosmology"
All based on dynamic changing curved geometry. HEP theorists taking GR more seriously now than, say, in 1972.

Think about a massive star collapsing to form a black hole. Are you going to model that whole process from beginning to end using a fixed unchanging flat space with ripples running around on it? Perturbative methods of calculation very good for some things. Not a full picture of reality. The full picture has to be able to handle extremes, highly dynamic changing geometry, extreme density, extreme moments of expansion. "Graviton" picture is inconvenient not to say unworkable. So (as Atyy indicates) the fashion among researchers has swung towards nonperturbative models. (which is where the relativists have been all along.)


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