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Non-geometric approach to gravity impossible?

by waterfall
Tags: gravity, impossible, nongeometric
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waterfall
#55
Mar1-12, 12:45 AM
P: 381
Quote Quote by atyy View Post
See post #44
You said there that "Weinberg exhibits harmonic coordinates for the FRW universe in his text book. I'm not sure whether the positive cosmological constant changes things.".

I'm talking about the Field Theory of Gravitation. Which is about Fields. What you meant above was that the FRW universe is covered by harmonic coordinates and can be modelled as spin-2 fields on flat spacetime. Now Field Theory of Gravitation is the formulism for this. Here one must separately model how space expands. In the other thread, someone said Field Theory of Gravitation doesn't have space expansion because this belongs to the curved spacetime formalism. Note the distinctions there are two formalisms involved. We must not mix them.
atyy
#56
Mar1-12, 12:56 AM
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Quote Quote by waterfall View Post
You said there that "Weinberg exhibits harmonic coordinates for the FRW universe in his text book. I'm not sure whether the positive cosmological constant changes things.".

I'm talking about the Field Theory of Gravitation. Which is about Fields. What you meant above was that the FRW universe is covered by harmonic coordinates and can be modelled as spin-2 fields on flat spacetime. Now Field Theory of Gravitation is the formulism for this. Here one must separately model how space expands.
Why?
waterfall
#57
Mar1-12, 12:58 AM
P: 381
Quote Quote by atyy View Post
Why?
Mentz in message #31 in http://www.physicsforums.com/showthr...=582440&page=2 stated:

"I think your logic is wrong in that not all curved spacetime is expanding. The expanding spacetimes of GR are a special class where spatial parts of the metric depend on t.

Also field gravity is not the same as GR. They are two different theories, both claim to explain the observed cosmological phenomena but in different ways. In fact I don't think FTG needs expanding space but supposes a fractal distribution of mass.

So you can't talk about splicing them together in the way you suggest."
atyy
#58
Mar1-12, 01:06 AM
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Quote Quote by waterfall View Post
Mentz in message #31 in http://www.physicsforums.com/showthr...=582440&page=2 stated:

"I think your logic is wrong in that not all curved spacetime is expanding. The expanding spacetimes of GR are a special class where spatial parts of the metric depend on t.

Also field gravity is not the same as GR. They are two different theories, both claim to explain the observed cosmological phenomena but in different ways. In fact I don't think FTG needs expanding space but supposes a fractal distribution of mass.

So you can't talk about splicing them together in the way you suggest."
I don't know what he means by FTG.
waterfall
#59
Mar1-12, 01:08 AM
P: 381
Quote Quote by atyy View Post
I don't know what he means by FTG.
Of course he means Field Theory of Gravitation (FTG). What else. Anyway. I'll ask him more thoroughly and references.
Mentz114
#60
Mar1-12, 05:02 AM
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Quote Quote by waterfall View Post
Of course he means Field Theory of Gravitation (FTG). What else. Anyway. I'll ask him more thoroughly and references.
There seems to be one worker in this field, Yurij V. Baryshev, and I gave reference to a review kind of paper (arXiv:gr-qc/9912003 v1) in an earlier post. There are about six papers in the arXiv on FTG, which makes it a rather insignicant subject.

But this theory is not as good as GR in explaining observations, and some authorities say it always leads to GR in any case.
waterfall
#61
Mar1-12, 05:18 AM
P: 381
Quote Quote by Mentz114 View Post
There seems to be one worker in this field, Yurij V. Baryshev, and I gave reference to a review kind of paper (arXiv:gr-qc/9912003 v1) in an earlier post. There are about six papers in the arXiv on FTG, which makes it a rather insignicant subject.

But this theory is not as good as GR in explaining observations, and some authorities say it always leads to GR in any case.
to Atyy, here's the reference (in the above paper) which says that in Field Theory of Gravitation, one must not use the same concept as space expansion in General Relativity.
Instead one must use the FTG version which is in the following terms:
Cosmology is another field of application of gravitation theory. Present data about large scale galaxies distribution contradict to the main point of Friedmann cosmology — its homogeneity. It turned out that galaxies form a fractal structure with dimension close to 2 at least up to the distance scales bout 200 Mpc. This leads to a new possibilities in cosmology (see an analysis of FTG cosmological applications in the review of Baryshev et al., 1994). One of the main difference between FTG and GR is that the field approach allows the existence of the infinite stationary matter distribution (Baryshev, Kovalevskij, 1990). In a stationary fractal distribution the observed redshift has gravitational and Doppler nature and is not connected with space expansion as in Friedmann model.
Now if cosmological observations prove beyond the shadow of a doubt that space indeed expand. Then spin-2 field over flat spacetime as a priori is falsified. If so. Then all quantum gravity theories that use gravitons in this terms like string theories are falsified. Think of the implications if space indeed expand. What do you think atyy?
atyy
#62
Mar1-12, 07:49 AM
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Quote Quote by waterfall View Post
to Atyy, here's the reference (in the above paper) which says that in Field Theory of Gravitation, one must not use the same concept as space expansion in General Relativity.
Instead one must use the FTG version which is in the following terms:


Now if cosmological observations prove beyond the shadow of a doubt that space indeed expand. Then spin-2 field over flat spacetime as a priori is falsified. If so. Then all quantum gravity theories that use gravitons in this terms like string theories are falsified. Think of the implications if space indeed expand. What do you think atyy?
You can evaluate Baryshev and his FTG for yourself.

See post #44.
waterfall
#63
Mar1-12, 12:34 PM
P: 381
Quote Quote by atyy View Post
You can evaluate Baryshev and his FTG for yourself.

See post #44.
I don't know what you were trying to say with your one line sentence written in riddles. I've tried reading his paper again. Maybe you were thinking FTG (Field Theory of Gravitation) is different from Weinberg Spin-2 over flat spacetime valid in spacetime covered by harmonic coordinates. What you didn't seem to get is it is identical to FTG as the following paragraph shows:

"The field theory of gravitation is based on the principle of universality of gravitational interaction and has some forms of the principle of equivalence as its particular cases. In FTG there are Minkowski background space and usual concepts of gravity force, gravity field EMT and quanta of gravity field - gravitons. Within FTG there is no infinite force at gravitational radius and compact massive stars could have masses much more than OV-limit. FTG is actually a scalar-tensor theory and predicts existance of tensor (spin 2) and scalar (spin 0) gravitational waves. Astrophysical tests of FTG will be available in near future. It is quite natural that fundamental description of gravity will be found on quantum level and geometrical description of gravity may be considered as the classical limit of quantum relativistic gravity theory."

How does the above differ to Weinberg formulation. They are the same.

Hope you can read the paper yourself instead of writing in one line riddles that is so difficult to understand.
atyy
#64
Mar1-12, 01:31 PM
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Quote Quote by waterfall View Post
I don't know what you were trying to say with your one line sentence written in riddles. I've tried reading his paper again. Maybe you were thinking FTG (Field Theory of Gravitation) is different from Weinberg Spin-2 over flat spacetime valid in spacetime covered by harmonic coordinates. What you didn't seem to get is it is identical to FTG as the following paragraph shows:

"The field theory of gravitation is based on the principle of universality of gravitational interaction and has some forms of the principle of equivalence as its particular cases. In FTG there are Minkowski background space and usual concepts of gravity force, gravity field EMT and quanta of gravity field - gravitons. Within FTG there is no infinite force at gravitational radius and compact massive stars could have masses much more than OV-limit. FTG is actually a scalar-tensor theory and predicts existance of tensor (spin 2) and scalar (spin 0) gravitational waves. Astrophysical tests of FTG will be available in near future. It is quite natural that fundamental description of gravity will be found on quantum level and geometrical description of gravity may be considered as the classical limit of quantum relativistic gravity theory."

How does the above differ to Weinberg formulation. They are the same.

Hope you can read the paper yourself instead of writing in one line riddles that is so difficult to understand.
I'll pass on this.

Wald, p383, we may view the full Einstein equation (γab not assumed to be "small") as the sum of this free piece, plus a nonlinear self-interacting term, ie. we may view Einstein's equation as an equation for a self-interacting spin-2 field ...
waterfall
#65
Mar1-12, 05:15 PM
P: 381
Quote Quote by atyy View Post
I'll pass on this.

Wald, p383, we may view the full Einstein equation (γab not assumed to be "small") as the sum of this free piece, plus a nonlinear self-interacting term, ie. we may view Einstein's equation as an equation for a self-interacting spin-2 field ...
But the Field Theory of Gravitation (FTG) also involves self-interacting spin-2 field on flat spacetime, it just summarizes the findings of others like Ward. It is not a new theory. In other words, Wald proposal also comprises the Field Theory of Gravitation (FTG). Please address this first by accepting or not accepting and why (in clearer terms because I'm always confused by your replies).
atyy
#66
Mar1-12, 05:24 PM
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Quote Quote by waterfall View Post
But the Field Theory of Gravitation (FTG) also involves self-interacting spin-2 field on flat spacetime, it just summarizes the findings of others like Ward. It is not a new theory. In other words, Wald proposal also comprises the Field Theory of Gravitation (FTG). Please address this first by accepting or not accepting and why (in clearer terms because I'm always confused by your replies).
Sorry to seem unhelpful. I'm just telling you what I learnt from the textbooks. As Baryshev states, his view is not the textbook view - he says it is different from Misner, Thorne & Wheeler, as well as Zeldovich & Novikov. I merely added the cite from Wald to give a more modern citation, in case you think MTW and ZN are out of date. To that one can also add Straumann, Carlip, & Donoghue. Perhaps Baryshev is right, but since it takes more time, energy and interest than I have to evaluate Baryshev, I'm just saying I'm not interested in discussing Baryshev's work at this moment. But if you are interested in reading it for yourself, I'd certainly like to hear when you are done whether you think his view, or the textbook view is right.
waterfall
#67
Mar1-12, 05:29 PM
P: 381
Quote Quote by atyy View Post
Sorry to seem unhelpful. I'm just telling you what I learnt from the textbooks. As Baryshev states, his view is not the textbook view - he says it is different from Misner, Thorne & Wheeler, as well as Zeldovich & Novikov. I merely added the cite from Wald to give a more modern citation, in case you think MTW and ZN are out of date. To that one can also add Straumann, Carlip, & Donoghue. Perhaps Baryshev is right, but since it takes more time, energy and interest than I have to evaluate Baryshev, I'm just saying I'm not interested in discussing Baryshev's work at this moment. But if you are interested in reading it for yourself, I'd certainly like to hear when you are done whether you think his view, or the textbook view is right.
Thanks for being clear that there are two views. I thought they were the same. But then they are both about spin-2 fields on flat spacetime. The differences may be subtle. So you are saying that in the MTW views, it is compatible with expanding space. Ok. I have most of these references on ebooks. I'll go over them again noting the distinctions as well as read Bayshev for the third time.
Mentz114
#68
Mar1-12, 06:03 PM
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Can you point me to some e-lit that shows the MTW treatment ?
waterfall
#69
Mar1-12, 06:18 PM
P: 381
Quote Quote by Mentz114 View Post
Can you point me to some e-lit that shows the MTW treatment ?
See: http://www.scribd.com/doc/81449908/F...time-Gravitons

find this starting line:

"5. Einstein's geometrodynamics viewed as the standard field theory for a field of spin 2 in an "unobservable flat spacetime" background...".

Please share how it differs to your description of Baryshev's as when you described it in the other thread:

"FTG is a classical field theory that begins with the Lagrangian which has three terms, one each for the field, one for the matter and crucially one for the interaction between the field and the matter. The exchange boson, if the theory was quantized would be spin-2. All this is done in Minkowski spacetime."

atyy.. since you are familiar with the MTW approach, please share how it differs to the above FTG theme. Thanks.
PAllen
#70
Mar1-12, 06:40 PM
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Quote Quote by waterfall View Post
See: http://www.scribd.com/doc/81449908/F...time-Gravitons

find this starting line:

"5. Einstein's geometrodynamics viewed as the standard field theory for a field of spin 2 in an "unobservable flat spacetime" background...".

Please share how it differs to your description of Baryshev's as when you described it in the other thread:

"FTG is a classical field theory that begins with the Lagrangian which has three terms, one each for the field, one for the matter and crucially one for the interaction between the field and the matter. The exchange boson, if the theory was quantized would be spin-2. All this is done in Minkowski spacetime."

atyy.. since you are familiar with the MTW approach, please share how it differs to the above FTG theme. Thanks.
All of the authors atyy cites believe that spin-2 field theory is identical to GR in physical predictions at least up to the event horizon, and possibly beyond (except, of course, for Hawking radiation). Baryshev believes that spin 2 field theory predicts that the event horizon doesn't exist, and therefore that Bekenstein-Hawking thermodynamics of black holes is incorrect. You can see this more clearly from some of Baryshev's other papers.

See, for example: http://arxiv.org/abs/0809.2328

Almost all other authors on spin 2 field theory would disagree with every prediction of the above paper, believing that spin 2 field theory would agree with GR instead. As with atyy, I am not in a position to judge Baryshev on the merits.

One comment on the disagreement is noted in the following:

http://arxiv.org/abs/1106.2476 :

"Finally, let us mention that approaches exist that treat gravity as simply a spin-2
field on
flat space [114, 115]. It has been conjectured that one could reconstruct the
Einstein-Hilbert action in such an approach by considering consisitency conditions order
by order in perturbation theory. This will, of course, be an invalid treatment when
gravity is strong, and in cosmology."

Most authors disagree with this paragraph and argue that such recovery of the Einstein-Hilbert action is imperative, and that the comment on invalidity is itself invalid.
Haelfix
#71
Mar1-12, 06:55 PM
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P: 1,677
There is a sublety regarding scaling when you go from the linear to the full nonlinear theory around certain solutions. So when doing perturbation theory around say the Schwarschild solution you naively run into an inconsistency and that is what Baryshev is picking up on.

What he fails to mention is that this problem was dealt with long ago by Vanshtein.
"To the problem of nonvanishing gravitation mass”, Phys. Lett. B, 39, 393–394, (1972)

But anyway, this is way beyond the scope of this thread and is just arguably going to confuse things more than they already are.
waterfall
#72
Mar1-12, 07:07 PM
P: 381
Quote Quote by Haelfix View Post
There is a sublety regarding scaling when you go from the linear to the full nonlinear theory around certain solutions. So when doing perturbation theory around say the Schwarschild solution you naively run into an inconsistency and that is what Baryshev is picking up on.

What he fails to mention is that this problem was dealt with long ago by Vanshtein.
"To the problem of nonvanishing gravitation mass”, Phys. Lett. B, 39, 393–394, (1972)

But anyway, this is way beyond the scope of this thread and is just arguably going to confuse things more than they already are.
No. It only makes things clearer now. Thanks.


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