Alternatives to QFT

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Have you missed the message of juanrga in post #182 where he shared the paper :
http://www.worldscinet.com/ijmpd/17/1703n04/free-access/S0218271808012085.pdf

FROM GRAVITONS TO GRAVITY: MYTHS AND REALITY

Abstract:

What the above may mean is that any quantum gravity theory that uses spin-2 field can't recreate General Relativity. So it's like a no-go theorem for any field approach to gravity and a yes-go theorem for GR being geometry forever. No?
No. The work emphasizes some mistakes in the usual textbooks claim that GR is fully equivalent to a massless spin-2 theory, when it is not.

It is possible to derive GR (geometrodynamics) from a field theoretic approach to gravitation, but as an geometric approximation. Somewhat as geometrical optics is an approximation to physical optics based in fields.
 
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You need to stop inserting your own assumptions and wording into what other people write... He was decidedly talking about classical physics, not string theory! My point earlier, is that you can't go around throwing terms around out of context without making a complete logical mess of the discussion.
But Bill was talking about string theory as shown in this thread http://groups.google.com/group/sci.physics/browse_thread/thread/83c972d2fcf3d124/552d4f6aa73d1501?lnk=gst&q=bill+hobba+spacetime+unknown+strings# where I pointed out earlier and it is a thread I've read over a dozen times and has me thinking about it from time to time for 5 years already with no resolution in sight... here are the conversations:

Someone asked Bill there:

> But in string theory, spacetime still has curvature.

Bill replied: "No it doesn't. It emerges as a limit - but the underlying geometry of space-time - if it has one - is not known."

Someone asked Bill again:

> Are you implying that in string and superstring theory, spacetime is flat and what caused gravity >are gravitons?

Bill replied: "It has long been known that a quantum theory of gravity as spin two particles in a flat space-time leads to GR eg the link I seem to have to give over and over:
http://arxiv.org/abs/gr-qc/9512024 "

Bill clearly stated that in string theory, spacetime has no curvature and it is the spin two particles in a flat spacetime that lead to GR!

So Bill is clearly talking about String theory and not classical physics. Now since spin-2 fields in flat spacetime in classical physics is not completely right. Then how could he bring it to string theory? This is the part I can't understand.

Bill, can you clarify this or someone can state once and for all that he has some misunderstanding here (and clarify it), at least to settle the issues because I've been thinking for this for over 5 years already.

Or if you still can't understand my point. Just answer this:

Does as Bill put it, a "quantum theory of gravity as spin two particles in a flat space-time leads to GR"??


What is clear, is that if a quantum theory contains gravitons in the usual way (which is quantum physics, not classical physics) with the correct couplings, you do end up with a classical limit that looks approximately GRish. But details matter here...

Further, just b/c you have gravitons, does not mean you have the correct theory of quantum gravity. You really do need a formalism or theory that describes the physics in all relevant physical regimes, not just those that are covered by weak coupling. SO what do I think?
I think string theory captures a part of the correct physics of quantum gravity, in particular in those regimes where the perturbative picture holds or where a duality is possible. I do not understand the rest and so I simply do not know more than that one way or the other.

As for the graviton myth or reality paper, I linked a direct response by Stanley Deser, one of the original creators of the spin2 linearized formalism.
 
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Lest anyone puts words in my mouth those posts are many years old.

My position is this. Spin 2 particles imply linearised gravity and linearised gravity implies full GR. There may be other issues involved - let see what emerges when people who are into this sort of stuff discuss it. There is something in the back of my mind where I have seen this discussed before and really it didn't lead anywhere.

Thanks
Bill
 

atyy

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Does as Bill put it, a "quantum theory of gravity as spin two particles in a flat space-time leads to GR"??
Yes, it leads to classical GR restricted to spacetimes that can be covered by harmonic coordinates. This quantum theory only works for energies below the Planck scale. The quest for quantum gravity is to find a theory that works near and above the Planck scale.
 
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Lest anyone puts words in my mouth those posts are many years old.

My position is this. Spin 2 particles imply linearised gravity and linearised gravity implies full GR. There may be other issues involved - let see what emerges when people who are into this sort of stuff discuss it. There is something in the back of my mind where I have seen this discussed before and really it didn't lead anywhere.

Thanks
Bill
But can you apply it to strings theory and say that a "quantum theory of gravity as spin two particles in a flat space-time leads to GR"?

In classical physics. This http://www.worldscinet.com/ijmpd/17/1703n04/free-access/S0218271808012085.pdf shows spin 2 particles in flat spacetime CAN'T lead to GR.

How is it that in a quantum theory of gravity like String theory, spin 2 particles in flat spacetime CAN lead to GR while in classical physics, It CAN'T (as juanrga emphased in his shared paper)?
 

fzero

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> Are you implying that in string and superstring theory, spacetime is flat and what caused gravity >are gravitons?

Bill replied: "It has long been known that a quantum theory of gravity as spin two particles in a flat space-time leads to GR eg the link I seem to have to give over and over:
http://arxiv.org/abs/gr-qc/9512024 "

Bill clearly stated that in string theory, spacetime has no curvature and it is the spin two particles in a flat spacetime that lead to GR!
Looking at what you quote, it seems that someone else said that. Flat spacetime is just a convenient starting point. It is also possible to study strings in other backgrounds.

Does as Bill put it, a "quantum theory of gravity as spin two particles in a flat space-time leads to GR"??
Actually the classical theory of a spin 2 particle (along with the appropriate linear gauge invariance) leads to equations of motion that are precisely what would have been obtained from Einstein's equation. If you study the quantum theory (as an effective theory) you will find higher-order corrections to Einstein's equations.
 
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Yes, it leads to classical GR restricted to spacetimes that can be covered by harmonic coordinates. This quantum theory only works for energies below the Planck scale. The quest for quantum gravity is to find a theory that works near and above the Planck scale.
But according to juanrga in his shared paper http://www.worldscinet.com/ijmpd/17/1703n04/free-access/S0218271808012085.pdf

"There is more to gravity than gravitons. (There is sufficient evidence to assume that gravity is not a fundamental field but an emergent phenomenon like elasticity."

Please read the paper written by India top physicist which disproves that in classical GR, spin-2 fields in flat spacetime can lead to GR. If it doesn't apply classically. You can't apply it in quantum gravity classical limit.
 

atyy

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But according to juanrga in his shared paper http://www.worldscinet.com/ijmpd/17/1703n04/free-access/S0218271808012085.pdf

"There is more to gravity than gravitons. (There is sufficient evidence to assume that gravity is not a fundamental field but an emergent phenomenon like elasticity."

Please read the paper written by India top physicist which disproves that in classical GR, spin-2 fields in flat spacetime can lead to GR. If it doesn't apply classically. You can't apply it in quantum gravity classical limit.
That paper is discussing subtleties. Padmanabhan still agrees that classical GR = spin 2: "Then we need to assume that the spin 2 field ... This assumption will lead consistently to Einstein’s theory and seems to be the most viable option, if we want to obtain standard gravity coupled to matter, starting from the graviton action."

When he says gravity is more than gravitons, he is talking about quantum gravity near the Planck scale - there Padmanabhan favours emergent gravity like string theory: "There is more to gravity than gravitons. (There is sufficient evidence to assume that gravity is not a fundamental field but an emergent phenomenon like elasticity. ..."
 

fzero

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But according to juanrga in his shared paper http://www.worldscinet.com/ijmpd/17/1703n04/free-access/S0218271808012085.pdf

"There is more to gravity than gravitons. (There is sufficient evidence to assume that gravity is not a fundamental field but an emergent phenomenon like elasticity."

Please read the paper written by India top physicist which disproves that in classical GR, spin-2 fields in flat spacetime can lead to GR. If it doesn't apply classically. You can't apply it in quantum gravity classical limit.
He doesn't disprove anything of the sort. He notes that on a manifold with boundary, the linearization of the Einstein-Hilbert action includes boundary terms. This has been known for 40 years and the resolution of the problem is to add a boundary term to the EH action http://en.wikipedia.org/wiki/Gibbons-Hawking-York_boundary_term. The bulk+boundary action is taken as the definition of GR on a manifold with boundary and its linearization agrees with the spin 2 theory.
 
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But can you apply it to strings theory and say that a "quantum theory of gravity as spin two particles in a flat space-time leads to GR"?

In classical physics. This http://www.worldscinet.com/ijmpd/17/1703n04/free-access/S0218271808012085.pdf shows spin 2 particles in flat spacetime CAN'T lead to GR.

How is it that in a quantum theory of gravity like String theory, spin 2 particles in flat spacetime CAN lead to GR while in classical physics, It CAN'T (as juanrga emphased in his shared paper)?
Before continuing misinterpreting what I really said, please read what I wrote in #201. Thanks.
 
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Before continuing misinterpreting what I really said, please read what I wrote in #201. Thanks.
Thanks. I understood things now more clearly than ever.

I believe with a little fixing, the quantum spin-2 field would be the primary entity and the geometry merely as a result of the symmetry in the math of the quantum field theory.

It's better than believing gravity is only geometry as General Relativity folks love to express.
Therefore I'm more inclined now toward string theory especially M-Theory which may involve what Witten describes as an incredible quantum symmetry where strings are just temporary constructs or a dual bit.

I'm not sure about Loop Quantum Gravity. If it's about geometry and reverse engineering it to get to the quantum parts or spin networks. Then it has less elegance.
 
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And no, GR ist not just the theory of a spin 2 graviton; string theorists know this probably better than anyone. How often needs it to be repeated that gravitons corresponds to "small ripples on a water surface" and not to the whole ocean including vortices etc.

The amount of misconceptions, desinformation and plain nonsense propagated here is really staggering!

In quantum field theory, the quanta like photons and gravitons are just momentum and energy of the fields. Of course you need the entire fields to do the work although what you can measure are simply the photons and gravitons but the fields which you take as the ocean underneath the "small ripples on a water surface" need to be active with the properties it needs.

Now in the case of our spin-2 gravitons discussions. To be sure I understood the concept. Let me explain it to you. What they meant when they say spin-2 field over flat spacetime equaled curved spacetime and gravity is not that spin-2 gravitons is enough to pull off those trick. But the ocean or gravitational fields derived from a quantum gravity has the properties and right coupling to pull of the gravity act. Then the spin-2 gravitons are just manifestation of this hidden gravitational fields. Therefore to avoid confusion. I think the proper things to say is instead of:

"Spin 2 gravitons + flat spacetime = General Relativity". One must say this:

"Spin 2 gravitons (with underlying gravitational field produced from excitations of strings or LQG or others) + flat spacetime = General Relativity."

Are we clear on this now. You guys used the former descriptons all throughout hence you confused even others like "surprised". Right?
 
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In post #99 a week ago:

Strange idea! Who told you that? Do the gravitons also make it appear that space is expanding? And expanding at different rates at different times and places? Do they make the expansion appear accelerate by various amounts, but it isn't really accelerating?

:biggrin: Sounds like someone sold you a load of bunkum, WF.

We had a long detour on string theory and spin-2 graviton thing because I was asking Marcus above (in post #98) if Loop Quantum Gravity was also about spin-2 graviton on flat spacetime and up to now it isn't answered because Marcus reactions to this spin-2 graviton idea is the above.

Well. So how do spin-2 gravitons (plus gravity fields) over flat spacetime explains Big Bang expansion? I guess we can consider the spin-2 fields as unique in that the fields can expand. Remember the Inflaton is also a field.. so it fills the gravity fields with inflatons expanding the fields with the effect as like producing spacetime curvature (but not really). Isn't it?

About Loop Quantum Gravity. So we can also consider it as spin networks producing the right coupling of gravity and hence can also be consider as having graviton spin-2 field over flat spacetime. Meaning spacetime only appears curved in LQG but not really curved?? This was what I was asking Marcus prior to his reply above whether one can consider LQG as like String theory where it is about spin-2 gravity fields over flat spacetime with the curvature geometry as not really a priori. But Marcus, like fellow poster surprised, misunderstood the concept as I didn't add the gravity field (behind spin 2) idea so didn't answer it. So let me ask this again now so someone can answer this LQG question above and we can close this thread clean. Thanks.
 

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