Exploring Alternatives to QFT: A Critique of Non-Interacting Quantum Fields

[marcus]

WF it seems to me that you do not a one "map" and a "territory", you have two maps. One is GR, which has been tested to exquisite accuracy in a lot of subtly different ways and fits nature remarkably well. The other map is something you (or Bill Hobba? don't know of him) have made up---it does not correspond to string theory or anything else I know. In this map, things called "gravitons" are responsible for all the geometric effects including those I mentioned. Expansion, inflation, accelerated expansion, black hole collapse, the gravitational field outside the BH horizon. I mentioned some others...

Your second map, that you call "territory" would have to be formulated exactly in order to be tested and would have to be tested (as GR has been) and my guess is would turn out to be a dud. 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.

Basically I'm skeptical of your second map. Ask Hobba to give you a reference to the paper by Steve Carlip and see exactly what Carlip said. I doubt a Euclidean cosmology (with "gravitons") has ever been formulated in a way that comes near matching what we observe. But I think it is probably dear to your heart and you are not going to change your ideas. So AFAICS we have to agree to disagree on that. Agreed?

 

[atyy]

What distinguishes strings and LQG are not gravitons. Any theory of quantum gravity must have gravitons. Gravitons are quantized excitations of the gravitational field at low energies. The difference between strings and LQG is whether the gravitational field still exists at high energies. Strings says no, canonical LQG tries to say yes. In this respect, canonical LQG is similar to Asymptotic Safety.

 

[waterfall]

WF it seems to me that you do not a one "map" and a "territory", you have two maps. One is GR, which has been tested to exquisite accuracy in a lot of subtly different ways and fits nature remarkably well. The other map is something you have made up---it does not correspond to string theory or anything else I know. In this map things called "gravitons" are responsible for all the geometric effects including those I mentioned. Expansion, acceleration, black hole collapse, the gravitational field outside the BH horizon.

Your second map, that you call "territory" would have to be formulated exactly in order to be tested and would have to be tested (as GR has been) and my guess is would turn out to be bunk. Everything 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.

Basically I'm skeptical of your second map. I doubt it has ever been formulated in a way that comes near matching what we observe. But I think it is probably dear to your heart and you are not going to change your ideas. So AFAICS we have to agree to disagree on that. Agreed?

Of course I didn't make up the second map. It's from Bill Hobba as detailed in post #105 above. And he didn't invent it either.. but got it from Steve Carlip. So basically.

1. We have pure GR
2. Gravitons in Superstrings can cause curvature (whether intrinsic we don't know as per Carlip or Hobba)
3. In LQG.. is it trying to re-create GR? or is the effect like in the second where it is emulating the curvature (when there may not even be intrinsic curvature)? This is what I simply want to know. I'm not taking any sides. Just want to know what is really the case. I can't get this specific answer from any Rovelli papers so hope someone familiar with it can directly address this whole thing.

 

[marcus]

See my post #106, when I heard you were channeling sci.physics and someone named Hobba *paraphrasing* Steve Carlip I made corrections in #106.

Atyy's post is also relevant. Get the Carlip exact page reference if you want. I like Carlip, heard him give a seminar talk here at Berkeley a year or two ago. He's written many good QG papers. One recent one I liked comparing Loop, Triangulations, Safe, Horava, and even classical. My guess is he would have said the two models are indistinguishable ONLY in an approximate limited local sense, not globally. Reservations often get lost in paraphrase (esp. when the paraphraser is not giving links to references.) So if you want to pursue it with Hobba, get his online Carlip link with a page reference, so you don't have to go thru reams of stuff.

I'll leave you to work this out with whoever is interested in representing the cosmos (and its wonders) in a fixed flat eternal Euclidean space. Good luck

 

[waterfall]

Marcus, first of all. I'm unbiased. I'm only interested in what the evidence or theoretical evidences lead.

Second I still can't understand exactly what Bill Hobba is saying. He uses textbooks as referenced (mentioned below) so it's not his own creation but actually from the string theories themselves. For example the following conversations at sci.physics in the thread "Non-geometric approach to gravity impossible?"?

Hobba said and with reference:

"Gravity in flat space-time , otherwise known as linaerised gravity, is easily constructed based on EM - See Ohanian and Ruffini - Gravitation and Space-time. Trouble it it contains the seeds of its own destrcution. It can be shown that particles moves as is space-time had an infinitesimal curvature and its gauge invarience is infinitesimal coordinate transformation. The obvious consequence leads immediately to GR."

"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 "

"I suggest you think a bit clearer. A membrane as a continuum and treated by the methods of continuum mechanics emerges as a limit from the atomic structure of an actual membrane - yet does not imply it is a continuum at the level of individual atoms. The same with GR. Gravity as space-time curvature emerges from spin two gravitons when the underlying geometrical background is not known, but usually assumed to be Minkowskian flat, so the methods on QFT theory can be applied."

Someone asked Bill:

> How do the gravitons of the entire Earth conspire and coordinate
> in such a way that gravitational mass is the same as inertial
> mass as well as forming consistent geometry.

Bill replied: "It is a prediction of the model, the same way as Euclid's fifth conspires to
ensure the angles of a triangle add up to 180%, or even the rules of arithmetic conspire to ensure the amount of your bank balance is really the number of dollars and cents you actually have. Absolutely foundational and fundamental understanding of science and scientific modelling.

As previously explained, the specifics in this case, are when you mathematically analyse the linear equation, it shows, even though you assume space-time is flat, particles move as if it had an infinitesimal curvature. Also its gauge symmetry is infinitesimal coordinate transformations. In developing the linear equations an assumption was made - namely since gravity interacts with all mass-energy, and gravity itself has energy, it must interact with itself - this means the equations are non linear. So the assumption of the linear equations is gravity is weak enough that its interaction with itself can be ignored. To remove that restriction, the very reasonable hypothesis is made, that space-time is curved, and the equations are invariant - exactly as the analysis of the linear equations
suggest. When this is done the mathematics shows that GR inevitably results - exactly as Euclid's fifth forces the angles of a triangle to add up to 180%. It is this assumption that does the 'conspiring' you refer to. It is required for the equations to make reasonable sense. If you can not comprehend that a theories logical consistency allows a theory to make predicitons that seem almost like magic, just like Euclidian geometry does, then you have not grasped what 10 year olds who are taught Euclidean geometry are able to grasp, and I am afraid physics is beyond you.


The details can be found in Gravitation and Space-time by Ohanian and
Ruffini
https://www.amazon.com/dp/0393965015/?tag=pfamazon01-20


Bill"

This are all standard Superstring concepts, no? Bill Hobba is a member of physicsforums so hope he can clarify.

 

[waterfall]

I was able to track where Steve Carlip stated it. It's in the thread sci.physics.research which is moderated like physicsforums.

http://groups.google.com/group/sci....Einstein+Field+Equations+and+Flat+Space+Time#

> Do you have a reference that goes into this in greater detail? I am
> taking a break from work and have a bit of time to check into some
> things that have been on my mind. I will be going through Feynmans
> Lectures on Gravitation


That's a good place to start. Two standard papers are by Deser,
Gen. Rel. Grav. 1 (1970) 9 and Class. Quant. Grav. 4 (1987) L99,
which deal with classical calculations. You might also look at a
paper by Boulware and Deser, Ann. Phys. 89 (1975) 193, for a more
quantum field theoretical argument (based on earlier work by
Weinberg). You might also look at two articles, by Duff and Deser,
in the book _Quantum Gravity: An Oxford Symposium_ (edited
by Isham, Penrose, and Sciama, Clarendon Press, 1975).

Steve Carlip "

Now I'm confused enough to maybe start a thread at the relativity forum. But if you see this Bill Hobba. Please comment on all this like how many string theories take this view as well as latest from Steve Carlip.

 

[waterfall]

Looking at this matter further. I found out it was not even original claim by Steve Carlip but direct from Misner, Thorne, & Wheeler's book "Gravitation". I saw the following in Physicsforums:

https://www.physicsforums.com/showthread.php?t=278874

"Is spacetime really curved? Embedded somewhere?

Message #4:

"There's a fascinating analysis due to Deser ["Self-interaction and
gauge invariance", General Relativity & Gravitation 1 (1970), 9-18;
see also his later paper "Gravity from self-interaction in a curved
background", Classical and Quantum Gravity 4 (1997), L99-L105],
summarized in part 5 of box 17.2 of Misner, Thorne, & Wheeler's book.

Quoting from that latter summary:

"The Einstein equations may be derived nongeometrically by
noting that the free, massless, spin-2 field equations
[[for a field $\phi$]]
[[...]]
whose source is the matter stress-tensor $T_{\mu\nu}$, must
actually be coupled to the \emph{total} stress-tensor,
including that of the $\phi$-field itself.
[[...]]
Consistency has therefore led us to universal coupling, which
implies the equivalence principle. It is at this point that
the geometric interpretation of general relativity arises,
since \emph{all} matter now moves in an effective Riemann space
of metric $\mathcal{g}^{\mu\nu} = \eta^{\mu\nu} + h^{\mu\nu}$.
... [The] initial flat `background' space is no longer observable."

In other words, if you start off with a spin-2 field which lives on a
flat "background" spacetime, and say that its source term should include
the field energy, you wind up with the original "background" spacetime
being *unobservable in principle*, i.e. no possible observation can
detect it. Rather, *all* observations will now detect the effective
Riemannian space (which is what the usual geometric interpretation of
general relativity posits from the beginning)."

Comment?

 

[waterfall]

Check out the full arguments here in Misner, Thorne, Wheeler "Gravitation":

http://www.scribd.com/doc/81449908/Flat-spacetime-Gravitons

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

(body of arguments)

ending at
"
...[The] initial flat 'background' space is no longer observable." In other words, this approach to Einstein's field equation can be summarized as "curvature without curvature" or - equally well - as "flat spacetime without flat spacetime"!"

Marcus and other Quantum Gravity fellows. This is not my idea nor Hobba nor Carlip but from the grand textbook on Gravitation therefore please address it. I need to how LQG make use of the concept (if at all)... or rather I still don't know how to tie it to LQG.

 

[atyy]

Check out the full arguments here in Misner, Thorne, Wheeler "Gravitation":

http://www.scribd.com/doc/81449908/Flat-spacetime-Gravitons

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

See my post #107.

 

[marcus]

Check out the full arguments here in Misner, Thorne, Wheeler "Gravitation":

http://www.scribd.com/doc/81449908/Flat-spacetime-Gravitons

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

(body of arguments)

ending at
"
...[The] initial flat 'background' space is no longer observable." In other words, this approach to Einstein's field equation can be summarized as "curvature without curvature" or - equally well - as "flat spacetime without flat spacetime"!"

Marcus and other Quantum Gravity fellows. This is not my idea nor Hobba nor Carlip but from the grand textbook on Gravitation therefore please address it. I need to how LQG make use of the concept (if at all)... or rather I still don't know how to tie it to LQG.

Someone should express appreciation for the *scholarship*. In your past 3 or 4 posts you have provided links so one can see where you are coming from! I found the discussion in the PHYSICSFORUMS thread which you link to here helpful. But you only quote post #4. Read on thru to the end of the thread.

One of the guys is making the distinction between local and global. There are derivations and equivalences you can establish in a local neighborhood which do not necessarily extend over the whole. Topological considerations enter---the difference between an infinite plane, a sphere, and a donut. And so on.
The Einstein Field Equation of GR is local. It's only part of the GR picture that comes from studying global solutions. One or more of the people in that Physicsforums thread brought out that distinction and mentioned socalled FRW solutions.

I think maybe it was Poincaré who said something like no mathematics is TRUE it's only CONVENIENT. If no experimental observation can distinguish between two models then it is meaningless to ask which is true. You simply use the one that is more convenient.

In this case one of the approaches has the inconvenience that it applies locally but not necessarily globally (I mentioned some things earlier like collapse to a black hole.) From what I've seen of Steve Carlip's papers, or for that matter Wheeler, Misner etc, the approach where you have geometry is found to be more convenient.

I tend to think of the other way as a mathematical curiosity pointed out by some people in the 1970s which might occasionally be used in "effective" field theory (I don't know that it is but certainly could be.) It would necessarily be used in limited local situations, I should think. Maybe some other posters know of instances.

Looking at this matter further. I found out it was not even original claim by Steve Carlip but direct from Misner, Thorne, & Wheeler's book "Gravitation". I saw the following in Physicsforums:

https://www.physicsforums.com/showthread.php?t=278874

"Is spacetime really curved? Embedded somewhere?

Message #4:

"There's a fascinating analysis due to Deser ["Self-interaction and
gauge invariance", General Relativity & Gravitation 1 (1970), 9-18;
see also his later paper "Gravity from self-interaction in a curved
background", Classical and Quantum Gravity 4 (1997), L99-L105],
summarized in part 5 of box 17.2 of Misner, Thorne, & Wheeler's book.

Quoting from that latter summary:

"The Einstein equations may be derived nongeometrically by
noting that the free, massless, spin-2 field equations
[[for a field $\phi$]]
[[...]]
whose source is the matter stress-tensor $T_{\mu\nu}$, must
actually be coupled to the \emph{total} stress-tensor,
including that of the $\phi$-field itself.
[[...]]
Consistency has therefore led us to universal coupling, which
implies the equivalence principle. It is at this point that
the geometric interpretation of general relativity arises,
since \emph{all} matter now moves in an effective Riemann space
of metric $\mathcal{g}^{\mu\nu} = \eta^{\mu\nu} + h^{\mu\nu}$.
... [The] initial flat `background' space is no longer observable."

In other words, if you start off with a spin-2 field which lives on a
flat "background" spacetime, and say that its source term should include
the field energy, you wind up with the original "background" spacetime
being *unobservable in principle*, i.e. no possible observation can
detect it. Rather, *all* observations will now detect the effective
Riemannian space (which is what the usual geometric interpretation of
general relativity posits from the beginning)."

Comment?

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.

You've got great search technique! You are supplying great links to sources. Congratulations and thumbs up for that. Thanks for taking trouble.

Hopefully others will comment.

 

[martinbn]

Any theory of quantum gravity must have gravitons.

I've heard something like that many times, why is it?

 

[atyy]

One of the beautiful things about gravity as spin 2 on flat spacetime is that you can derive the equivalence principle. In the curved spacetime view, this has to be postulated.

http://phys.columbia.edu/~nicolis/GR_from_LI_2.pdf
http://arxiv.org/abs/1007.0435v3 (section 2.2.2 and Appendix A)

 

[waterfall]

Someone should express appreciation for the *scholarship*. In your past 3 or 4 posts you have provided links so one can see where you are coming from! I found the discussion in the PHYSICSFORUMS thread which you link to here helpful. But you only quote post #4. Read on thru to the end of the thread.

What's weird is that beginning post #4, they were not originally discussed here at physicsforums but the moderators somehow acquired them and appended here from the sci.physics.research thread called:

"Einstein Field Equations and Flat Space Time Options"

See the original thread here including the replies of Steve Carlip which the above cut out:

http://groups.google.com/group/sci....equations+may+be+derived+nongeometrically+by#

One of the guys is making the distinction between local and global. There are derivations and equivalences you can establish in a local neighborhood which do not necessarily extend over the whole. Topological considerations enter---the difference between an infinite plane, a sphere, and a donut. And so on.
...
Hopefully others will comment.

Well. I had this misconception for the past 5 years. I thought it was standard in string theory and know now it is not. Thanks for pointing that out. It will be a new chapter of understanding for me.

 

[martinbn]

One of the beautiful things about gravity as spin 2 on flat spacetime is that you can derive the equivalence principle. In the curved spacetime view, this has to be postulated.

http://phys.columbia.edu/~nicolis/GR_from_LI_2.pdf
http://arxiv.org/abs/1007.0435v3 (section 2.2.2 and Appendix A)

Atyy, was that a response to my question? I should probably explain. I've heard that any quantum field theory, which contains massless particle of spin 2 contains gravity. i would be interested to see that too, but to me it seems that it applies only to quantum field theries, at least as stated, and it doesn't say anything about other type of theories. For example string theory, it is not exactly quantum field theory, right? And often it is said that it is a theory of quantum gravity, but why? Does the statement apply here?

 

[atyy]

Atyy, was that a response to my question? I should probably explain. I've heard that any quantum field theory, which contains massless particle of spin 2 contains gravity. i would be interested to see that too, but to me it seems that it applies only to quantum field theries, at least as stated, and it doesn't say anything about other type of theories. For example string theory, it is not exactly quantum field theory, right? And often it is said that it is a theory of quantum gravity, but why? Does the statement apply here?

No, that wasn't a reply. Basically, gravity as spin 2 already works at low energies. Any new theory must reduce to a working old theory in the appropriate regime. Carlip has some references in here http://arxiv.org/abs/gr-qc/0108040, try searching for "Donoghue".

Hence, one way to see if LQG works is to see if it reproduces the graviton propagator http://arxiv.org/abs/0905.4082.

 

[Haelfix]

I am a little uneasy with the quote in 118 for a number of reasons. The local vs global thing is a bit of a red herring.

First of all, working with linearized gravity does not preclude cosmological or vacuum black hole solutions in any way, nor does it require an R^4 topology. Those solutions are readily studied, and in fact entire textbooks have been written on those solutions (see eg Weinberg 'Gravitation')

However it is true that GR does not in general, uniquely constrain the topology of spacetime. That is additional structure is necessary to fix the exact physics (eg by appealing to experiment).

But not so fast! Working with the standard formulation has the exact same problem! That is why for instance in the case of cosmology, it is still an open question what the exact topology of the universe is like. There is no extra physical information that one formulation gives over the other, which is why they are isomorphic mathematically.

The real fundamental difference (between any of the tens of different formulations of GR) is that in some cases using one formulation allows you to solve problems in a more straightforward way.

You wouldn't want to appeal to the geometric theory to solve the classical black hole merger problem for instance. You want a heavy dose of linearized perturbation theory to tackle that (and a very good computer)!

However trying to prove singularity theorems alla Hawking-Penrose, is more or less completely opaque if all you could see were infinite series of curvature invariants.

So anyway, this whole story is pretty well understood classically. The real question is what happens when you introduce quantum mechanics? And indeed, theorists have tried quantizing pretty much every single formulation of gravity out there, so far unsuccessfully and indeed it is perhaps the case that they give unitarily inequivalent theories.

 

[suprised]

.
Hence, one way to see if LQG works is to see if it reproduces the graviton propagator http://arxiv.org/abs/0905.4082.

No, that's by far not enough. The propagator captures only the quadratic piece of the effective action. But the Einstein action involves quite a number of extra vertices, and all those need to be reproduced as well; in other words, not just the free part, but also all the interactions must come out right. Obviously no sensible person would try to prove this term by term, rather one should find an indirect argument as to why all terms must come out right.

In string theory it is worldsheet conformal invariance that guarantees that all terms come out right, in this sense GR emerges automatically. I don't know of any such principle in LQG that would guarantee the correct outcome.

.. The difference between strings and LQG is whether the gravitational field still exists at high energies. Strings says no, canonical LQG tries to say yes.

I guess it is the other way around.

 

[atyy]

No, that's by far not enough. The propagator captures only the quadratic piece of the effective action.

Yes, I agree.

I guess it is the other way around.

Did I say it backwards? I meant strings introduces new degress of freedome, canonical LQG doesn't.

 

[atyy]

Atyy, was that a response to my question? I should probably explain. I've heard that any quantum field theory, which contains massless particle of spin 2 contains gravity. i would be interested to see that too, but to me it seems that it applies only to quantum field theries, at least as stated, and it doesn't say anything about other type of theories. For example string theory, it is not exactly quantum field theory, right? And often it is said that it is a theory of quantum gravity, but why? Does the statement apply here?

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

 

[waterfall]

So standard string theory assumes there is spacetime curvature and the gravitons are just quanta of the gravitational field much like in QFTs where the photons are quanta of the electromagnetic field or the electrons qunta of the electron field?

For 5 years. I actually thought all string theories use the mentioned concept of flat spacetime plus spin 2 = curved spacetime idea. So absolutely no active working string theorists like Witten ever use or entertain the concept?

But still Lee Smolin kept emphasizing strings occurred in the backdrop of a fixed spacetime background. When he said fixed. It includes spactime curvature but the strings not part of spacetime versus the idea in LQG where the spin networks are spacetime itself (and not in it)?

How come string theorists continue with the strings in a fixed background idea.. maybe because they still hope that perhaps nature is like that? But the idea of General Relativity is already based on no prior geometry or background independence. Maybe string theories thought the strings in a fixed background is more fundamental and GR just unnatural? Hope someone can elaborate on all this. Thanks.

 

[atyy]

So standard string theory assumes there is spacetime curvature and the gravitons are just quanta of the gravitational field much like in QFTs where the photons are quanta of the electromagnetic field or the electrons qunta of the electron field?

For 5 years. I actually thought all string theories use the mentioned concept of flat spacetime plus spin 2 = curved spacetime idea. So absolutely no active working string theorists like Witten ever use or entertain the concept?

But still Lee Smolin kept emphasizing strings occurred in the backdrop of a fixed spacetime background. When he said fixed. It includes spactime curvature but the strings not part of spacetime versus the idea in LQG where the spin networks are spacetime itself (and not in it)?

How come string theorists continue with the strings in a fixed background idea.. maybe because they still hope that perhaps nature is like that? But the idea of General Relativity is already based on no prior geometry or background independence. Maybe string theories thought the strings in a fixed background is more fundamental and GR just unnatural? Hope someone can elaborate on all this. Thanks.

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."

 

[waterfall]

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]

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]

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 occurred 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]

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]

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 anyone 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 regardless 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]

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]

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]

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]

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]

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]

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]

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]

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]

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".