Verlinde, LQG, entropy and gravity as a fundamental force vs emergent

[ensabah6]

Verlinde and Jacobson's early work, strongly implies that gravity is emergent.

Anyhow, one of Jacobson's and Verlinde's claim in his paper is that since gravity is not a fundamental force, it does not make physical sense to quantize it canonically. So the LQG program is misguided, quantizing GR, Gravity is geometry, does not give you the fundamental degrees of freedom. I'm not sure what ramifications gravity-entropy emergent argument has

1- does the Weinberg-Witten theorem apply?
2- Verlinde does not believe in gravitons as fundamental but in "quasiparticles"
3- What are the fundamental forces? If EM can be shown to be entropic does it cease to be fundamental? What about E-W? Strong?
4- how does this affect background independence, gravity as geometry, gravity is spacetime?
5- what are particles in this framework?

 

[atyy]

I believe Verlinde and Jacobson are wrong.

http://arxiv.org/abs/gr-qc/0308048

"This led me at first to suggest that the metric shouldn’t be quantized at all. However I think this is wrong. Condensed matter physics abounds with examples of collective modes that become meaningless at short length scales, and which are nevertheless accurately treated as quantum fields within the appropriate domain."

"Similarly, there exists a perfectly good perturbative approach to quantum gravity in the framework of low energy effective field theory[2]. However, this is not regarded as a solution to the problem of quantum gravity, since the most pressing questions are non-perturbative in nature: the nature and fate of spacetime singularities, the fate of Cauchy horizons, the nature of the microstates counted by black hole entropy, and the possible unification of gravity with other interactions."

Thus he favours emergence still, but it is unclear whether this is because of the thermodynamic argument.

 

[Fra]

4- how does this affect background independence, gravity as geometry, gravity is spacetime?

Not to suggest this is what V & J think, but in my preferred "flavour" of this direction, BI should be replaced by the concept of "democracy of backgrounds". IE. true BI can not be established by an inside observer, but this does not mean there IS a unique background. Just that the backrounds related to the choice of observer, is unvoidable and instead what we should do is understand the equilibration process where EFFECTIVE BI is established in the sense of a steady sate or equiblirium.

I think this partly resonates with Ted's idea of seeing GR as an "equation of state" - equilibrium state that is. But there seeems to be many subtle flavours and possible subdirections in this new overall trend.

/Fredrik

 

[tom.stoer]

... it does not make physical sense to quantize it canonically. So the LQG program is misguided, quantizing GR, Gravity is geometry, does not give you the fundamental degrees of freedom.

Compare the three programs of "perturbative quantization of GR", string theory and LQG. All these approaches allow for a canonically quantization, but they end up with completely different degrees of freedom. There is no "unique approach of canonical quantization", so you will certainly never end up with "unique fundamental degrees of freedom".

Look at QCD; depending on certain choices you end up with a perturbative framework containing plane-wave ghosts and gluons or with a ghost-free lattice gauge theory.

Quantization of a classical theory is always like constructing a house from a fuzzy architectural drawing. W/o context know-how, additional instructions etc it will never work. The basic reason is that the drawing is an imprecise 2-dim. reduction of a 3-dim. object; so it is never one-to-one. The big problem the physicists have compared to the construction worker is the they have never the house they want to construct :-)

Regarding

1- does the Weinberg-Witten theorem apply?
2- Verlinde does not believe in gravitons as fundamental but in "quasiparticles"
3- What are the fundamental forces? If EM can be shown to be entropic does it cease to be fundamental? What about E-W? Strong?
4- how does this affect background independence, gravity as geometry, gravity is spacetime?
5- what are particles in this framework?

1- no, it does not apply; if you look at the proof you will find that the framework of LQG is totally different from the framework which is assumed for the Weinberg-Witten theorem.
2- Neither do I, nor does the LQG community. Gravitons are a concept (not necessarily a physical entity) used to build a perturbative quantization scheme similar to ordinary QFT; they do not show up in LQG.
3- I do not see how other forces are affected by these ideas
4- not at all; in order to make Verlindes approach work you need fundamental degrees of freedom which can produce entropy; so you definitely need "something", some degrees of freedom
5- which framework do you mean? and about which particles are you talking: electrons, protons, ...?

 

[Fra]

3- I do not see how other forces are affected by these ideas
4- not at all; in order to make Verlindes approach work you need fundamental degrees of freedom which can produce entropy; so you definitely need "something", some degrees of freedom

I agree that the paper of Verline gives no clear hints at this point, but I definitely see possible developments which would involve all forces from entropic reasoning, but tihs would also imply that one does not need fundamental degrees of freedom, effective degrees of freedom would do fine. One would also expect here a natural hiearchy in the interactions. IT's only one one freezes the picture at agiven level, that only one force at a time is affected.

I think there is a larger potential in these ideas that what yet is seen.

/Fredrik

 

[tom.stoer]

... one does not need fundamental degrees of freedom, effective degrees of freedom would do fine

w/o fundamental degrees of freedom you will neither have effective degrees of freedom nor entropy; that's why you need at least something

 

[Demystifier]

I think the Verlinde paper opens more questions than it solves. And that's one of the reasons why it receives so many citations.

Other reasons are:
- Verlinde is famous (if the paper was written by somebody unknown, nobody would care)
- the idea is technically simple so that everyone can understand it and contribute
- holography (which is the main not-well-justified assumption of the approach) is cool and modern

 

[czes]

Cramer's Transactional Interpretation of the QM shows that background's space is created of the interacted information (product of the wave functions).
The idea that space, time and gravity are emergend is old (Sakharow 1968). Some other suggested it also (Barbour, Zeh, Rovelli).

 

[Fra]

w/o fundamental degrees of freedom you will neither have effective degrees of freedom nor entropy; that's why you need at least something

I'm not sure if I disagree or if we just use the word differently.

I assume that with "fundamental" you mean "observer independent"?

If so, without observer indepednent degrees of freedom, yes it's true we don't have observer indepdent information measures or entropy, which is exactly my point.

This is sort of bad news since it makes things more complicated, but if it's reflecting the nature of things, I think our model should reflect it.

/Fredrik

 

[Fra]

that's why you need at least something

This is true, but I think loosely an arbitrary fluctuation could fine. Such a thing need a minimum of motivation.

It's like, we need to START with something, to be able to relate to anything, but this something does not need to be "fundamental" as in observer independent. Why would it?

/Fredrik

 

[tom.stoer]

If you want to develop a theory you have to use a mathematical expression for this "something"; so at least for this stage of the theory I would call it fundamental.

I wouldn't say that it should be an arbitrary fluctuation. Why not spin? bits? strings? logical expressions? Currently we do not know, but I would vote for an algebraic structure

 

[Fra]

If you want to develop a theory you have to use a mathematical expression for this "something"; so at least for this stage of the theory I would call it fundamental.

Ok, with this definition of fundamental, we are in agreement after all. "fundamental" is a somewhat well used word too.

I guess the disctinction I tried to make, is that I acknowledge from start, that the theory itself evolves. Thus, what is fundamental today, may not be in 50 years. BUT while that is obvious, I do mean it in a deeper way. I mean that this is suggestive also for the inside view - ie. we both agree that electrons don't write papers with mathematics, but instead, maybe we can agree that the electrons "understanding" is implicit in it's own action forms, and in this sense different physical subsystems may "see" different "fundamental degrees of freedom", but that this may even be the key to understand their interactions, and then not ONLY gravity, but all "interactions".

I wouldn't say that it should be an arbitrary fluctuation. Why not spin? bits? strings? logical expressions? Currently we do not know, but I would vote for an algebraic structure

The qualifier I would choose here is degree of speculation. In the usual sense at least, a bit would be simpler than a string, since a string is an entire continuum since it takes more information to specify a random string than a random bit.

This is why I think say a string could be self organised from simpler starting points (points). But that's certainly no objective assessment, it just mine :)

I vote for sets, and sets of sets which are related by transformations (representing different compressions) can respect information capacity. Then selection selects certain traits of these sets. One can assign algebraic structures to some of these things too if one likes, and also geometrical properties once we reach a continuum. Somehow many of these views are apparently isomorphic. some people love geometry and want to reformulate everything in terms of geometry, some love algerbra etc. I guess my preferences is to want to reformulate everything in terms of inferrable coherent structures.

/Fredrik

 

[marcus]

The title of the thread is:
Verlinde, LQG, entropy and gravity...

On that topic it's clear that the Verlinde paper has fed energy into the LQG program.

So what are the reasons for this?

The Loop program is basically a search for the fundamental degrees of freedom underlying geometry+matter, which asks "how to build qft without background geometry?"
Obviously the first requirement is that such a qft reproduce GR in the appropriate limit.

The program has uncovered various possible guises of the fundamental dof, various candidates. These show a tendency towards a topological character, particularly when we talk about spin networks and constrained BF theory (A possible nickname: be-ef or"beef").
BF is a topological quantum field theory (TQFT) built using differential forms on a continuum which has no geometry. The spin foam LQG approach has from inception been closely allied to beef tqft, and indeed derives from it.

Both spin networks and SO(4,1) BF provide fundamental degrees of freedom which can be used to flesh out Verlinde's entropic force idea.

The latter case is I think especially interesting. Here's the thread on it:
https://www.physicsforums.com/showthread.php?t=377015
The Kowalski-Glikman paper discussed there shows SO(4,1) beef degrees of freedom which clearly deserve study as possibly explaining the entropic force and justifying Velinde's heuristic idea. The pace of research has picked up so we should know later this year how it's going to work out.

Atyy picked up an important Ted Jacobson quote from 2003. Jacobson's vision is the guiding light in all of this, I think. He suggests that quantizing geometry ("quantizing the metric") can be a valid approach. A fruitful way of engaging with the problem of uncovering fundamental dof. As the saying goes: on s'engage et puis on voit.

The 2003 Jacobson quote that Atyy found is:
http://arxiv.org/abs/gr-qc/0308048
"This led me at first to suggest that the metric shouldn’t be quantized at all. However I think this is wrong."

From a 2010 perspective, with e.g. Kowalski-Glikman's paper in hand, one can more than agree with Jacobson. It is wrong (and quantizing geometry the right move) for several reasons not only for those Jacobson mentioned.

 

[atyy]

Both spin networks and SO(4,1) BF provide fundamental degrees of freedom which can be used to flesh out Verlinde's entropic force idea.

Does that mean LQG is emergent? BF+matter --> BF+constraint=LQG

Edit: I just saw your other thread, will continue the discussion there instead.

 

[Finbar]

If GR is the equilibrium equation of state don't we have to worry about the non-equilibrium dynamics?

 

[ensabah6]

Compare the three programs of "perturbative quantization of GR", string theory and LQG. All these approaches allow for a canonically quantization, but they end up with completely different degrees of freedom. There is no "unique approach of canonical quantization", so you will certainly never end up with "unique fundamental degrees of freedom".

Look at QCD; depending on certain choices you end up with a perturbative framework containing plane-wave ghosts and gluons or with a ghost-free lattice gauge theory.

Quantization of a classical theory is always like constructing a house from a fuzzy architectural drawing. W/o context know-how, additional instructions etc it will never work. The basic reason is that the drawing is an imprecise 2-dim. reduction of a 3-dim. object; so it is never one-to-one. The big problem the physicists have compared to the construction worker is the they have never the house they want to construct :-)

Regarding

1- no, it does not apply; if you look at the proof you will find that the framework of LQG is totally different from the framework which is assumed for the Weinberg-Witten theorem.
2- Neither do I, nor does the LQG community. Gravitons are a concept (not necessarily a physical entity) used to build a perturbative quantization scheme similar to ordinary QFT; they do not show up in LQG.
3- I do not see how other forces are affected by these ideas
4- not at all; in order to make Verlindes approach work you need fundamental degrees of freedom which can produce entropy; so you definitely need "something", some degrees of freedom
5- which framework do you mean? and about which particles are you talking: electrons, protons, ...?

Hi I wasn't referring to LQG but Verlinde argument gravity as equation of state of entropy on these and on continuous spacetime

 

[ensabah6]

Thermodynamics of Spacetime: The Einstein Equation of State
Authors: Ted Jacobson

http://arxiv.org/abs/gr-qc/9504004

Viewed in this way, the Einstein equation is an equation of state. This perspective suggests that it may be no more appropriate to canonically quantize the Einstein equation than it would be to quantize the wave equation for sound in air.

"canonically quantize the Einstein equation" sounds like LQG program

 

[marcus]

Hi I wasn't referring to LQG but...

'Sabah I think your initial mistake was what you said in the first post of the thread:

...the LQG program is misguided, quantizing GR, Gravity is geometry, does not give you the fundamental degrees of freedom...

This is a non sequitur---sounds kind of out-to-lunch. In the Loop program, quantizing geometry is a strategy for discovering/developing fundamental degrees of freedom. Dof candidates, I should stress, and it has uncovered several alternative descriptions so part of the game is to show relatedness between them.

 

[marcus]

Thermodynamics of Spacetime: The Einstein Equation of State
Authors: Ted Jacobson

http://arxiv.org/abs/gr-qc/9504004

Viewed in this way, the Einstein equation is an equation of state. This perspective suggests that it may be no more appropriate to canonically quantize the Einstein equation than it would be to quantize the wave equation for sound in air.

"canonically quantize the Einstein equation" sounds like LQG program

You didn't read post #2 in this thread, apparently. In 2003 Jacobson changed his mind and corrected himself. He gave one reason (one of several possible ones) that it makes good sense to quantize the Einstein equation.

 

[ensabah6]

You didn't read post #2 in this thread, apparently. In 2003 Jacobson changed his mind and corrected himself. He gave one reason (one of several possible ones) that it makes good sense to quantize the Einstein equation.

At the time I posted the thread I had that quote in mind.

 

[Fra]

If GR is the equilibrium equation of state don't we have to worry about the non-equilibrium dynamics?

I personally think we do.

But as I see it, part of the non-equiblirium evolution is unpredictable as in uncertain, to a given inside observer. This is why I think this should be considered in a context of evolving law. But this scenario I think might help a somewhat large inside observer (say a human based lab) understand, the action of other small interntal observer (particle physics).

So as far as I see, the possible implication is this idea, is more than cosmology only. It's just that in cosmology, WE are like real small inside observer, and this is why part of the grand scape evolution will remain unpredictable, instead maybe the current models can be explain as the one unique EXPECTED evolution, given just this. But in particle experiments we play the role of an intermediate observer. We are _effectively_ an outside observer, relative to a particle experiment arena, still we have uncertainty but this is of another kind, which a more defined uncertainty.

I think might explain what probability is more appropriate to particle physics, than it is to cosmology. Since there is no sense is histories of universes. Instead the logic to understand the "expectations" of things that happen once only are somewhat different.

/Fredrik

 

[marcus]

'Bah, since you evidently didn't catch this, let me repeat what I said a few posts back

Atyy picked up an important Ted Jacobson quote from 2003. Jacobson's vision is the guiding light in all of this, I think. He suggests that quantizing geometry ("quantizing the metric") can be a valid approach. A fruitful way of engaging with the problem of uncovering fundamental dof. As the saying goes: on s'engage et puis on voit.

The 2003 Jacobson quote that Atyy found is:
http://arxiv.org/abs/gr-qc/0308048
"This led me at first to suggest that the metric shouldn’t be quantized at all. However I think [what I said in 1995] is wrong."

From a 2010 perspective, with e.g. Kowalski-Glikman's paper in hand, one can more than agree with Jacobson. It is wrong (and quantizing geometry the right move) for several reasons not only for those Jacobson mentioned.

 

[ensabah6]

'Bah, since you evidently didn't catch this, let me repeat what I said a few posts back

I agree.

 

[marcus]

If GR is the equilibrium equation of state don't we have to worry about the non-equilibrium dynamics?

If you read the papers you see an awareness of this emerging and techniques (maybe for now ad hoc assumptions) for coping. You probably remember the earlier research on BH quasi-normal modes. Very brief quickly damped oscillations. You may have had that in mind! I was thinking of something else:

A premise that seems applicable is that at Planck scale everything happens very fast, it almost does not make sense to analyse or break down any further into smaller steps.
Something similar entered in the 1970s with Bekenstein's thought experiment. He declared he would consider the particle to have entered the horizon when it was within one Compton wavelength of the horizon.

It may sound ad hoc to you, but it makes sense to me. If you can't localize the particle any better than within one Compton, then it doesn't make sense to analyze the process of its entering the black hole horizon any finer than that. So the entry of a particle into the BH is in some sense akin to a "quantum jump".

I think it is probably inevitable that some (if not all) non-equilibrium transitions----in the thermodynamics of geometry---are likely to be treated this way. Everything macroscopic (like Bekenstein gradually lowering the particle down to the horizon on a strong rope) is going to be considered "slow", so that it is constantly in equilibrium. And critical transitions are likely, I suspect, to be blurred or fuzzed by a kind of Bekenstein Compton doctrine, or a "quantum jump" ansatz.

To me, this feels OK. But I can understand thinking one should worry about non-equilibrium geometric processes.

 

[Finbar]

Thermodynamics only makes sense when you coarse grain the microscopic degrees of freedom. General relativity and it's effective perturbative quantization can also make sense as long as we don't look at energies E~M_pl.

 

[tom.stoer]

1- does the Weinberg-Witten theorem apply?
2- Verlinde does not believe in gravitons as fundamental but in "quasiparticles"
3- What are the fundamental forces? If EM can be shown to be entropic does it cease to be fundamental? What about E-W? Strong?
4- how does this affect background independence, gravity as geometry, gravity is spacetime?
5- what are particles in this framework?

Did you get my answers?

1- no, it does not apply; if you look at the proof you will find that the framework of LQG is totally different from the framework which is assumed for the Weinberg-Witten theorem.
2- Neither do I, nor does the LQG community. Gravitons are a concept (not necessarily a physical entity) used to build a perturbative quantization scheme similar to ordinary QFT; they do not show up in LQG.
3- I do not see how other forces are affected by these ideas
4- not at all; in order to make Verlindes approach work you need fundamental degrees of freedom which can produce entropy; so you definitely need "something", some degrees of freedom
5- which framework do you mean? and about which particles are you talking: electrons, protons, ...?

 

[tom.stoer]

@ensabah6: I am still not sure what you mean. You talk about canonical quantization, LQG etc. but then you say that do not have LQG in mind ...

Let me explain: I am referring to LQG to provide one example how to "canonically quantize the Einstein equation". There may be others ... mbe LQG, but that there must be some theory with some fundamental degrees of freedom in order to apply thermodynamic arguments and entropy.

 

[ensabah6]

@ensabah6: I am still not sure what you mean. You talk about canonical quantization, LQG etc. but then you say that do not have LQG in mind ...

Let me explain: I am referring to LQG to provide one example how to "canonically quantize the Einstein equation". There may be others ... mbe LQG, but that there must be some theory with some fundamental degrees of freedom in order to apply thermodynamic arguments and entropy.

HI sorry for the misunderstanding, If Verlinde's account that gravity is not a fundamental force, but an equation of state of entropy, what would be its ramifications, for example, on continuum spacetime used in string theory, for example.

 

[tom.stoer]

I have to correct myself

2- Neither do I, nor does the LQG community. Gravitons are a concept (not necessarily a physical entity) used to build a perturbative quantization scheme similar to ordinary QFT; they do not show up in LQG.

There seems to be an approximation of LQG where a graviton propagator can be derived; but this is not a fundamental object.

 

[MTd2]

Maybe it is not gravity that needs to be quantized, but geometry.