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Verlinde, LQG, entropy and gravity as a fundamental force vs emergent 
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#1
Feb1110, 12:52 AM

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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 gravityentropy emergent argument has 1 does the WeinbergWitten 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 EW? Strong? 4 how does this affect background independence, gravity as geometry, gravity is spacetime? 5 what are particles in this framework? 


#2
Feb1110, 01:05 AM

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I believe Verlinde and Jacobson are wrong.
http://arxiv.org/abs/grqc/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 nonperturbative 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. 


#3
Feb1110, 01:27 AM

P: 2,799

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 


#4
Feb1110, 02:42 AM

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Verlinde, LQG, entropy and gravity as a fundamental force vs emergent
Look at QCD; depending on certain choices you end up with a perturbative framework containing planewave ghosts and gluons or with a ghostfree lattice gauge theory. Quantization of a classical theory is always like constructing a house from a fuzzy architectural drawing. W/o context knowhow, additional instructions etc it will never work. The basic reason is that the drawing is an imprecise 2dim. reduction of a 3dim. object; so it is never onetoone. The big problem the physicists have compared to the construction worker is the they have never the house they want to construct :) Regarding 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 definitly need "something", some degrees of freedom 5 which framework do you mean? and about which particles are you talking: electrons, protons, ...? 


#5
Feb1110, 02:59 AM

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


#6
Feb1110, 03:06 AM

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#7
Feb1110, 03:42 AM

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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 notwelljustified assumption of the approach) is cool and modern 


#8
Feb1110, 04:17 AM

P: 216

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). 


#9
Feb1110, 05:54 AM

P: 2,799

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 


#10
Feb1110, 06:02 AM

P: 2,799

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 


#11
Feb1110, 07:16 AM

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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 


#12
Feb1110, 08:18 AM

P: 2,799

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". 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 


#13
Feb1110, 11:57 AM

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PF Gold
P: 23,274

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: beef 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: http://physicsforums.com/showthread.php?t=377015 The KowalskiGlikman 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/grqc/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. KowalskiGlikman'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. 


#14
Feb1110, 12:17 PM

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


#15
Feb1110, 02:44 PM

P: 343

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



#16
Feb1110, 03:12 PM

P: 716




#17
Feb1110, 03:16 PM

P: 716

Thermodynamics of Spacetime: The Einstein Equation of State
Authors: Ted Jacobson http://arxiv.org/abs/grqc/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 


#18
Feb1110, 03:34 PM

Astronomy
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PF Gold
P: 23,274




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