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

  1. 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?
  2. jcsd
  3. atyy

    atyy 11,185
    Science Advisor

    I believe Verlinde and Jacobson are wrong.

    "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.
    Last edited: Feb 11, 2010
  4. 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.

  5. tom.stoer

    tom.stoer 5,489
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    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 :-)

    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 definitly need "something", some degrees of freedom
    5- which framework do you mean? and about which particles are you talking: electrons, protons, ...?
  6. 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.

  7. tom.stoer

    tom.stoer 5,489
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    w/o fundamental degrees of freedom you will neither have effective degrees of freedom nor entropy; that's why you need at least something
  8. Demystifier

    Demystifier 5,592
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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 not-well-justified assumption of the approach) is cool and modern
    Last edited: Feb 11, 2010
  9. 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).
  10. 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.

  11. 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?

  12. tom.stoer

    tom.stoer 5,489
    Science Advisor

    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
  13. 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".

    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.

  14. marcus

    marcus 24,214
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    2014 Award

    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:
    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:
    "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 :biggrin: not only for those Jacobson mentioned.
    Last edited: Feb 11, 2010
  15. atyy

    atyy 11,185
    Science Advisor

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

    Edit: I just saw your other thread, will continue the discussion there instead.
  16. If GR is the equilibrium equation of state don't we have to worry about the non-equilibrium dynamics?
  17. Hi I wasn't referring to LQG but Verlinde argument gravity as equation of state of entropy on these and on continuous spacetime
  18. Thermodynamics of Spacetime: The Einstein Equation of State
    Authors: Ted Jacobson

    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
  19. marcus

    marcus 24,214
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    'Sabah I think your initial mistake was what you said in the first post of the thread:
    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.
  20. marcus

    marcus 24,214
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    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.
  21. At the time I posted the thread I had that quote in mind.
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