Diversity of emergent gravity programs?

[Fra]

Diversity of "emergent gravity" programs?

Does any of you knowledgeable people know of any an reviews of various "emergent gravity" programs? Ie. something that focuses on the different ways to do it.

It appears that many emergent gravity programs, inspired by condensed matter physics somehow work in a fixed context, a fixed universal microstructure that suppodly represents the degrees of freedom of a deeper reality from which dimensionality and other stuff emerges from.

But what emergent gravity approaches are there that does not start with and universal background structure but instead pictures a kind of emergent gravity from a similarly emergent and evolving context?

Anyone know what the closest fits among existing programs this would be?

Edit: Wth background structure I'm not referring specifically to background spacetimes of any dimensionality. I'm talking about generic information structures, supposedly placeholders encoding the the deeper reality.

/Fredrik

 

[ccdantas]

http://arxiv.org/abs/gr-qc/0604075

 

[Naty1]

In the introduction to the above paper, diagram 1.1 declares spacetime is fundamental to both string theory and loop quantum gravity. Can anyone explain how spacetime in both these approaches is "fundamental"? I thought string theory assumed a fixed spacetime while in LQG a dynamic spacetime emerges from the theory analogous to general relativity.

I interpret Fra's question to be directed at the latter type.

From the referenced paper:

...In this section (1.2) we review to approaches to quantum gravity that differ
in the way they view time. The first approach comes from solid state
physics; the second comes from quantum information theory...
(regarding the first)Unfortunately the mass of the graviton is not generically zero. Instead the parameters of the model have to be chosen in a special way to make the mass vanishingly small

Isn't this just the kind of fine tuning that makes so many approaches unsatisfactory? (Not that some inights can't be gained.)

Second:
1.2.2

A completely different approach is the one proposed by S. Lloyd [3]. For
him the universe is one giant quantum computation. The problem of
quantum gravity is then to show how a quantum computation gives rise
to a spacetime...Given a quantum computation we arrive at a superposition of discrete
spacetimes. The question arises of what the right calculation is.

Sounds a bit closer ??

We want to stress one point here that all the proposals so far have in
common. For all of them spacetime and matter arise together. They
can not be separated.

Now we are talking! It's about time this is explicitly stated as it is not always appreciated on this forum.

I got lost past this point, but the Conclusions 1.4, page 12 are worth a look.

 

[Naty1]

In Leonard Susskind'sTHE BLACK HOLE WARS, 2008, he said in passing that Einsteins relativity equations can be derived from...and gave two or so fundamental theories...

I can't for the life of me find that passage but maybe someone else can. This would seem to be another possibility relating to your question...I'll look more but it seems to have slipped away.

 

[Naty1]

My first search (ever) at arXiv at http://arxiv.org/find/physics

using "emergent spacetime" turned up 25 titles...

but these are articles rather than reviews...


might be worth checking out.
emergent general relativity turned up 4 including the above referenced one..

maybe " emergent gravity" ??

This one sounded interesting:

http://arxiv.org/abs/0902.0035

6 pages, A contribution for the Proceedings of the Workshop on Cosmology and Particle Astrophysics 2008

Dark Energy and Emergent Spacetime
Authors: Hyun Seok Yang
(Submitted on 2 Feb 2009 (v1), last revised 30 Apr 2009 (this version, v3))
Abstract: A natural geometric framework of noncommutative spacetime is symplectic geometry rather than Riemannian geometry. The Darboux theorem in symplectic geometry then admits a novel form of the equivalence principle such that the electromagnetism in noncommutative spacetime can be regarded as a theory of gravity. Remarkably the emergent gravity reveals a noble picture about the origin of spacetime, dubbed as emergent spacetime, which is radically different from any previous physical theory all of which describe what happens in a given spacetime. In particular, the emergent gravity naturally explains the dynamical origin of flat spacetime, which is absent in Einstein gravity: A flat spacetime is not free gratis but a result of Planck energy condensation in a vacuum. This emergent spacetime picture, if it is correct anyway, turns out to be essential to resolve the cosmological constant problem, to understand the nature of dark energy and to explain why gravity is so weak compared to other forces.

 

[friend]

Does any of you knowledgeable people know of any an reviews of various "emergent gravity" programs? Ie. something that focuses on the different ways to do it.

Emergent from what? I think that the only thing that classical spacetime can "emerge" from is some sort of quantum formula. So I think your question is the same as which efforts start with a quantum formula of gravity to produce classical spacetime. I'm not aware of any programs that seek to actually justify the quantum formulation of gravity/geometry. Don't they all just assume a quantum theory of gravity and seek to be justified only if they produce GR at the classical level? But I think this is a mistake, because we don't know if GR needs to be modified a bit to resolve all the issues. If we are simply trying to reproduce unmodified GR, then our quantum assumptions might be flawed. The more straightforward approach is to start with quantum assumption that are fully justifiable that do lead to GR whether in its classical form or a modified version. But any theory that can actually justify ANY quantum description from more fundamental principles would probably justify QM, GR, QG, and is probably a good candidate for a TOE.

 

[Fra]

http://arxiv.org/abs/gr-qc/0604075

Thanks! I will print and read. I have applauded part of Dreyers reasoning before, so I am pleased to see that you found a review written by him!

/Fredrik

 

[Fra]

We want to stress one point here that all the proposals so far have in
common. For all of them spacetime and matter arise together. They
can not be separated.

Now we are talking! It's about time this is explicitly stated as it is not always appreciated on this forum.

I agree completely. I haven't read the review yet but this comment is signature of Dreyers reasoning and I I fully share that view, that pure gravity theories w/o matter isn't just incomplete, I don't think it can be consistent to separate it, in particular not if you are into the emergent reasoning.

/Fredrik

 

[Fra]

Emergent from what?

Yes that's the question, that caused me to ask for a review.

But any theory that can actually justify ANY quantum description from more fundamental principles would probably justify QM, GR, QG, and is probably a good candidate for a TOE.

Yes, I don't think we should look for an emergen pure gravity. The emergence, should explain (this is at leat my ambition): quantum logic, the observer matter actions in nature (~standard model), and the spacetime and gravity actions.

I have my own expectations, but with the review I hope to see an overview of what current programs that might be closest to my own preferred reasoning.

My preferred answer to emergent from what, is a information theoretical contextual model, where the context represents an observer, which is always evolving in an unknown environment. The explanation of laws (including gravity) should then be pretty much analogous to a darwinian type of information processing. Unfortunately I have not yet found a current program that does this. I think Dreyers internal view is a key, but more keys are needed. The evolving logic of smoling, is what is needed if you reject a universal deep microstructure. Because the only satisfactory solution is then to let the context itself be a result of evolution. All we need to explain is some of the logic of this evolutionary game, and I think will need to be done in terms of interactions between information structure, in a way that is described only from the inside, without reference to an external context. Or at least with a minimal connection to an external context. Current models have _massive_ external context, and if you "pick a context", no wonder you end up with fine tuning and initial value problems. I think smolin has tried a few times to convey that point.

/Fredrik

 

[Fra]

http://arxiv.org/abs/gr-qc/0604075

About Olaf Dreyer, it is interesting to note that he got large FQXi grants both 2006 and 2008.

http://www.fqxi.org/grants/large/awardees/list

/Fredrik

 

[Fra]

I read Dreyers review, but it was a very brief and limited scope review.

I'm curious to see what Dreyer comes up with in the future.

The traits about his reasoning I like is

- do NOT assume Einsteins Equations, they are likely to follow once the physics is properly formulated from the inside view (He states this as a conjecture, although clearly this "conjecture" must be seen as defining his program, ie. the task is to if not proove, at least provide strong arguments showing how his inside view yields relativity)

- His strong emphasis on insisting on that matter and spacetime belong together, and thay any separatiion likelt to fail. He suggests that the power of this approach is most obvious on tow points, understanding the meaning and origin of hte cosmological constant, and resolving the problem of time.

These are two points where I am in agreement. But that said, since his programs is very much starting up, it's too early to judge. Some of his ways of putting things makes me think that I might have disagreement on somethings.

In the introduction to the above paper, diagram 1.1 declares spacetime is fundamental to both string theory and loop quantum gravity. Can anyone explain how spacetime in both these approaches is "fundamental"? I thought string theory assumed a fixed spacetime while in LQG a dynamic spacetime emerges from the theory analogous to general relativity.

I interpret Fra's question to be directed at the latter type.

One of the things where I'm not quite sure what Dreyer means is with his use of fundamental. I think that with fundamental he is referring to "observer independent" or rather that all observers would share the same fundamentals.

I would otherwise perhaps distinguish between objectively fundamental (which in essence is a kind of realist view of thte matter) or subjectively fundamental, as fundamental to one observers action, but not necessarily to another observer.

Anyway, LQG assumes with a fixed dimensionality of the spacetime manifold. I would certainly expect the dimensionality of space to be explained.

But this really raises the question friend asked - emergent from what? I didn't feel Dreyer addressed that enough.

I would personally want to make the arguments Dreyer tries to make, stronger in the sense that it's not just about wether spacetime is fundamental, and wether time is fundamental, it's WHAT (if anything) is fundamental?

Here is a simplified perspective of mine...

In string theory, the string actions is somehow a fundamental starting point. An assumption of the "microstructure" of the world. The choice of this action is ambigous at best. But it's still unclear to me where the observer is. To take string theory, and think of it in terms of ordinary QM or QFT doesn't make senes to me, and it's not just the technical problems.

In LQG, they try to start from a 4D spacetime and make a ordinarly quantum theory out of it, whatever that means - I mean where is the observer?? This is IMO, directly in contradiction to what I think of as the "intrinisic measurement perspective" - I am not sure to what extent Dreyer share this.

CDT starts with the Einsteins action, but again, where is the observer? and where did Einsteins action come from?

I think several approaches does not take the measurement perspective serious. I think we need an intrinsic inside view of measurement. Where the context of the measurement is the observer. And an observer is a real physical system. Normally matter - Deryer calls this "coherent degrees of freedom".

So a given system of coherent degrees of freedom, with relations, could be the "observer". And it's in this context, the entire theory - including the probability space - must be encoded. This is why I think we need to reconstruct the continuum. I don't know to what extent Deryer has this in mind.

What I'm suggesting is that an observer with a fixed number of coherent degrees of freedom, can not make sense out of what the heck a continuum is, and even less a continuum probability? Instead, there are only a certiain combinatorics, and this will be reflected in the systems action. IE the system will behave "as if he doesn't make sense out of the continuum".

This would suggest not only an inside reconstruction of the ordinary spacetime stuff, but also the specific stuff of measurement theory, in particular probabilities, or measure of expected change. These should be intrinsically justified mesures.

IF we can pull that off, we will reconstruct spacetime and the instrinic measures, and thus actions (and thus the properties of matter) in parallell, just like Dreyer suggests.

But even in this somewhat now constrained direction, there are still mroe choces to be made than the ones Dreyer mentions. I think Dreyer works on somemthing he calls quantum space, I'll try to see if he has published anything or if it's still in progress. I think his future research should reveal his choices.

/Fredrik

 

[Fra]

About strings and LQG, to clarify. With them taking spacetime as fundamental Deryers means that the notion of the spacetime manifold are put in manually form square one. Ie. there is no REASON for it, it is just assume there is a spacetime. In ST it's a higher dimensional spacetime, but a given one nevertheless - at least at the present.( I can see some hopes on there but it's not my caes to argue for that.)

Edit: OTOH, in defense of string theory, IF string theory ever gets to properly interpret what these higher dimensional stuff is, and IF it's simply interpreted to not describe in an universally observer invariant way the UNIVERSE as a whole, but instead just the observers view of the universe, and that as different observers interact, they might well disagree on the spacetimes, and this simply results in interactions and selection for spacetimes. I think it's in this direction some dreams are where string theory could eventually make sense, and if that's so, i think it would be fair to say that ST is more emergent than LQG. I definitely see the logic. But I think the reformuilation necessary for that is so large, that it would be unfair to label that string theory since it would be totally alien to the original reasoing that gave rise to string theory.

In contrast a fully emergent spacetime, does not just start with a manifodl and gets emergent geometry - this is a PARTLY emergent behaviour. In that sense LQG is more emergent than ST since it assumes to background metric for the formulation.

But Dreyers wants more. Start with just degrees of freedom, with no manifold structure at all, and let the manifold define itself from the bare degrees of freedom, in this case the dimensionality should be fully dynamical and not be hardcoded. it's matter of evolution if we get a 2D, 4D or 11D or whatever manifold, or for that matter if we get a clean manifold at all! It's not a priori obvious at all that these old manifold abstractions are the way to do.

I personally think that we will get groups of different manifolds interacting, but that this system of manifolds is not the universe, but rather it's the observer, which is indirectly an image of the universe - but an instrinsic such.

The degree of fundamental, or degree of emergence and emergent from what is similar to the background independence debate. The point is that most BI approaches, still have background structures, even though if it's not literally a background metric. But then i never got what's the obsession with metrics. Metric information is just one among all inforamtion, all that should be subject to measurement processes.

/Fredrik

 

[Fra]

About Olaf Dreyer, it is interesting to note that he got large FQXi grants both 2006 and 2008.

http://www.fqxi.org/grants/large/awardees/list

/Fredrik

About Dreyer.

http://www.fqxi.org/data/articles/Dreyer_Olaf.pdf

A brief fqxi news article on Dreyers new ideas on a new approach and his grants.

This doesn't answer any questions of his approach though, but it briefly describes the new angle.

I couldn't see that there are much published yet. I found some other papers from him but they seem to more or less contain the same info. One might hope that in the future, some interesting papers may come from him.

/Fredrik