Is Entropic Gravity the Future of Physics?

[Demystifier]

This seems to be a strong argument against the Verlinde proposal that gravity is an entropic force:
http://xxx.lanl.gov/abs/1004.0877

What do you think?

 

[MTd2]

What is the problem with gravity having a negative temperature? A lot of systems also display this feature and this is something that should be expected from gravity, given that by definition of Verlinde's gravity also has a maximum entropy bound.

http://en.wikipedia.org/wiki/Negative_temperature

EDIT.:

Hmm, I just read the paper, and the authors consider this situation, but the fundamental quantity for gravity is entropy, not temperature. Also in matter, entropy is not a fundamental quantity and because of this, the relation between non gravity fields and gravity fields does not follow usual wisdom.

EDIT2:

Dr. Gero does not scare me. I am a SSJ4. Or 3, if you don't consider that cannon.

 

[Naty1]

I skimmed the paper but the math is above my paygrade...I hope some one will offer some additional insights...

 

[vacuumcell]

...the fundamental quantity for gravity is entropy, not temperature.

I haven`t read the paper, but my understanding is that any physical system with entropy must also have temperature and vice versa.

 

[Demystifier]

but my understanding is that any physical system with entropy must also have temperature and vice versa.

That is certainly wrong. A system out of thermal equilibrium does not have a temperature, but still has an entropy. Of course, one should distinguish thermodynamic entropy from a more general notion of statistical entropy.

 

[Naty1]

Here is a NY Times review of Verlinde's theory:
http://www.nytimes.com/2010/07/13/science/13gravity.html?_r=1&8dpc

Lots of comments from other physicsts...but I did not see anyone claiming the idea is "dead".

 

[Demystifier]

Here is a NY Times review of Verlinde's theory:
http://www.nytimes.com/2010/07/13/science/13gravity.html?_r=1&8dpc

Lots of comments from other physicsts...but I did not see anyone claiming the idea is "dead".

They were probably not aware of the paper above. In fact, this paper perhaps did not even exist when they said what they said.

 

[Max™]

If you want to keep inflation, holographic concepts, and the Einstein Field Equations, then yeah... right now the outlook ain't so good for Verlinde.

The math isn't too bad, mostly trying to find ways to incorporate a co-moving holographic screen, the various constraints on bit values, and the entropic gravity assumption in a manner which produces a derivation of the EFE's.

 

[Dmitry67]

Gravity as Entropic force, MWI and Born Rule

I read very interesting article:

http://arxiv.org/PS_cache/arxiv/pdf/1001/1001.0785v1.pdf

Starting from rst principles and general assumptions Newton's law of gravitation is shown to arise naturally and unavoidably in a theory in which space is emergent through a holographic scenario. Gravity is explained as an entropic force caused by changes in the information associated with the positions of material bodies. A relativistic generalization of the presented arguments directly leads to the Einstein equations. When space is emergent even Newton's law of inertia needs to be explained. The equivalence principle leads us to conclude that it is actually this law of inertia whose origin is entropic.

After reading the article, analyze the entropy from MWI perspective. In MWI, the entropy is linked to Born rule, if we ignore Born rule and look at ALL branches, the second law of thermodynamics is not valid - there are many branches when hot bodies absorb heat from the environment, etc.

Hence (if we assume the conclusions of the article) gravity doesn't work in such odd branches. Gravity works only in branches where entropy increases = where Born rule is respected. Voila!

 

[MTd2]

They were probably not aware of the paper above. In fact, this paper perhaps did not even exist when they said what they said.

The NY article seemed much more pessimistic than that paper. People there simply didn't acknoledge there was any clear idea behind Verlinde. Even he somehow acknoledged that.

But what I thought strange in the article it is that Verlinde claimed he would try to come up something from string theory to formalize his idea, whereas in the same article he says gravity doesn`t exist as a force. Well, the existence of gravitons was the justification to reinterpret string theory as a quantum gravity theory.

Besides these all, I am happy with his idea.

 

[Naty1]

The NY article seemed much more pessimistic than that paper. People there simply didn't acknoledge there was any clear idea behind Verlinde. Even he somehow acknoledged that.

In earlier discussions in other threads here, I think it was acknowledged that Verlinde's paper was an introduction with conceptual and mathematical ideas rather than a firm irrefutable and detailed step by step proof.

 

[rod_worth]

I've read Verlinde's paper "On the Origin of Gravity and the Laws of Newton". The first 23 pages are on how to derive Newton and Einstein's equations of gravity "as an entropic force caused by a change in the amount of information associated with the positions of bodies of matter." In the last couple of pages he then applies it to string theory. In my opinion, those 23 pages can stand without the remaining 4. He states early on (3.3) that: "Our starting point was that space has one emergent holographic direction. The additional ingredients were that (i) there is a change of entropy in the emergent direction (ii) the number of degrees of freedom are proportional to the area of the screen, and (iii) the energy is evenly distributed over these degrees of freedom."

I'm very well read, but I would say I understand about half of that. None-the-less, I was able to relatively easily follow his logic. The entire theory is based on the holographic principle, and I must admit, the logic he uses is rather elementary which is remarkable. I truly think he is on to something by deriving gravity from entropy and temperature (i.e. energy)

His paper leaves me with just one question which stems from the fact that I am very transparent about my die-hard hatred of string theory (I don't want to get into it), so my QUESTION becomes this: "CAN THE HOLOGRAPHIC PRINCIPLE EXIST AND/OR BE CORRECT WITHOUT A STRING THEORY AND/OR QUANTUM GRAVITY INTERPRETATION OF IT?"

 

[MTd2]

"CAN THE HOLOGRAPHIC PRINCIPLE EXIST AND/OR BE CORRECT WITHOUT A STRING THEORY AND/OR QUANTUM GRAVITY INTERPRETATION OF IT?"

Looks like so. If gravity is just a side effect of entropy, looking for a quantum theory of gravity is just wrong, or so it seems. So, no to string theory or LQG.

 

[ensabah6]

Looks like so. If gravity is just a side effect of entropy, looking for a quantum theory of gravity is just wrong, or so it seems. So, no to string theory or LQG.

Verlinde hopes to merge entropy with string theory

http://www.nytimes.com/2010/07/13/science/13gravity.html

At a workshop in Texas in the spring, Raphael Bousso of the University of California, Berkeley, was asked to lead a discussion on the paper.

“The end result was that everyone else didn’t understand it either, including people who initially thought that did make some sense to them,” he said in an e-mail message.

“In any case, Erik’s paper has drawn attention to what is genuinely a deep and important question, and that’s a good thing,” Dr. Bousso went on, “I just don’t think we know any better how this actually works after Erik’s paper. There are a lot of follow-up papers, but unlike Erik, they don’t even understand the problem.”

The Verlinde brothers are now trying to recast these ideas in more technical terms of string theory, and Erik has been on the road a bit, traveling in May to the Perimeter Institute and Stony Brook University on Long Island, stumping for the end of gravity. Michael Douglas, a professor at Stony Brook, described Dr. Verlinde’s work as “a set of ideas that resonates
with the community, adding, “everyone is waiting to see if this can be made more precise.”

Until then the jury of Dr. Verlinde’s peers will still be out.


"“We’ve known for a long time gravity doesn’t exist,” Dr. Verlinde said, “It’s time to yell it.” "


IF so then is quantizing it a viable project?

 

[MTd2]

Well, trying to quantize gravity does not make sense in this case, much less using string theory. He could mean try to quantize space time, without gravity, but that is not string theory, I think...

 

[ensabah6]

Well, trying to quantize gravity does not make sense in this case, much less using string theory. He could mean try to quantize space time, without gravity, but that is not string theory, I think...

in GR there are gravitational waves and in QM where there are waves there are particles.

 

[marcus]

IF so then is quantizing it a viable project?

Quantizing "it" means quantizing geometry.

in GR there are gravitational waves and in QM where there are waves there are particles.

Welldefined particles only arise from fields in special (such as flat) circumstances.
I think your statement is wrong generally speaking---perhaps you meant it as a joke. Fields can undulate and evolve dynamically all sorts of ways without that being resolved into particles.

 

[rod_worth]

"CAN THE HOLOGRAPHIC PRINCIPLE EXIST AND/OR BE CORRECT WITHOUT A STRING THEORY AND/OR QUANTUM GRAVITY INTERPRETATION OF IT?"

I just watched a lecture by Raphael Bousso from UC Berkeley on "The World as a Hologram" on YouTube, and the answer to my own question is 'yes'. Black hole entropy apparently tells us much about the structure of nature, and these arguments are not based on string theory; it's based on 'information' contained on a surface, like the surface of a room; for example, how many 'letters' (information) you can fit on a page (surface area). It's simply a different way of looking at the world, and is independent of 'assumptions' (watch the video) made by theories such as string theory. Taking that into account, Verlinde's argument seems even more impressive.

 

[Naty1]

"CAN THE HOLOGRAPHIC PRINCIPLE EXIST AND/OR BE CORRECT WITHOUT A STRING THEORY

At least some very prominent scientisits think so...you can get some really good insights into both aspects of this question via Leonard Susskind's THE BLACK HOLE WAR where he discusses development of both ideas. And Kip Thorne's BLACK HOLES AND TIME WARPS clearly develops the idea of entropy, information and black hole horizons without resort to string theory.

But Susskind's interpretation of some horizon effects via string theory is very insightful.

 

[marcus]

If gravity is just a side effect of entropy, looking for a quantum theory of gravity is just wrong, or so it seems. So, no to ... LQG.

Huh? Smolin's January 2010 paper deriving Newton from LQG in Verlinde's manner.

Showed LQG can provide a basis for the "entropic force" picture of Newtonian gravity.

So as far as we know LQG is compatible with Verlinde's derivation of Newton.

LQG and allied QG approaches are about geometry. For geometry to have entropy it MUST have microscopic degrees of freedom. This opens up a big field of investigation (where Loop is already an active program) namely what are the underlying degrees of freedom of geometric relationships. String researchers can be expected to follow suit.

Focus on spatial measurements like area and volume, not on "fundamental force" or "gravitons". The spin networks of LQG are the eigenvectors of the area and volume observables. So you are looking at a clear bid for the role of underlying DoF.

Of course different depictions, or pictorializations, are not necessarily exclusive. Feynman pointed out that your picture of quantum reality simply indicates how you plan to calculate. Rovelli expressed similar view. Two pictures/calculations can be compatible if leading to the same expectation values, same operator spectra.

 

[ensabah6]

Huh? Smolin's January 2010 paper deriving Newton from LQG in Verlinde's manner.

Showed LQG can provide a basis for the "entropic force" picture of Newtonian gravity.

So as far as we know LQG is compatible with Verlinde's derivation of Newton.

LQG and allied QG approaches are about geometry. For geometry to have entropy it MUST have microscopic degrees of freedom. This opens up a big field of investigation (where Loop is already an active program) namely what are the underlying degrees of freedom of geometric relationships. String researchers can be expected to follow suit.

Focus on spatial measurements like area and volume, not on "fundamental force" or "gravitons". The spin networks of LQG are the eigenvectors of the area and volume observables. So you are looking at a clear bid for the role of underlying DoF.

Of course different depictions, or pictorializations, are not necessarily exclusive. Feynman pointed out that your picture of quantum reality simply indicates how you plan to calculate. Rovelli expressed similar view. Two pictures/calculations can be compatible if leading to the same expectation values, same operator spectra.

Verlinde's argument requires the holographic principle. Is there a well stated version of this in LQG? Smolin makes assumptions like "smooth space" in his paper that remains unproven.

 

[Naty1]

Smolin's Jan 2010...

Showed LQG can provide a basis for the "entropic force" picture of Newtonian gravity.

It appears we have at the origin of a universe (or mutiverse) some combination of physical entities...entropy, time, geometry, time, causality, energy and maybe a few other things...maybe uncertainty,quantum foam, gravity etc,etc...I don't even understand if they would in theory be observable at the instant of origination (like a big bang)...

If anyone has determined that one or several of these is "fundamental" meaning it is the first to emerge and precipitates the others, I have not seen that yet. My only personal hesitancy is that quantum theory, incomplete though it may be, suggests there is no space and time at the tinest scales...Planck size stuff...so I personally wonder if geometry/time is the first to emerge...

We'll have to wait for a complete theory see is any of these entities naturally leads to the others...or whether that process itself is quantum in nature in which case maybe one chance leads to one type of universe, another virtually identical moment leads to a different result.

 

[MTd2]

Huh? Smolin's January 2010 paper deriving Newton from LQG in Verlinde's manner.

Showed LQG can provide a basis for the "entropic force" picture of Newtonian gravity.

Verlinde said that there is no gravity in the NYT article. So that pretty much rules out LQG and ST, even though there are coinciding results.

 

[rod_worth]

Verlinde said that there is no gravity in the NYT article.

In Verlinde's paper, he does not 'do away with gravity' in the sense that there "is no gravity" as you stated. He simply realizes what it 'truly' is, if you will, and states on page 9, "-the origin of gravity: it is an entropic force!"

Here is a quick summary of how he gets from temperature (thermodynamics) to deriving Newton's 'force' of gravity. He states (pg 8), "Of course it is well known that acceleration and temperature are closely related. Namely, as Unruh showed, an observer in an accelerated frame experiences a temperature

(Boltzman's constant)*Temperature = ('h'bar*acceleration)/2*$$\pi$$*c".

By simply using the F=ma relationship, he continues later on the same page, "The key statement is simply that we need to have a temperature in order to have a force." How he gets from this to Newtons gravity is contained within his paper in relatively easy to understand math, I just don't feel like re-writing his paper here when you can all read the detail in his paper for yourselves. Cheers!

 

[my_wan]

I haven't finished going over the OP paper, but if you define gravity as an entropic force then it seems to me that the way to overcome the negative temperature issue is to fully relativise temperature. Note how in GR the temperature would not define the gravitational force, rather the rate of change in temperature does. In standard GR it's called depth of field, and the classical acceleration force is the rate of change in the depth of field.

This also entails that an absolute temperature is meaningless, as the local temperature, as defined by a local observer is always constant, irrespective of depth. Only the rate of change in [temperature] the region of an observer is meaningful. If the expansion and contraction (geometry) was defined by global variations of temperature, which locally defined the intervals (perspective) unique to an observer, then what is a negative temperature from one perspective is merely a ground state temperature for another. This would naturally wash out the supposed vacuum catastrophe for any local observer.

If we take gravity as an entropic force as anything more than a toy model, then these relativistic symmetries must be honored. Thus temperature would add, from an observer perspective, in a relativistic manner. SR does this as an inverse relation between ranges 0 to 1 and 1 to c, where 1 defines an observers local perspective (temperature). This is a bit paradoxical for event horizons of black holes.

 

[marcus]

Huh? Smolin's January 2010 paper deriving Newton from LQG in Verlinde's manner.

Showed LQG can provide a basis for the "entropic force" picture of Newtonian gravity.

So as far as we know LQG is compatible with Verlinde's derivation of Newton.

LQG and allied QG approaches are about geometry. For geometry to have entropy it MUST have microscopic degrees of freedom. This opens up a big field of investigation (where Loop is already an active program) namely what are the underlying degrees of freedom of geometric relationships. String researchers can be expected to follow suit.

Focus on spatial measurements like area and volume, not on "fundamental force" or "gravitons". The spin networks of LQG are the eigenvectors of the area and volume observables. So you are looking at a clear bid for the role of underlying DoF.

Of course different depictions, or pictorializations, are not necessarily exclusive. Feynman pointed out that your picture of quantum reality simply indicates how you plan to calculate. Rovelli expressed similar view. Two pictures/calculations can be compatible if leading to the same expectation values, same operator spectra.

Verlinde's argument requires the holographic principle. Is there a well stated version of this in LQG?...

The area-entropy relation is required and LQG has this, pointed out as far back as 1995.

http://arxiv.org/abs/1001.3668

"In this paper I show that a version of Verlinde’s argument can be run in loop quantum gravity (LQG). This strengthens the case for taking LQG as a candidate for a quantum theory of gravity. Previously it has been shown that the theory has massless spin two excitations[12], which have the correct propagators in the low energy limit[13], but there has not before been a direct demonstration that the theory has a limit which yields Newtonian gravity.

Indeed, once one sees Verlinde’s strategy, its application to LQG is immediate, as it relies on a realization of the area-entropy relation within LQG that has been known for many years[15]. Before giving the details, it may be helpful to sketch the reasons for this.

The story goes back to papers of Crane[16], which anticipated ’t Hooft’s formulation of the holographic principle. Crane proposed that in quantum cosmology, Hilbert spaces should be associated not with the whole universe, but with any choice of a boundary that splits the universe into two parts. The idea was that the observers and their measuring instruments live on one side of the boundary, and they observe the quantum gravity dynamics on the other side by means of measurements made on the boundary, and recorded in a boundary Hilbert space..."
================

BTW this may be of wider interest: I see that in a footnote here Smolin points out that Padmanabhan also gets Newton's law, but in a different way:
Footnote 1: "Verlinde and Padmanabhan’s argument both lead to Newton’s law but the logic is different. Both use the equipartition relation, but Verlinde employs the notion of an entropic force, while Padmanabhan gets the gravitational acceleration by inverting Unruh’s relation between temperature and acceleration."
=================
I'm not sure that all the concern over the recent paper of Miao Li, Yi Pang (http://arxiv.org/abs/1004.0877) is warranted. Newton's law is not the ultimate grail after all. There are doubtless several ways to derive it. Ted Jacobson derived the more fundamental Einstein equations back in 1995. It turns out that one can derive Newton from well-established results in LQG. No doubt Newton's law can be derived in several other ways.

The aim of Verlinde was not to derive inflation. It was simply to derive Newton gravity. A much simplified approximation to GR (the framework in which inflation is normally formulated). If the picture Verlinde used to derive Newton happens to run into trouble when you try to force inflation into it, so what? What is the big deal? It is just a derivation of Newton.

If Verlinde's overpublicised gambit runs into trouble (and I am not yet convinced that it does) this does not shoot down the whole Thermodynamic Gravity business. Probably the work of both Jacobson and Padmanabhan is deeper and more fundamental in any case. People pay disproportionate attention to Verlinde's public relations campaign with all the exaggerated hype.
And if there is some difficulty with Verlinde's derivation, give him time to modify it before pronouncing it "dead".

 

[atyy]

The aim of Verlinde was not to derive inflation. It was simply to derive Newton gravity. A much simplified approximation to GR (the framework in which inflation is normally formulated). If the picture Verlinde used to derive Newton happens to run into trouble when you try to force inflation into it, so what? What is the big deal? It is just a derivation of Newton.

Verlinde does attempt to derive the Einstein equations. http://arxiv.org/abs/1001.0785 : "A relativistic generalization of the presented arguments directly leads to the Einstein equations."

And Li and Pang claim a no-go for the derivation of the Einstein equations http://arxiv.org/abs/1004.0877 : "we prove that it is impossible to find a proper new definition of temperature to derive the Einstein equations."

Of course, the deep relation between gravity and thermodynamics is not in question, regardless of Verlinde's claim.

BTW, Crane's hypothesis that Smolin talks about is also discussed by van Raamsdonk http://arxiv.org/abs/0907.2939 "The mathematical structure that we observe in section 2 shares some features with an approach to quantum gravity called “relational quantum cosmology” [11], which also involves associating quantum states in a number of different systems with a single quantum spacetime. The association of specific Hilbert spaces to particular causal patches is also implicit in Bousso’s discussion of holography in general spacetimes [17, 18], and it is central to the holographic space-time proposal of Banks and Fischler [8]. The present work is significantly less ambitious than these others in the sense that we are not attempting to present a complete mathematical framework for quantum gravity. Rather, we wish to point out some mathematical structure that already appears in concrete examples from string theory, in the hopes that this structure may be part of a complete framework that could apply to the description of more general quantum spacetimes."

There's a general feeling that string theory is only a specific example of a theory of quantum gravity, and maybe there are things in string theory that are common to other quantum gravities, eg. Bredberg et al http://arxiv.org/abs/1006.1902 : "It is our hope that the attempt here to generalize fluid/gravity duality away from the stringy context to its most essential ingredients may be useful in understanding this triangle. Much of the current work on fluid/gravity duality attempts to learn about matter systems from gravity: we would like to reverse the arrow towards an understanding of quantum gravity. In this regard, there may be interesting connections with previous work in this direction including in particular [43,44,45]."

 

[marcus]

...
And Li and Pang claim a no-go for the derivation of the Einstein equations http://arxiv.org/abs/1004.0877 : "we prove that it is impossible to find a proper new definition of temperature to derive the Einstein equations."

Of course, the deep relation between gravity and thermodynamics is not in question, regardless of Verlinde's claim.
...

As a general claim about deriving Einstein from Thermo, Li and Pang do not prove that it is impossible as you will have been quick to notice

If you just quote from their abstract out of context, it sounds like they did. But what they say in their conclusions paragraph only applies to Verlinde's original paper's approach.

==quote Li and Pang conclusions==
In conclusion, we have shown that in the original derivation of the Einstein equations by
Verlinde, a negative temperature must be introduced for an accelerated expanding region,
thus introducing vexing physical problems. One may try to modify Verlinde’s derivation
by modifying the definition of temperature, and we have shown that no modification is
appropriate to generate the correct Einstein equations.
==endquote==

You and I have been over this ground before, I expect. Jacobson proved you could derive Einstein from Thermo in 1995, and his derivation has been repeatedly cited and never challenged AFAIK. If Li and Pang want to offer a significant no-go result they should take up the matter with Jacobson (not bother with Verlinde). In case anyone is joining the discussion late, I will fetch the 1995 abstract.

Of course, the deep relation between gravity and thermodynamics is not in question, regardless of Verlinde's claim.

Yes.

http://arxiv.org/abs/gr-qc/9504004
Thermodynamics of Spacetime: The Einstein Equation of State
Ted Jacobson
8 pages, 1 figure. Phys.Rev.Lett. 75 (1995) 1260-1263
(Submitted on 4 Apr 1995)
"The Einstein equation is derived from the proportionality of entropy and horizon area together with the fundamental relation $$\delta Q=TdS$$ connecting heat, entropy, and temperature. The key idea is to demand that this relation hold for all the local Rindler causal horizons through each spacetime point, with $$\delta Q$$ and $$T$$ interpreted as the energy flux and Unruh temperature seen by an accelerated observer just inside the horizon. This requires that gravitational lensing by matter energy distorts the causal structure of spacetime in just such a way that the Einstein equation holds. 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."

As a side remark, Rovelli's April paper points out that in LQG one does not quantize the Einstein equation. The aim is to find a consistent quantum theory of geometry which has the right classical limit. The original program of quantizing GR was a good starting point and provided scaffold/signposts. The analogy is that one wants to find a theory of the molecules in air and their motion from which the wave equation for sound in air arises in largescale limit. The April paper 1004.1780 has a brief clear statement of overall philosophy near the beginning.

 

[MTd2]

Marcus, the NYT is more recent than his paper, so I suppose he has a clearer picture on his mind. If he just says that there is no gravity, it means there is no gravity to quantize. So, it looks like one should look for how quantized fields affect space-time and forget quantum gravity.

 

[marcus]

Marcus, the NYT is more recent than his paper, so I suppose he has a clearer picture on his mind. If he just says that there is no gravity, it means there is no gravity to quantize. So, it looks like one should look for how quantized fields affect space-time and forget quantum gravity.

MTd2, my sense of the realities of research differs widely from yours. Perhaps, as with physical theories, we should make predictions about the future course of research. :-D

It looks to me as if you think Verlinde is important, a kind of weathervane. You pay attention to what he says to the media, like NYT. You try to interpret this, as if you believed that it would somehow show which way the wind is blowing or which are the fruitful directions for research.

Or you may be joking, I can't tell.

I tend to disregard Verlinde especially the attention-getting things he says to someone like Dennis Overbye, quotable stuff. From my perspective what is interesting here comes from Ted Jacobson, Lee Smolin, I expect Carlo Rovelli to have something to say about the relation of thermodynamics and gravity, and Thanu Padmanabhan. Maybe Smoot, maybe also Jerzy Kowalski-Glikman.

BTW did you see how the Rovelli Smerlak paper* (the geometric temperature = the speed of time) is doing in the 2nd quarter MIP poll?

The thermodynamics of geometry is a deep subject that needs to be intelligently developed, not just exploited as a last-minute gimmick.

A question for you, MTd2. Geometry has a temperature. Padmanabhan always points this out. The recent Rovelli Smerlak paper deals with geometric temperature a lot.
If geometry has temperature, how can it not have microscopic degrees of freedom?
Maybe you will have some simple answer. If so, I would like to hear it!

In any case, if you seriously credit the idea that one should not quantize geometry, then you must be willing to make predictions. Perhaps you would like to predict a decline in QG publication/cites. And an increase in research where one looks at "how quantum fields affect classical geometry".

Heh :-D

Or were you actually joking?

*Nearly half those in the poll so far voted for the Rovelli Smerlak paper, and it was out of a field of 15 candidates. Plus neither you nor I voted for it. Somehow a bunch of other people decided it was important. That is a thermodynamics+geometry paper that I think is truly interesting----only realized it recently.
https://www.physicsforums.com/showthread.php?t=413838
Rovelli Smerlak
Thermal time and the Tolman-Ehrenfest effect: temperature as the "speed of time"
http://arxiv.org/abs/1005.2985

 

[Dmitry67]

Question

How holographic principle is defined if we don't use null surfaces of the region - but regular ones, timelike? So normal boundaries we use (a box, a sphere...)

The problem I see: when 'region' is spacially extended, the position of the boundary (and the very notion of the region) becomes frame-dependent (in SR)

It is even worse in GR and very curved spaces. Talking about the Dark Energy and expansion (this is why I am asking this question) I don't see how 'region' can be bigger than a Hubble volume.

P.S. I assume that Dark Energy is enropic force associated with entropy of geometry of our spacetime?
If our space is so surprisingly flat, then it should tend to transform into more chaotic state...

 

[Fra]

I've been low on time lately, but IMHO, beeing am a advocate of a view that includes kind of "entropic forces" (as another expression of rational action which is the idea that the action of any systems basically follows "random" motion in a evolving state space) but I still see view Verlinde's reasoning so far so still lack the deeper pictures.

I think it's possible withfor alternative developments of the initiated route and the ultimate version of the arugments are still not on the table.

It seems all different people try to interpret or construct a connection to their advantage. I am so far not so much liking smolins connection. I would rather want to see a smolin connection to verlind á la R. Unger, rather than á la LQG.

/Fredrik

 

[MTd2]

Hey Marcus! Well, I was not joking. But saying geometry should not be quantized is not my idea, but Verlide's! That's what I understood. He could come out saying whatever he wanted in NYT, so that layman could understand, except for things like "X" does/doesn't exist. It`s like saying "cancer is not a unique desease" or "HIV causes AIDS". This is something that can be understood by anyone...

 

[Fra]

In any case, if you seriously credit the idea that one should not quantize geometry, then you must be willing to make predictions. Perhaps you would like to predict a decline in QG publication/cites. And an increase in research where one looks at "how quantum fields affect classical geometry".

I apologize if I miss the point as I haven't had time to read all the papers discussed but I seek at least two ways to interpret "one should not quantize geometry or gravity".

research where one looks at "how quantum fields affect classical geometry".

- One possibility is that "gravity" is simply a classical thing, that exists only (in a sensible way) in a classical limit and in the true quantum domain there is no gravity? In the sense that statistical laws doesn't exist at microlevel, and gravity is only emergent at classical and macroscopic level?

- The other possibility (to which I adhere) is that it's perhaps just to say that it's possibly the wrong approach to the problem of QG to start to look for a regular "quantization procedure" of a classical thing or field. But this is also a critique agains the entire framework of how QM is usually introduce - ie from a classical model, then applying a "quantization" trick. Instead, perhaps a proper reconstruction of a measurement theory, that does NOT start from a classical counterpart, will show that gravity is emergent from the "new quantum framework" itself, as a kind of entropic force. But in this view, IMO at least, the same must apply to ALL forces. ALL forces should be "entropic", the distinction between the various forces would then probably be a matter of classifying components of a generic "entropic force".

Note that even Ariel Caticha, who is working on inference models (a sort of version of entropic reasoing, but still different from Verlinde!) also have said that if he is right the entire approch to the "quantum gravity" problem are likely to need new grip.

This latter view is what I think, and it's why I think "quantizing gravity" in the regular sense is doubtful. But that's not to say that gravity is just classical, it rather means (to me at least) something different.

/Fredrik

 

[marcus]

David Berenstein is a string theorist who is still, AFAIK, pursuing string theory. He hasn't turned to last minute career-saving gimmicks of the "anything to get me out of this rut!" variety. Here's a 13 July 2010 blog post of his.

======quote======
Dirac Sea Shore — Gravity does not exist?

I happened onto an article on the New York Times abut Erik Verlinde’s take on gravity as an Entropic force. The article was written by Dennis Overbye who most of the time does a good job of covering high energy physics. Erik’s work dates from earlier this year and can be found here. To tell the truth, I don’t understand what he’s trying to say in that paper and to me it feels like it’s almost certainly wrong.

However, I don’t want to discuss that paper. What I want to discuss is the following provocative quote

“We’ve known for a long time gravity doesn’t exist,” Dr. Verlinde said, “It’s time to yell it.”

I don’t believe this is taken out of context, so we should take it at face value. The statement is obviously wrong, so it sounds like ultra-post-modern pap and makes all physicists working on the subject of quantum gravity look like crazy mad men. I’m sure this sells newspapers, but that is not the point.

When asked for a sound byte can’t people at least say something that is correct and not just provocative?

The proper way to write that statement is that “Gravity is not really a fundamental force “, which is more correct and does not deny gravity its proper place as something that has been observed in nature, however it is less catchy. If we apply the same criteria as used in the above construction, all of the following statements are also correct:

Hydrodynamics does not exist (it only happens for collections of atoms, but not for individual ones)
Space and time do not exist (often used when talking about quantum gravity being emergent from somewhere else)
All emergent phenomena do not exist (they are not fundamental after all).
I do not exist (I’m an emergent phenomenon)...
===endquote===
http://diracseashore.wordpress.com/2010/07/13/gravity-does-not-exist/
Incidentally, Berenstein may be looking for his own way out of the woods. See for example http://arxiv.org/abs/1001.4509