1. Jul 9, 2010

### 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?

2. Jul 9, 2010

### 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.

Last edited: Jul 9, 2010
3. Jul 11, 2010

### Naty1

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

4. Jul 11, 2010

### vacuumcell

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

5. Jul 12, 2010

### Demystifier

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.

6. Jul 13, 2010

### Naty1

7. Jul 14, 2010

### Demystifier

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

8. Jul 14, 2010

### 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.

9. Jul 14, 2010

### Dmitry67

Gravity as Entropic force, MWI and Born Rule

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

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 doesnt work in such odd branches. Gravity works only in branches where entropy increases = where Born rule is respected. Voila!

10. Jul 14, 2010

### MTd2

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 doesnt 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.

11. Jul 14, 2010

### Naty1

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.

Last edited: Jul 14, 2010
12. Jul 14, 2010

### 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?"

13. Jul 14, 2010

### MTd2

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.

14. Jul 14, 2010

### ensabah6

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?

15. Jul 14, 2010

### 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...

16. Jul 14, 2010

### ensabah6

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

17. Jul 14, 2010

### marcus

Quantizing "it" means quantizing geometry.

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.

18. Jul 14, 2010

### rod_worth

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.

19. Jul 14, 2010

### Naty1

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.

20. Jul 14, 2010

### 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.

21. Jul 14, 2010

### ensabah6

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.

22. Jul 14, 2010

### Naty1

Smolin's Jan 2010....
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.

23. Jul 14, 2010

### MTd2

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.

24. Jul 14, 2010

### rod_worth

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!

Last edited: Jul 14, 2010
25. Jul 14, 2010

### 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.