Friday, February 12, 2010

Gravity as an Entropic Force

UPDATE, Feb. 18, 2008 - Lubos Motl has tossed another log on his Verlinde stake-burning here. The paper it refers to was just released today by Nature Magazine by Holger Muller, Achim Peters, and Stephen Chu, "A precision measurement of the gravitational redshift by the interference of matter waves". Since I do not subscribe to Nature, I cannot read the paper. Do any subscribers to Nature here care to comment, before Lubos weighs in?

I never thought I would see a cutting edge theory that would unite Lubos Motl and Peter Woit in agreement (they hate it), but here it is:

From Wikipedia, entry: Gravity as an entropic force:

The hypothesis of gravity being an entropic force has a history that goes back to the studies on black hole thermodynamics by Bekenstein and Hawking in the mid-70s of the twentieth century. These studies suggest a deep connection between gravity and thermodynamics. In 1995 Jacobson demonstrated that the Einstein equations describing relativistic gravitation can be derived by combining general thermodynamic considerations with the equivalence principle.[1] Subsequently, other physicists have further explored the link between gravity and entropy.[2]

Verlinde's statistical description of gravity as an entropic force leads to the correct inverse square distance law of attraction between classical bodies.

In 2009, Erik Verlinde disclosed a conceptual theory that describes gravity as an entropic force.[3] This theory combines the thermodynamic approach to gravity with 't Hooft's holographic principle. If proven correct, gravity is not a fundamental force, but an emergent phenomenon which arises from the statistical behaviour of microscopic degrees of freedom encoded on a holographic screen.[4]

Verlinde's suggestion of gravity being an entropic phenomenon attracted considerable media[5][6] and weblog[7][8][9][10] exposure, and led to immediate follow-up work in cosmology[11][12], the dark energy hypothesis[13], cosmological inflation[14] and loop quantum gravity[15]. Also, a specific microscopic model has been proposed[16] that indeed leads to entropic gravity emerging at large scales.


Steven Colyer said...

A different way to look at the laws of Thermodynamics are as follows:

- First law (conservation of Energy):
You cannot win.

- Second law (Entropy never decreases*):
You can only lose.

- Third law (achievement of Absolute zero is impossible):
You can't get out of the game.

(Cheerful, ain't it? But that's our Universe! Kinda makes you wonder if the "deity" everyone talks about isn't the one with 2 horns, a spiked tail, and bat-wings rather than the one with the halo around his/her/its head. Hmm.)

I wish I could take credit for that, but that's from page 39 of "50 Physics Ideas You Really Need To Know" described 3 articles below this one.

* There's also a Zeroth Law, which is boring, and yes quantum fluctuations can make Entropy increase on rare occasions.

Jérôme CHAUVET said...

Oh Gosh! I just realize that this e-print, which I found interesting and innovative, was in fact directly linked to Verlinde's theory... Well, it's seems like not all superstring theorists are lame!

Jérôme CHAUVET said...

In fact, this idea sounds quite natural to me... It must be my education in biochemistry : this is the kind of explanation I would give firstly, in fact.

Good that some physicists look through the little holes in the walls between physics and other fields.

Cross-fertilization will be the key.

Steven Colyer said...

There are now 24 papers (as of today) that cite Verlinde's paper. It's anyone's guess how many will link to it eventually. Woit bemoans the "high school mathematics" and seems quite cranky in his latest replies section (link to his name in the main article). Lubos just goes ballistic, as Motls often will. Seems when a String Theorist leaves the faith, this is a greater sin than being a mere infidel.

Jérôme CHAUVET said...

24 papers soon!... But the paper is only in pre-published form since not long ago!

I guess this can be considered as being a peer-reviewing by facts, or better say an "effective peer-reviewing", which I like better than the old system, i.e., peer-reviewing by authoritary statement.

Steven Colyer said...

What we have is pre-printing by sponsorship, but Cornell has gotten full of itself, or Jacques Distler has gotten to them, or somebody. I heard Lee Smolin can't sponsor pre-prints on arXiv anymore. Now do NOT quote me on that, that is a rumor and take all those things with huge grains of salt. NOR am I dissing Distler, nor am I accusing him personally (it could be Susskind). All I'm saying is that IF Smolin can't sponsor someone, then some String theorist, somewhere, has gotten to Cornell, and that's sad.

But as you say, better than the old days. Even Susskind's first submission on strings was rejected by Physical Review D, in response to which he got roaringly drunk. Good old Lenny, always good for a laugh. :-)

Back on topic ...

In Lee Smolin's "Three Roads to Quantum Gravity" two of the roads are Loop Quantum Gravity and Superstrings Theory, but the 3rd and least mentioned is Black hole Thermodynamics, so this subject is not new.

As a Mechanical Engineer (Pi Tau Sigma member, thank you), I can ASSURE you THIS subject (Entropy) is OUR baby, so I'll be all over this in the next month. While Peter Woit is handing out beaded necklaces to drunken college girls in "New Orleans at Mardi Gras this weekend, I'll be dragging out my old textbook and notes and going over this stuff from first principles (good old Rudolph Clausius).

The question is, who will be having the most fun?

Jérôme CHAUVET said...

Yes, there are strange things like
this, where none of the two owns the paper, but Lee Smolin's last upload is okay: he can endorse.

The name of the superstring theorist, which makes ink flow about his unofficial censorship policy is in fact Ginspar.

Jérôme CHAUVET said...

From reading his post on the subject, I say Woit is rather nasty on Verlinde's approach. But why?... Because it comes from simple and well-known laws...

The stupid part is about the statement that the needed Mathematics for a correct explanation of gravity should be complicated, i.e., not using simple Mathematics. Where is that men living? If Maths are not complicated, then it can't be correct... Waooo, what a scientific statement, indeed.

Steven Colyer said...

Yes, well Peter is a former particle physicist who knows his Standard Model and works in Columbia's Math Department, so he knows his stuff. But I think I know what you mean. The Standard Model has its flaws, to be sure. Its Mathematics are an order of magnitude tougher than Quantum Mechanics, and Stringy Theory Fantasy is an order of magnitude harder yet, but that's not what I'm talking about.

What I mean is the Standard Model has considerable problems: to whit, it can't explain the masses of the fundamental particles (it ignores gravity so duh), it can't (as far as we know) be reconciled with General Relativity, yet, if ever (ain't looking good), and it can't explain why SU(3)xSU(2)xU(1) symmetry explains the quantum world's behavior, though it does kick ass on the "How?" It will probably be discovered to be a special-case approximation of something greater, something yet to be discovered. Peter's high on the mathematics of Langlands, for example, as is Witten, his hero (in a non-stringy way).

In the long run Peter is taking the "as things get smaller the math gets more complicated" attitude to the limit, and if he can't see the forest for the trees, then he's wrong. But maybe he's not wrong and has a point as well.

In any event I strongly suggest reading his "Not Even Wrong" book if you haven't done so. He lays out the case for the need for new and better Math very strongly and quite logically, and leaves the reader wanting for more.

I too want "reality" to get simpler, not more complex, the deeper we dig. I want this very much. But we do not know at present whether or not it does.

Jérôme CHAUVET said...

The piece of sentence I was pointing out was this one from Woit's site:

"[...]based on vague ideas that often use not much more than a few pieces of high-school level physics and mathematics."

To me it means that though one does not know the correct answer at all, one is however supposed to know what it looks like prior to its effective discovery.

Thinking so is nothing else but speculation, at best conjecturing.

It's just like saying that Newton's gravitational force formula "F = G.(m1.m2/r^2)" (imagine it was discovered yesterday) does provide NO relevant information about gravitation because it is too simple to encompass all the complexity we know to exist throughout the universe.

If we had a Standard Model as good as the one we have in 2010, but no idea of what gravitation is, then the formula above would be essential though being so poor regarding the mathematics of the present time.

This is in fact the oldest issue in theory ever: How far can we generalize our models?

The fact is that nobody ever tested Newton's law on EVERY object in the Universe, as there are too many of them. Should then one reject this idea for this reason?

I fear that so many theorists forgot that the only to validate a theory is to see if it stands for long the test of experience.

As long as it works, one is not supposed to reject it. Whithout the test of experience, the theory is only an option.


Jérôme CHAUVET said...

And many options are better than a few.


Jérôme CHAUVET said...

Here, Motl has proposed an experience which would demonstrate that Verlinde's theory can't hold. The state degeneracy advocated by the theory would impair interferences of the double-slit experiment in gravitational field, of which we know it is wrong.

If Motl is right, then Verlinde's theory has no future...

Jérôme CHAUVET said...

The problem in Motl's interpretation is perhaps that the huge entropy in the position of the neutron does not necessarily means entropy in the wavelength (which makes the interference patterns the way it is). I mean, if Verlinde's theory does provide a protection on that point, then it's okay.

Maybe I should dig in Verlinde's article. If you find the time to read it and tell me what you think, thank you :)

Steven Colyer said...

Good thoughts and points, Jérôme, except be very, VERY careful in reading anything Motl says about String Theory, or theories that threaten it. Lubos is quite obviously NOT objective about these subjects. He considers himself the keeper of the faith and the defender of the realm so to speak on these subjects.

But OBJECTIVITY is what Science is all about! I don't mind people defending their positions, that's fine and very often careers in the form of continued employment depend on doing so. But there is a fine line between a reason and an excuse, and when a Scientist loses objectivity, is it fair to call him or her a Scientist anymore, regardless of the sheepskins on the wall or the awards in the bank?

So rather than being the "White Knight" that he thinks he is, he may be in fact a "devil" in the sense he obfuscates issues by mixing truths and untruths, and no not intentionally. I would never accuse Lubos or anyone else of intentionally lying. But "cognitive dissonance", that is to say lying to oneself, that is to say rationalization, is a failing we must all of us be on constant guard against regarding our own thoughts, as it seems to be a natural human flaw. Among many.

So yes, if there is an experiment that can prove Jacobson/Verlinde wrong as Lubos suggests, let it be done.

And yes as I stated, I will probably devote the next month of my life to this subject, which means other subjects that interest me, such as QCD, Quantum hall effect and other solid-state condensate Physics, Nonlinear dynamics, and your paper, will have to be put on my backburner.

Again. (sigh)

Phil Warnell said...

Hi Steven,

I can see that you’re thoughts on things more generally are coming to congeal. I would hope howver that this doesn’t have you to believe things have been resolved. The fact is no matter how well Verlinde has argued as there being no need of a continuous background, it still rests upon it requiring to be relevant to what’s known as the holographic screen. This to me is simply to have the holistic continuum of space-time denied as then only to be as a result rep[laced by another called instead the holographic screen. Simply put a piston can neither rise nor fall in corresponding reaction unless compared to what is changing in entropy is in some way confined by something that has it to be a closed system. So whether this be referred to as space-time or a holographic screen it must be of a form and character completely different then what it has to be contained. That is at least from how I can have it to be logically consistent.



Steven Colyer said...

Hi Phil,

You are awesome, Phil.

Hey Phil, two of the greatest things in my life that I look forward to doing are going through your blog and leaving at least one comment on every page, and the same after I finish yours: Plato's (the blogger) 400+ article blog as well.

I've been pretty busy though, with 2 teenagers at home and 2 more kids in college 12 miles away, so I have been a bit remiss in doing that stuff which is my passion.

I respectfully ask for your patience. Your weblog "What Is Einstein's Moon?" is worthy. Right up there with BackReAtion and Cosmic Variance and Not Even Wrong, in my book. Plato's "Dialogos of Eide" as well.

Phil Warnell said...

Hi Steven,

I confess I’m flattered you would find any of my thoughts valuable and of course as I find the comments you have offered. Yet I wouldn’t have anyone to think my contributions are as valuable as what can be gathered from serious blogs such as Backreaction or Cosmic Variance, as they are written by those that have a firm grasp of their respective subjects, as well as having made their own mark upon them. My writings are intended simply to provoke the thought process more generally as to what science has revealed and holds the promise of being capable of that many fail to realize. What I find interesting for instance is not so much how science gives us ways to find solution or solve problems, yet rather how it serves to lend a better understanding of the character of nature rather than its subsequent actions.

For example even though Newton was able to put together as to codify the actions of gravity, he had to concede that he could not explain why nature has such action when he said in the conclusion of his great treaties:

“But hitherto I have not been able to discover the cause of those properties of gravity from phænomena, and I frame no hypotheses; for whatever is not deduced from the phænomena is to be called an hypothesis; and hypotheses, whether metaphysical or physical, whether of occult qualities or mechanical, have no place in experimental philosophy.”

However we find Einstein was neither discouraged nor deterred by Newton’s failure, as to go further to discover the cause that Newton was unable to find and have it consistent with science’s demands. So for me it demonstrates that science is not limited as many imagine to only being able to answer the what and the how questions, for which we find common utility, yet also the why which has us understand the true value to be found only in the power reason.



Steven Colyer said...

UPDATE, Feb. 18, 2008 - Lubos Motl has tossed another log on his Verlinde stake-burning here. The paper it refers to was just released today by Nature Magazine by Holger Muller, Achim Peters, and Stephen Chu, here. Since I do not subscribe to Nature, I cannot read the paper. Do any subscribers to Nature here care to comment, before Lubos weighs in?

Jérôme CHAUVET said...

any subscribers to Nature here care to comment, before Lubos weighs in?

You should ask Bee or Stefan for the article. They work in research labs and automatically have access to Nature's article.


Steven Colyer said...

Got it, thanks. I'll be surfing Lubos' blog on the subject for a bit. That'll teach me where to zero in.