The paper: http://arxiv.org/abs/1001.0785 I read over the paper, and it is surprisingly readable for mere mortals. I don't know enough physics to comment on its accuracy as a model of the "real" world, but even as an abstract model it is very beautiful. Basically, Verlinde turns Bekenstein on his head. Verlinde rearranges the world so that gravity falls out of information/entropy instead of the other way around. Bekenstein showed that the universe is "holographic", meaning that _all_ the information about what is going on within a simple region of space is encoded in bits on its surface, and the _amount_ of such information is proportional to its surface area. Verlinde goes backwards and shows that gravity is an emergent concept from the statistics of microstates. The closest high school physics analog is an osmotic force across a membrane: the force arises because it is far more probable that the more concentrated fluid becomes less concentrated than the other way around. Interestingly, the Heisenberg constant h plays very little part in Verlinde's derivation, which treats entropy, temperature, etc. as continuous variables. This makes a lot of sense, as gravity itself is a macroscopic phenomenon. From a blog about Verlinde's paper: http://motls.blogspot.com/2010/01/erik-verlinde-comments-about-entropic.html Thursday, January 14, 2010 Erik Verlinde: comments about the entropic force of gravity Logic of the paper ... The starting point is a microscopic theory that knows about time, energy and number of states. That is all, nothing more. This is sufficient to introduce thermodynamics. From the number of states one can construct a canonical partition function, and the 1st law of thermodynamics can be derived. No other input is needed, certainly not Newtonian mechanics. Time translation symmetry gives by Noether's theorem a conserved quantity. This defines energy. Hence, the notion of energy is already there when there is just time, no space is needed. Temperature is defined as the conjugate variable to energy. Geometrically it can be identified with the periodicity of euclidean time that is obtained after analytic continuation. Again there is nothing needed about space. Temperature exists if there is only time. It is possible to introduce other macroscopic variables that are associated with a finite but still large subset of the microstates. Let us denote such a variable by x, at this point this is just some arbitrary choice. It can be any macroscopic variable that singles out a collection of the microscopic states. So specifying x in addition the to energy gives more detailed description of the microscopic states, but nothing more. So it is not even necessary to think about x as a space coordinate. Nevertheless one can define a number of microstates denoted by Omega(E,x) for given energy E and for a given value x for this macroscopic variable x. Next one can introduce a formal variable called F and introduce in the partition function as the thermodynamical dual to x. Following just standard statistical physics (I avoid the word mechanics, since Newton's law is not necessary) one can obtain the 1st law of thermodynamics. dE = T dS - F dx. This makes clear that F is a generalized "force", but it has nothing to do with Newton's law yet. It is defined in terms of entropy differences. The macroscopic force that is obtained in this way has no microscopic origin in terms of microscopic field. The force is entirely a consequence of the amount of configuration space and not mediated by anything. There is no space yet. The meaning of the statement that space is emergent is that the space coordinates x can be viewed as examples of such macroscopic variables. They are not microscopically defined, but just introduced as a way of singling out part of the available micro states. It is my impression that not all readers have understood or appreciated this essential point. Hence, if the number of states depend on x there can be an entropic force, when there is a finite temperature. This is all, nothing more. Again, for this point I don't need to assume Newtonian mechanics. It does not exist yet in this framework. The other central point paper is that if one chooses a macroscopic coordinate x that corresponds to a fixed position in a non-inertial frame, that Newton's law of inertia F = ma will be the consequence of such an entropic force. This has to be. There is no other way it can arise, simply because x is not a microscopic variable. It is obvious. Nevertheless, it is a fundamental new insight that has not been noted before. This is not an empty or circular statement. It says something about the way that the function Omega(E,x) should behave as a function of x. All this can be derived and defined without the input of Newtonian mechanics. The other formulas presented in the paper are just there to illustrate that indeed it is possible to get gravity from this kind of reasoning, and that it is consistent with the ideas of holography. But the main point concerns the law of inertia. The derivation of the Einstein equations (and of Newton's law in the earlier sections) follows very similar reasonings that exist in the literature, in particular Jacobson's. The connection with entropy and thermodynamics is made also there. But in those previous works it is not clear WHY gravity has anything to do with entropy. No explanation for this apparent connection between gravity and entropy has been given anywhere in the literature. I mean not the precise details, even the reason why there should be such a connection in the first place was not understood. My paper is the first that gives a reason why. Inertia, and hence motion, is due to an entropic force when space is emergent. This is new, and the essential point. This means one HAS TO keep track of the amount of information. Differences in this amount of information is precisely what makes one frame an inertial frame, and another a non-inertial frame. Information causes motion. This can be derived without assuming Newtonian mechanics. So the logic of the part of the paper dealing with inertia is: microscopic theory without space or laws of Newton ? thermodynamics ? entropic force ? inertia. The part that deals with gravity assumes holography as additional input. But this is just like what has been done before. It is also not the main point of the paper. Gravity in a way does not exist in Einstein's theory either. But one would like to recover the gravity equations. The logic here is thermodynamics + holographic principle ? gravity. The obvious question is of course, where does the holographic principle come from? Of course, it was extracted from the physics of black holes. But the holographic principle can be formulated without reference to black holes or gravity. Hence, it can be taken as a starting point, from which one then subsequently derive gravity. Again, this part is in essence not new. Jacobson followed exactly the same logic. This way of turning the logic of an existing argument around is done more often in physics, and it is known to lead to much more clear formulations of a theory. The example that comes to mind is the way that Dirac used the result of Heisenberg that p and q do not commute, which was obtained in some roundabout way, and made it in to the starting point for quantum mechanics. This is how it is being taught today. Anyhow, I hope this clarifies some points, and removes some of the misunderstandings. The essential new points of the paper I have noticed another point of the paper that is not appreciated in blog discussions. For many years, there have been previous works in the literature that discuss the similarity between gravity and thermodynamics. In particular in Jacobson's work there is a clear statement that if one assumes the first law of thermodynamics, the holographic principle, and identifies the temperature with the Unruh temperature, that one can derive the Einstein equations. This is a remarkable result. Yet it is already 12 years old, and still up to this day, gravity is seen as a fundamental force. Clearly, we have to take these analogies seriously, but somehow no one does. I studied the previous papers very well, and know about them for years. Many people have. We have seen a recent increase in papers following Jacobson, and extending his work to higher derivative gravity, and so. But from all of these papers, I did not pick up the insights I presented in this paper. What was missing from those papers is the answer to questions like: why does gravity have anything to do with entropy? Why do particles follow geodesics? What has entropy to do with geometry? The derivation of Jacobson does not take in to account the fact that the mass of an object and therefore its energy can change due to the displacement of matter far away from it. There is action at a distance hidden in gravity, even relativistically. The ADM and Komar definitions of mass make this non-local aspect of gravity very clear. This non-local aspect of gravity is precisely what the holographic principle is about. Jacobson's argument is ultra local, and assumes the presence of stress energy crossing the horizon. But there is no statement about an entropic force that is influencing particles far away from the horizon. My point of view is an attempt to take a much more global view, and map out the information over a bigger part of space, even though initially I can only do that for static space times. The statement that gravity is an entropic force is more then just saying that "it has something to do with thermodynamics". It says that motion and forces are the consequence of entropy differences. My idea is that in a theory in which space is emergent forces are based on differences in the information content, and that very general random microscopic processes cause inertia and motion. The starting point from which this all can be derived can be very, very general. In fact we don't need to know what the microscopic degrees of freedom really are. We only need a few basic properties. For me this was an "eye opener", it made it from obscure to obvious. It is clear to me know that it has to be this way. There is no way to avoid it: if one does not keep track of the amount of information, one ignores the origin of motion and forces. It clarifies why gravity has something to do with entropy. It has to, it can not do otherwise. When I got the idea that gravity and inertia emerge in this way, which is close to half a year ago, I was really excited. I felt I had an insight that makes clear what gravity is. But I decided not to publish too quickly, also to allow time to make it more precise. But also to see if the idea that gravity is entropic would still appear to me as new as exciting as my first feeling about it. And it does. Now, almost half a year later, I still feel that way. For instance, the similarity between the entropic force for a polymer and gravity is a real clue to something important. The fact that it fits in well with an adapted version of the work of Jacobson gives additional support. The derivation of the Einstein equations is not really new, in my mind, since it technically is very similar to the previous works. And I agree that the other line of the paper that discusses inertia is heuristic, and leaves some important gaps. But nevertheless I decided to publish it anyway, because I think this approach to gravity is the right one, it is different, very different from everything that is done today. Everyone who does not appreciate that this view is different from previous papers are missing an essential point. If space is emergent, a lot more has to be explained than just the Einstein equations. Geodesic motion, or if you wish, the laws of Newton have to be re-derived. They are not fundamental. This has not been discussed anywhere, not even noted that it is the case. If the previous papers had made the emergence of gravity so clear, why are people still regarding string theory as the final theory of quantum gravity? Somehow, not everyone was convinced that these similarities mean something, or at least, people had no clear idea of what they mean. Some people may think that when we develop string theory further that eventually we will learn about this. I am not sure that string theory is the way to go. In any case, not if we keep regarding the definition in terms of closed strings as being microscopically defined, may be equivalent to some other formulation. And not if we keep our eyes closed for emergent phenomena. Graviton's can not be fundamental particles in a theory of emergent space time and gravity. So what is the role of string theory, if gravity is emergent? I discussed this at some level in the paper. It should also be emergent, and it is nothing but a framework like quantum field theory. In fact, I think of string theory as the way to make QFT in to a UV complete but still effective framework. It is based on universality. Many microscopic systems can lead to the same string theory. The string theory landscape is just the space of all universality classes of this framework. I have more to say about it, but will keep that for a publication, or I will post that some other time. Of course, I would have liked to make things even more clear or convincing. In this paper, I use heuristic and you might say handwaving arguments. The issue of motion: why is the acceleration a that I introduced equal to the second time derivative of the position? If one assumes the equivalence principle, it is clear. Also coordinate invariance would be enough. But I do not have a very precise way of seeing how that emerges. How to go from just information to a Lorentzian geometry in which general coordinate invariance is manifest. Some assumptions have to be made. But again, this are questions that others have not been even started to think about. These are questions that have not been even addressed by previous works. But they are essential. When one really understands this well, there should be no doubt that gravity is emergent and forces are driven by entropy. This is the essential idea, which is really new and important, and which in my view justifies this level of reasoning, certainly in a first paper. It is clear that this is not the final paper on this subject. This is also my own view. I clearly did not answer all of the questions. In fact, my approach probably raises more questions than it answers. But it should be obvious that these questions are important, very fundamental and their answers should lead us in a completely new direction. Our theories will have to based on new paradigms. I find all this still very exciting and will continue to work in this direction. And remember, quantum mechanics was also not developed in one paper. Do you think de Broglie knew exactly what he was talking about? Leaps based in intuition are sometimes necessary. They are an important part of progress in science, even if they do not immediately give complete finished theories of Nature. At 06:57 PM 7/12/2010, Henry Baker wrote:

FYI -- You can't win, you can't break even, you can't get out of the game.

July 12, 2010

A Scientist Takes On Gravity

By DENNIS OVERBYE

It’s hard to imagine a more fundamental and ubiquitous aspect of life on the Earth than gravity, from the moment you first took a step and fell on your diapered bottom to the slow terminal sagging of flesh and dreams.

But what if it’s all an illusion, a sort of cosmic frill, or a side effect of something else going on at deeper levels of reality?

So says Erik Verlinde, 48, a respected string theorist and professor of physics at the University of Amsterdam, whose contention that gravity is indeed an illusion has caused a continuing ruckus among physicists, or at least among those who profess to understand it. Reversing the logic of 300 years of science, he argued in a recent paper, titled “On the Origin of Gravity and the Laws of Newton,” that gravity is a consequence of the venerable laws of thermodynamics, which describe the behavior of heat and gases.

“For me gravity doesn’t exist,” said Dr. Verlinde, who was recently in the United States to explain himself. Not that he can’t fall down, but Dr. Verlinde is among a number of physicists who say that science has been looking at gravity the wrong way and that there is something more basic, from which gravity “emerges,” the way stock markets emerge from the collective behavior of individual investors or that elasticity emerges from the mechanics of atoms.

Looking at gravity from this angle, they say, could shed light on some of the vexing cosmic issues of the day, like the dark energy, a kind of anti-gravity that seems to be speeding up the expansion of the universe, or the dark matter that is supposedly needed to hold galaxies together.

Dr. Verlinde’s argument turns on something you could call the “bad hair day” theory of gravity.

It goes something like this: your hair frizzles in the heat and humidity, because there are more ways for your hair to be curled than to be straight, and nature likes options. So it takes a force to pull hair straight and eliminate nature’s options. Forget curved space or the spooky attraction at a distance described by Isaac Newton’s equations well enough to let us navigate the rings of Saturn, the force we call gravity is simply a byproduct of nature’s propensity to maximize disorder.

Some of the best physicists in the world say they don’t understand Dr. Verlinde’s paper, and many are outright skeptical. But some of those very same physicists say he has provided a fresh perspective on some of the deepest questions in science, namely why space, time and gravity exist at all ­ even if he has not yet answered them.

“Some people have said it can’t be right, others that it’s right and we already knew it ­ that it’s right and profound, right and trivial,” Andrew Strominger, a string theorist at Harvard said.

“What you have to say,” he went on, “is that it has inspired a lot of interesting discussions. It’s just a very interesting collection of ideas that touch on things we most profoundly do not understand about our universe. That’s why I liked it.”

Dr. Verlinde is not an obvious candidate to go off the deep end. He and his brother Herman, a Princeton professor, are celebrated twins known more for their mastery of the mathematics of hard-core string theory than for philosophic flights.

Born in Woudenberg, in the Netherlands, in 1962, the brothers got early inspiration from a pair of 1970s television shows about particle physics and black holes. “I was completely captured,” Dr. Verlinde recalled. He and his brother obtained Ph.D’s from the University of Utrecht together in 1988 and then went to Princeton, Erik to the Institute for Advanced Study and Herman to the university. After bouncing back and forth across the ocean, they got tenure at Princeton. And, they married and divorced sisters. Erik left Princeton for Amsterdam to be near his children.

He made his first big splash as a graduate student when he invented Verlinde Algebra and the Verlinde formula, which are important in string theory, the so-called theory of everything, which posits that the world is made of tiny wriggling strings.

You might wonder why a string theorist is interested in Newton’s equations. After all Newton was overturned a century ago by Einstein, who explained gravity as warps in the geometry of space-time, and who some theorists think could be overturned in turn by string theorists.

Over the last 30 years gravity has been “undressed,” in Dr. Verlinde’s words, as a fundamental force.

This disrobing began in the 1970s with the discovery by Jacob Bekenstein of the Hebrew University of Jerusalem and Stephen Hawking of Cambridge University, among others, of a mysterious connection between black holes and thermodynamics, culminating in Dr. Hawking’s discovery in 1974 that when quantum effects are taken into account black holes would glow and eventually explode.

In a provocative calculation in 1995, Ted Jacobson, a theorist from the University of Maryland, showed that given a few of these holographic ideas, Einstein’s equations of general relativity are just a another way of stating the laws of thermodynamics.

Those exploding black holes (at least in theory ­ none has ever been observed) lit up a new strangeness of nature. Black holes, in effect, are holograms ­ like the 3-D images you see on bank cards. All the information about what has been lost inside them is encoded on their surfaces. Physicists have been wondering ever since how this “holographic principle” ­ that we are all maybe just shadows on a distant wall ­ applies to the universe and where it came from.

In one striking example of a holographic universe, Juan Maldacena of the Institute for Advanced Study constructed a mathematical model of a “soup can” universe, where what happened inside the can, including gravity, is encoded in the label on the outside of the can, where there was no gravity, as well as one less spatial dimension. If dimensions don’t matter and gravity doesn’t matter, how real can they be?

Lee Smolin, a quantum gravity theorist at the Perimeter Institute for Theoretical Physics, called Dr. Jacobson’s paper “one of the most important papers of the last 20 years.”

But it received little attention at first, said Thanu Padmanabhan of the Inter-University Center for Astronomy and Astrophysics in Pune, India, who has taken up the subject of “emergent gravity” in several papers over the last few years. Dr. Padmanabhan said that the connection to thermodynamics went deeper that just Einstein’s equations to other theories of gravity. “Gravity,” he said recently in a talk at the Perimeter Institute, “is the thermodynamic limit of the statistical mechanics of “atoms of space-time.”

Dr. Verlinde said he had read Dr. Jacobson’s paper many times over the years but that nobody seemed to have gotten the message. People were still talking about gravity as a fundamental force. “Clearly we have to take these analogies seriously, but somehow no one does,” he complained.

His paper, posted to the physics archive in January, resembles Dr. Jacobson’s in many ways, but Dr. Verlinde bristles when people say he has added nothing new to Dr. Jacobson’s analysis. What is new, he said, is the idea that differences in entropy can be the driving mechanism behind gravity, that gravity is, as he puts it an “entropic force.”

That inspiration came to him courtesy of a thief.

As he was about to go home from a vacation in the south of France last summer, a thief broke into his room and stole his laptop, his keys, his passport, everything. “I had to stay a week longer,” he said, “I got this idea.”

Up the beach, his brother got a series of e-mail messages first saying that he had to stay longer, then that he had a new idea and finally, on the third day, that he knew how to derive Newton’s laws from first principles, at which point Herman recalled thinking, “What’s going on here? What has he been drinking?”

When they talked the next day it all made more sense, at least to Herman. “It’s interesting,” Herman said, “how having to change plans can lead to different thoughts.”

Think of the universe as a box of scrabble letters. There is only one way to have the letters arranged to spell out the Gettysburg Address, but an astronomical number of ways to have them spell nonsense. Shake the box and it will tend toward nonsense, disorder will increase and information will be lost as the letters shuffle toward their most probable configurations. Could this be gravity?

As a metaphor for how this would work, Dr. Verlinde used the example of a polymer ­ a strand of DNA, say, a noodle or a hair ­ curling up.

“It took me two months to understand polymers,” he said.

The resulting paper, as Dr. Verlinde himself admits, is a little vague.

“This is not the basis of a theory,” Dr. Verlinde explained. “I don’t pretend this to be a theory. People should read the words I am saying opposed to the details of equations.”

Dr. Padmanabhan said that he could see little difference between Dr. Verlinde’s and Dr. Jacobson’s papers and that the new element of an entropic force lacked mathematical rigor. “I doubt whether these ideas will stand the test of time,” he wrote in an e-mail message from India. Dr. Jacobson said he couldn’t make sense of it.

John Schwarz of the California Institute of Technology, one of the fathers of string theory, said the paper was “very provocative.” Dr. Smolin called it, “very interesting and also very incomplete.”

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.

Over lunch in New York, Dr. Verlinde ruminated over his experiences of the last six months. He said he had simply surrendered to his intuition. “When this idea came to me, I was really excited and euphoric even,” Dr. Verlinde said. “It’s not often you get a chance to say something new about Newton’s laws. I don’t see immediately that I am wrong. That’s enough to go ahead.”

He said friends had encouraged him to stick his neck out and that he had no regrets. “If I am proven wrong, something has been learned anyway. Ignoring it would have been the worst thing.”

The next day Dr. Verlinde gave a more technical talk to a bunch of physicists in the city. He recalled that someone had told him the other day that the unfolding story of gravity was like the emperor’s new clothes.

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