In the New York Times this week there’s an article* by Dennis Overbye on Erik Verlinde and his paper on the idea that gravity is not a force at all, but a consequence of thermodynamics. You can think of it as an extreme take on one of the directions a lot of the research (that I’ve mentioned a number of times has been going on in string theory) has been pointing, although I think it is safe to say that there’s a lot to be done on making the statement a concrete one that you can do physics with. I think it contains the germs of the right thing we’re all reaching for, but does not quite get there yet. We’re now quite routinely formulating some of the key physics of gravity entirely as physics of a completely non-gravitational dual theory – this is the content of what we call holography – and in particular the quantum physics of black holes in those settings get holographically mapped to the thermodynamics of the non-gravitational physics. This is the basis of the tools that we’ve been applying to studying aspects of phenomena showing up in various experimental systems in nuclear physics and cold atomic physics (and studies of phenomena relevant to various condensed matter systems are also being done). I’ve told you about a lot of this in various posts. (Some of them are listed below.) Running this the other way, the model non-gravitational systems (certain gauge theories at strong coupling and with large rank gauge groups) can be thought of as examples of how gravity (and space-time itself) is really an emergent phenomenon, appearing simply as an economical description of physics that has no gravity in it at all. If you like, gravity is simply a consequence of pragmatism. For moderate or small coupling, and/or small rank, there’d be no need for it in these systems – you’d simply do standard field theory computations – but at some point it becomes better and better to think in terms of the language of gravity in a nice smooth space-time that emerges in the limit of strong coupling. You can ask – is it real? Is the space-time really there, and is the gravity in it (i.e., its dynamics) really real? Is there secretly a black hole (however approximate) lurking in a little fifth dimension that opens up near the experiments that touch upon the regime that has these dual descriptions (at least qualitatively)? See my earlier post on that issue. Real is a funny word.
Verlinde is not simply restating this familiar (to many of us working in string theory) and highly useful quantum gravity phenomenon. He’s, as I understand it, trying to start at a more basic level and find a way of doing away with gravity in a much more general setting. Have a look at the article and the paper. My feeling is that we do need to take the hints and examples given to us by holography and gauge/gravity dualities and see if there is a way of saying something stronger and more generally applicable, and so Erik (and the many others who have been thinking about the issues over the years) is on the right track. But there is a lot of work to do.
-cvj
*Thanks Shelley, Krzysztof.
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Is it not funny that people are using Einstein’s equations and trying to prove that gravity emerges out of it? like GR -> Black Holes ->
Holographic principle + thermodynamics -> entropy ->emergent gravity?
[…snip (link removed)…-cvj]
Here we go again. After 100 years of successful experimentation proving Einstein’s concepts, some people still love to reinvent gravity. We do not need to waste time on new concepts based on a hodge podge of other half baked ideas. Erik can wait till there is absolute evidence and proof for ideas like black holes, holographic principles etc.
The only thing that matters is to work on supplementing or extending
on Einstein’s concept of space-time curvature gravity and nothing else.
For example I explain DM using space-time curvature. […snip (link removed)…-cvj]
To Kim,
I think that Erik Verlinde, in his paper ‘On The Origin Of Gravity and The Laws Of Newton’ page 7, is really discussing bosonic string theory. So perhaps the notion of rest mass may not apply in this context.
I say this because, in his paper, he writes; mc = hbar/ deltax. Well, this m cannot possibly be rest mass travelling at velocity c. Or do I misunderstand? If indeed m is the relavistic mass of some particle with rest mass (u) we now have the task of trying to figure out what to do with this particle in a ‘non-relativistic space’. Sounds contradictory.
If I’m correct we have a massless particle in a non-relativistic space?? But, if we plough on…the link to bosonic string theory seems to be obtained this way; suppose that the area A is very small, microscopic – which it could be because he says “So we consider a small piece of an holographic screen, and a particle of mass m that approaches it from the side at which space time…” – then the particle m must have a very small cross-section in order to fit into the area A.
Well, a small cross-section (zero) and a large energy (because A is small, R is small) implies that m can/is be a particle with a lot of energy and small cross-section. This would give it a string-like appearance in 3-d space. So…a zero rest mass, string-like particle with loadsa energy.
However, I’m not an expert on String Theory. In my defence, Verlinde is supposed to be discussing ordinary Newtonian gravity.
There are other things in his paper which, in my view, raise more serious questions about the validity of his hypothesis as a physical theory. Perhaps some of these problems are inherited from the bosonic string theory approach and have since been resolved. Needless to say, I am yet not convinced his realisation of Newtonian gravity is consistent.
Hi Haywood,
Thank you very much! Glad you enjoyed it.
Cheers,
-cvj
You’re a patient man Clifford: responding to some of the above must be frustrating. I enjoyed reading this posting, it’s nice to see what you think about these things, given you are an active researcher.
All the best,
Haywood
Why use 1? In what units? I recommend thorough study of, for example, this book: http://www.amazon.com/Fundamentals-Physics-David-Halliday/dp/0471216437
Thank you.
-cvj
if i take charge to be 1 and the masses to be :
((9.1)*10^-31)*(1.67)*10^-27) is this the wrong values? it doesnt give 10^38
By the way where do these values derive from? When we speak of rest mass what does this mean? I thought the locatoin of electrons cant be pinned down so how do we know they are travelling near the speed of light?
Hello John,
Again, you are not listening to (reading) what I’ve said.
First, I never expressed any surprise or anything like that concerning whether people work on string theory or not. You just made that up. In fact, a constant theme I express is that people should work on a variety of approaches since none of us know what the answer is. It is on-going research.
What I despise is the arrogant attitude that some have that they (with little or no knowledge of what people are doing – compounded by knowingly ignoring things about the research that they are being shown since it undermines their belief system) can just walk in and declare everyone wrong, and that they know more about it than the research community does. It’s all about showing how smart they are, and little to do with making progress. Well, I grew tired of such games long ago in kindergarten. See your remark on the other thread as an example. It is just tiresome and boorish behaviour and does not deserve the time to prepare a serious answer.
Second, and following on from the above point, you are now in your most recent comment admitting to dismissing a large class of specific examples that do for gravity precisely what you claim cannot be done. Whether you like them or not, they are gravity. So they are counterexamples. So why stick to a dead point on principle instead of learning, modifying your view, and moving on? I’m happy to have a discussion that we can both learn from, but this is not the way to do it. It is just tedious and tiresome, with all due respect, and I have better things to do, and all you are doing is freezing out the contributions of others with this back and forth.
Good day to you.
-cvj
Where did I say it could not be calculated? The point is if you _did_ do the computation (and why not try?) you would not be confused as to where he deduced that… you’d know precisely where he got it from. Coulombs law vs Newton’s law. This is basic freshman physics that I am sure you know. This is an opportunity to put it into action.
I can’t understand why you just don’t do the computation. It is a good and instructive exercise… Physics is mostly about trying out computations and seeing how these things work. Have a go.
-cvj
hi,
Yes it is the same kim. http://www.theory.caltech.edu/people/jhs/
But here he writes: ‘the gravitational attraction between an electron and proton in a hydrogen atom is about 10^38 times weaker than the electric attraction. ‘ So how did he deduce that?
You wrote: ”go ahead and do the simple computation of the strength of the force to see why. ”
The distances and masses are very small but finite so why cant it be calculated?
Well, you know, one of the reasons why I ignore these (and i am aware of them, though I do not know many details), is because I am not surprised at all that within string theory you can construct working examples with ‘some form’ of gravity ! But that is not convincing me that you are on the right path. All I wanted to tell you is that you should not be surprised that some people chose not to work in string theory. If string theory manages to adress the questions one day I want to see an answer too, and John Schwarz certainly believes that at a more advanced stage of the theory this will become clear, then I shall be all ears (but I guess I am more pessimistic than Schwarz). My attitude here is quite similar to John Bell’s attitude to locally causal deterministic theories versus quantum mechanics : if one day someone comes up with such theory I will listen with interest, but I will certainly not look for one myself.
That is all, I am not saying string theory is nonsense: actually a whole spectrum of particles coming from one object is pretty cool, but in the form I started learning it, it definetly had some drawbacks which personally convinced me not to persue it.
Concerning this holograpic principle I have plenty of other worries: two others of them are (a) how to define this in a background independent way (b) the holographic principle is formulated for now in the language of (preferred) spaces, while it should be in a spacetime language.
Cheers,
John
It is nothing to do with beliefs. This is science I’m working on here. You say you are not convinced of this holographic principle and also that dismissing gravity as not fundamental is wrong when in both cases we have several examples of how this works. (I am not saying that it works, so far, for the gravity here in our world, but it works for things that all physicists would agree are _a_ form of gravity.) I’m pointing not to plausibility arguments but concrete computations in various systems. I’ve told you about them and you seem to be bent on ignoring the examples (not even addressing them) and carrying on with your own stories and what you consider to be killer arguments (that supposedly nobody’s been able to “counterpunch”). Nice arguments are fine, but you’re been presented with several specific working examples, but you carry on and ignore them. It is tiring.
Best,
-cvj
Clifford,
I haven’t made any claim in this tread or in another one that something CANNOT be done. If you are annoyed that I somehow did not choose to follow your beliefs, that is an entirely different story ! So, please do not make up stories which are not there.
Cheers,
John
John,
There’s a vast literature out there with many concrete examples of those things you’ve chosen not to believe can be done. Professionals and students alike are routinely working with them. I’ve suggested in another thread that you have a look at them. I’ve given several links and references to the literature. But you don’t seem interested in looking at any of it, so I’m pretty much done trying to discuss it with you since you’ve decided to stick to your own narrative at all costs. This is not a good use of my time.
Cheers,
-cvj
Actually, this reminds me about an argument which I have once given about unitarity versus black hole radiation (and nobody really knew a good counterpunch :-)) Hawking’s calculation reveals that the vacuum state in the asymptotic past is going to be seen as a thermal density matrix in the asymptotic future (indeed it is a density matrix, since you take a partial trace over all degrees of freedom living on the black hole horizon). Let me stress again that this is easily understood on general grounds : the asymptotic region is globally hyperbolic but has a boundary, the correct action does not only contain field densities in the bulk but there are inevitably boundary terms (associated to the black hole event horizon). That means that radiation will get in the universe (through the white hole horizon) and leak out (through the bh event horizon). So, your evolution will not be described by a global Hamiltonian defined on the Killing hypersurfaces of equal time. Consequently, your evolution won’t be unitary. Now, people hope (for somewhat idle reasons) that this unitarity will be restored once you take into account quantum gravity corrections. However, such corrections are expected to be small and I once argued that there is a very precise mathematical sense in which the distance between a thermal density matrix and any pure state is LARGE 🙂
John
Hi,
To start with, I am not really convinced by this holographic principle. First of all, it is of no surprise that it is the surface area of the BH which couples to the surface gravity in the classical ‘thermodynamic’ formula simply because the entire thing is calculated with respect to the outside observer and the latter has (in the limit) only acces to the BH horizon. Also all other quantities appearing in this formula (Komar, Bondi) Mass and angular momentum are non local things which do not depend upon the inside of the BH. Second, all backreaction calculations ‘t hooft made were semiclassical, so it is of no real surprise that the modified spectral properties to depend on the ‘perturbed’ BH horizon in this approximation. Third, I would not even know how to define the volume inside a BH since there is no notion of surface orthogonal Killing time present, so let along that I would know how to map the physics inside this volume to the boundary. And fourth, in a full theory of quantum gravity, there are no black holes and ‘in a semiclassical approximation’ one would expect the BH entropy to receive ‘volume corrections’ since the inside will become visible to the ‘asymptotic’ observer now by quantum fluctuations of the metric.
Anyway, these are just my five cents…
John
Ah. So you finally got around to checking this out. I share your sense that there is something “right” about it but it is not really the definitive description that one might hope for. What I have intuitively felt for many years is that there is an intimate relationship between gravity and what I will call “positional information”. That is where something is located is not an intrinsic property but information that must be stored on the holographic screen. And somehow that information is “coupled” (maybe a bad choice of words) with gravity. That is where I see things headed. This perspective resolves a number of issues. But I cannot quantify it in a meaningful way that I can calculate the consequences. I was hoping this paper would do that but it seems to fall short. What particularly bothers me about Verlinde’s characterization is that he seems to be saying that the emergent force would only manifest itself if a mass was in motion toward/away from the screen. But I could be misinterpreting what he is saying.
Anyway. Thanks for sharing your view.
e.
Hi,
They don’t. Assuming you are the Kim who has stated previously that they’ve been studying Physics and Mathematics, go ahead and do the simple computation of the strength of the force to see why.
Best,
-cvj
hi,
I have a question. How do physicists measure the gravitational force between a pair of electrons?
Some string theorists may be called gravity haters (and a large portion of it may be called gravity ignorami) in the same way as some relativists do not think highly of quantum theory. Concerning this particular approach, it has been thoroughly (and justifiably) destroyed by another blogging string theorist whose name I shall not mention here. I think dismissing gravity as not fundamental is possibly even worse than claiming that quantum mechanics can be rewritten as a deterministic theory. More importantly, I think the gravitational lessons are even more universal (and string theory ignores these for now, at least John Schwarz understands this as I could read on his webpage).
I don’t get it. In our 4D world the fundamental gravitational force is exerted via a graviton? YES or NO?
String theory says YES. I’m satisfied with that, period.
Thanks for this post, I was just trying to find someone who’d already explained the article a bit as people have been asking me about it! Though very unfortunately the article itself doesn’t do much explaining, just says “hey look at this!” so far as I can tell.