Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: yor_on on 06/02/2010 19:40:01
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So I found two totally fresh approaches to Gravity here.
A trial to explain Gravity "as an entropic force caused by changes in the information associated with the positions of material bodies." By Erik Verlinde (String theorist)
On the Origin of Gravity and the Laws of Newton (http://arxiv.org/pdf/1001.0785v1)
And
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Directly related but yet unconnected, coming out in parallel, by the theoretical physicist Thanu Padmanabhan. Equipartition of energy in the horizon degrees of freedom and the emergence of gravity (http://arxiv.org/pdf/0912.3165v2)
Can this be the final answer to gravity?
What do you think?
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I'm really enjoying Erik Verlinde, some of the things he say make eminent sense to me, like the way he looks at gravitons.
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"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 will necessarily take us in the right direction, if we keep regarding the definition in terms of closed strings as being microscopically defined, or may be equivalent to some other formulation. And not if we keep our eyes closed for emergent phenomena. Gravitons 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 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."
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It looks very interesting but it will take me a some time to understand it, at least to some level. I can't help but feel that there is an element of starting off with a premise, then going round a set of arguments that end up back at the original premise. I fear I have a lot to learn here; I don't know anything about holographic screens etc. and, although I can understand what is being said about entropic force, it is another area with which I am totally ignorant. I am not sure whether forces (even with the polymer) are as a result of a system trying to move to a high entropy state or whether the definition of the high entropy state is one that results from the sum of the forces in the system, if you see what I mean. I think this may be the key issue.
There is a lot of work needed to show how the theory can be made consistent with observations. It will be interesting to see how many people will be suficiently inspired to test it out.
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Okay I think I can see the 'holographic argument' now. Black_hole_thermodynamics (http://en.wikipedia.org/wiki/Black_hole_thermodynamics)
And it came from this "The holographic principle was inspired by black hole thermodynamics, which implies that the maximal entropy in any region scales with the radius squared, and not cubed as might be expected. In the case of a black hole, the insight was that the description of all the objects which have fallen in can be entirely contained in surface fluctuations of the event horizon. The holographic principle resolves the black hole information paradox within the framework of string theory."
And the idea it builds on is 'information density' of that holographic interface. That introduces a philosophical aspect of the theory to me. Here is a simple explanation using the idea of throwing coins to watch their outcome. First we have to define if the coin is 'fair' or 'crocked' :) From -> Entropy (information theory) (http://en.wikipedia.org/wiki/Information_entropy) and Holographic_principle#Black_hole_information_paradox (http://en.wikipedia.org/wiki/Holographic_principle#Black_hole_information_paradox) (So, now you got most of the info I've have, for now:)
"A fair coin has an entropy of one bit. However, if the coin is not fair, then the uncertainty is lower (if asked to bet on the next outcome, we would bet preferentially on the most frequent result)" like a crooked roulette table, okay?
"Consider tossing a coin with known, not necessarily fair, probabilities of coming up heads or tails.
The entropy of the unknown result of the next toss of the coin is maximized if the coin is fair (that is, if heads and tails both have equal probability 1/2). This is the situation of maximum uncertainty as it is most difficult to predict the outcome of the next toss; the result of each toss of the coin delivers a full 1 bit of information.
However, if we know the coin is not fair, but comes up heads or tails with probabilities p and q, then there is less uncertainty. Every time it is tossed, one side is more likely to come up than the other. The reduced uncertainty is quantified in a lower entropy: on average each toss of the coin delivers less than a full 1 bit of information.
The extreme case is that of a double-headed coin which never comes up tails. Then there is no uncertainty. The entropy is zero: each toss of the coin delivers no information."
But here is my question, where are the limits for this kind of observation? Can you by stating 'I start now' ignore the coin throws you did before? It seems to me that you can't. And if you draw out that conclusion to its limits, considering all coins thrown in human time :) How do you construct your 'system' of observation?
Thinking of it it has a clear relation to how time works, won't you agree?
As if we assume that by defining a freely chosen 'point in time' as our 'start' we will get different results from our coin throwing?
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Graham I think it's with you like with me, the more I look at it the more I seem to remember :) It's just that I haven't put it together his way. But to me it's more or less the same idea that I've had, except I'm sort of allergic to a 'holographic universe', as that sounds too much as something ethereal to me. I'm sure GoodElf could give us some input on this paper too. He's been thinking some time about a holographic approach.
I guess it's all about definitions though, and that what I react to now will make more sense when I understands that 'holographic idea' better..
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Here's Sarah's thoughts on it.
Tell her what you think (http://sarahaskew.net/2010/02/04/the-end-of-gravity-as-we-know-it/#comment-2115)
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And a interesting analysis (criticism in fact:) by Luboš Motl
independent review (http://motls.blogspot.com/2010/01/gravity-as-holographic-entropic-force.html) lifted from Sarah's blog (read it first)
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This statement by Erik Verlinde (http://motls.blogspot.com/2010/01/erik-verlinde-comments-about-entropic.html) I find really interesting :) as we just had an discussion about it here.
"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."
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A gravity story to take us out of Newton's orchard (http://www.newscientist.com/article/mg20527443.800-the-entropy-force-a-new-direction-for-gravity.html)
Awh I don't know :)
But it's very very cool, I need to know, now :)
As for Thanu Padmanabhan? I read some call it a direct copy of Ted Jacobson's work?
Thermodynamics of Spacetime: The Einstein Equation of State (http://arxiv.org/abs/gr-qc/9504004)
So now you will have to read both to see for yourself :)
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A rather strange thing though is that many seems offended over the simplicity of Erik's equations. Am I wrong supposing that they too look for that unifying theory that with one simple stroke explains it all :)
Or what are they looking for?
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Website of Erik Verlinde (http://staff.science.uva.nl/~erikv/page20/page18/page18.html)
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Reading Jacobson.
"In thermodynamics, heat is energy that flows between degrees of freedom that are not macroscopically observable. In spacetime dynamics, we shall define heat as energy that flows across a causal horizon. It can be felt via the gravitational field it generates, but its particular form or nature is unobservable from outside the horizon. For the purposes of this definition it is not necessary that the horizon be a black hole event horizon. It can be simply the boundary of the past of any set O (for “observer”)...That causal horizons should be associated with entropy is suggested by the observation that they hide information....So far we have argued that energy flux across a causal horizon is a kind of heat flow, and that entropy of the system beyond is proportional to the area of that horizon. It remains to identify the temperature of the system into which the heat is flowing. Recall that the origin of the large entropy is the vacuum fluctuations of quantum fields. According to the Unruh effect, those same vacuum fluctuations have a thermal character when seen from the perspective of a uniformly accelerated observer..."
So entropy is then defined as an energy flux that is expressed as heat. It's very nicely written in fact, clear and precise.
And Erik writes this "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."
And Thanu Padmanabhan doesn't seem to reference him at all in his work?
I better add that I don't think he's copying, but it would have been nice to see him reference Jacobson as he too must have read him?
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I now have work to do so not much time, however would like to comment on the coin analogy. Let us take a dice for example. Each face of a dice has a 1:6 chance of coming face up, and this would be the highest entropy state - with equal weighting. If the dice was really weighted so that it always came up with a six (say), this would be a low entropy condition. However I think it would be odd to say that the low entropy state was responsible for the biassed result; I would say that it was the weighting of the dice and the forces that acted on these weights that results in the dice/throwing/gravity system having low entropy. The order was introduced by someone fiddling the dice and creating a more ordered system. No forces are created by the entropic state but clearly the other way round, the entropic state is created by how the forces act on the newly configured dice. In the same way I have my doubts about the whole concept of entropic force. There is correlation, but I am unhappy about the concept that there is a magical need to go to high entropy that causes the forces, it is rather the other way round. This theory seems to crucially depend on gravity being a consequence of entropy, which does not seem fundamentally sound.
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It's speculation, yor_on. There's no evidence for it. It isn't often that I agree with Lubos Motl, so that's saying something.
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Maybe, of course, naturally :)
It's all speculations Farsight. What if a Godess stepped down tomorrow to inform us that we actually are on the back of a very slow moving turtle, and that all what we measured so far have been a superb joke of hers?
But Erik's math seems nice, and interesting, as far as I understand, and it has to be one of the most innovative theories I've ever seen. And as I said, I'm already partial towards the concept of 'emergences' :)
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Yes Graham, that idea surprised me too. If one consider entropy something that is bound to a arrow of time then the arrow must be there before entropy can exist. But if you decide that this arrow we see is just another name for entropy, and that entropy is the better definition for our 'events' then maybe?
I need to understand this one much better than I do, but it takes time to see how Erik thinks. Hopefully you will contribute to my understanding too :)
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https://www.quora.com/How-valid-is-Verlindes-theory-of-gravity-as-an-entropic-force/answer/Autymn-Castleton
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Sometimes I wonder if these physicists forgot about the equivalence principle, since gravity and fictious forces are one and the same, in order to understand why gravity is caused one needs to understand why movement resistance exist. And as far as I know, there is no logical sense in quantum field theory of a boson telling baryons to move or resist moving since QFT doesn't work that way, it suggest either we're very misleaded or very frustrate of our lack of understanding, a better theory is required in order to include gravity in it, one that will make sense of motion, distance, resistance to movement and probabilistic universe (or even harder: deterministic universe we are all familiar with)