Gravity is not entropic force ?

[MTd2]

But what this statement has to do with the holographic sreens and Verlinde's theory?

That in the paper, the neutron initially has no energy whatsoever, so, it doesn`t exist.

 

[MTd2]

these are the so-called virtual states, they are part of the vacuum, and as such do not create neither entropy nor temperature

They do create entropy, as they all contribute to the actual mass. In fact, if you look how similar the path integral looks like a partition function, that the virtual particles are in fact the microstates of a particle. That also brings a problem, similar to what was raised by Motl, that due a limit bound on information, some virtual particles may be killed, just like interference fringes.

I personally think this is a possible advantage. That is, what if there are "hole" particles and they are responsible for dark matter?

 

[CHIKO-2010]

That in the paper, the neutron initially has no energy whatsoever, so, it doesn`t exist.

Could you please be more specific where in the paper you find that "the neutron initially has no energy"?

 

[CHIKO-2010]

They do create entropy, as they all contribute to the actual mass. In fact, if you look how similar the path integral looks like a partition function, that the virtual particles are in fact the microstates of a particle. That also brings a problem, similar to what was raised by Motl, that due a limit bound on information, some virtual particles may be killed, just like interference fringes.

I personally think this is a possible advantage. That is, what if there are "hole" particles and they are responsible for dark matter?

Sorry, but what you have written above does not make any sense! You seems are confused with some basic stuff of quantum field theory.

 

[MTd2]

Could you please be more specific where in the paper you find that "the neutron initially has no energy"?

In the paper, a neutron was set with zero entropy. But if you are dealing with calculations inside the theory of verlinde, all entropy must be taken into account, including the one from the mass. So, the neutron cannot possibly be started with a 0 entropy because that would be ignoring the entropy of the mass and given that we are talking about gravity, gravity cannot be forgotten because it is the main part of the problem.

 

[MTd2]

Sorry, but what you have written above does not make any sense! You seems are confused with some basic stuff of quantum field theory.

Verlinde`s theory does not look like QFT for me.

 

[CHIKO-2010]

In the paper, a neutron was set with zero entropy. But if you are dealing with calculations inside the theory of verlinde, all entropy must be taken into account, including the one from the mass. So, the neutron cannot possibly be started with a 0 entropy because that would be ignoring the entropy of the mass and given that we are talking about gravity, gravity cannot be forgotten because it is the main part of the problem.

1. please read the paper more carefully. Neutron is NOT set with zero entropy in the paper.

2. More importantly, the energies corresponding to neutron states in eq. (18) are given in eq. (17). They all are non-zero!

 

[CHIKO-2010]

Verlinde`s theory does not look like QFT for me.

You commented on virtual particles producing entropy -- if you are talking about thermodynamics, what the virtual particles have to do with it? Virtual particles do not exist in classical physics, they do not exists even in quantum mechanics

 

[MTd2]

Although there is a typo in the line above eq. 15 (it uses eq. 15 to find eq. 15), I cannot agree with eq. 15. It assumes that the entropy increases to mc, from 0, using eq. 12.

 

[MTd2]

You commented on virtual particles producing entropy -- if you are talking about thermodynamics, what the virtual particles have to do with it?

Think of processes with the same outcome, but with different virtual processes, as like analogues of degenerate sates. It is just that since Verlnde`s gravity is not unitary, some of these processes are destroyed.

 

[CHIKO-2010]

Although there is a typo in the line above eq. 15 (it uses eq. 15 to find eq. 15), I cannot agree with eq. 15. It assumes that the entropy increases to mc, from 0, using eq. 12.

I think the remark above eq. 12 is indeed confusing. In deriving eq. 15 you do not need to assume that a state at z is a pure state, it can be a mixed state as well. Eq 12 is just a generic equation which tells you how the density matrix changes under the translation along z-direction. So, the end result does not depend on the assumption on the existence of pure neutron state, i.e. absolute entropy are not important in deriving (15) only the entropy difference given by eq. (11).

 

[MTd2]

I checked the origin of eq. 11, that's eq. 3.6 of Verlinde's paper.

Where is the holographic screen, for you? For me, it is of order ~10billion light years away, which is 10^23.The compton length is 10^-15, so the suppression, in 8, factor is about exp(-5*10^-8), that is, 50 parts in one billion.

BTW, now, I agree with you, as far as having 0 entropy, and gaining an entropy proportional to the relativistic mass, . It happens by crossing the holographic screen. It is an non causal region, so it is the same as the particle didn't exist.

 

[CHIKO-2010]

I checked the origin of eq. 11, that's eq. 3.6 of Verlinde's paper.

Where is the holographic screen, for you? For me, it is of order ~10billion light years away, which is 10^23.The compton length is 10^-15, so the suppression, in 8, factor is about exp(-5*10^-8), that is, 50 parts in one billion.


I guess z in (18) must be between the absorber and the mirror, for z higher than the absorber neutrons are not passing the slit, this is just due to the experimental setup. I do not understand where did you get ~10billion light years (?!) from when you just need to reproduce local gravitational field of Earth

 

[Fra]

If you judge Verlindes paper as an input among the sea of other papers I personally thing it's good. But even though I belong to those that think thta ALL forces eventually should be understood as a generalised form for "entropic" processes, I think Verlindes paper rather raises more questions than it answers (what is the origin and status of the holographic principle etc) are good ones, I like his paper.

I personally think some of the underlying ideas I read of verlinde such as he expressed in his original paper

"The universality of gravity suggests that its emergence should be understood from general principles that are independent of the specic details of the underlying microscopic theory. In this paper we will argue that the central notion needed to derive gravity is information. More precisely, it is the amount of information associated with matter and its location, in whatever form the microscopic theory likes to have it, measured in terms of entropy. Changes in this entropy when matter is displaced leads to an entropic force, which as we will show takes the form of gravity. Its origin therefore lies in the tendency of the microscopic theory to maximize its entropy. The most important assumption will be that the information associated with a part of space obeys the holographic principle [8, 9]."

I share the first parts, but this raises questiosn such as:

Where is the observer, and how does he account for the fact that entropy must be relative?
Also what inferences leads a generic observer to infer the holographic principle? and how can this be formulated in an intrinsic sense?

I think Verlindes quest for understanding gravity from general principles of information processing is sound, but he assumes too much baggage, some of it that may even also be wrong. But I don't think that invalidates the intent.

Maybe instead of rejecting the entire idea of inductive processes in physics based on incomplete first steps, maybe we should instead keep asking the deeper questions that Verlindes paper feeds.

How does an inside oberver construct intrinsic measures of "amount of information" stored say in an unknown environment?
How does the notion of "location" emerge from this picture?
What is "matter" in terms of this picture?
What status does the holographic principle have in terms of these much more basal terms?

I think we have to distinguish between two views, an observer beeing ITSELF living inside a strongly dynamical gravitational field, or an observer that from a safe distance or static field, observers gravitational interactions between subsytems in it's own environment. I think this relates to theories that contain gravity as an interaction on part with the others, and thoese theories that doesn't. IT seems clear to me that the second scenario does put gravity on par with any other interaction in priciple. But the first picture does not.

Similar things can be said about constructing measures of missing information, it's not a priori obvious, that two observers measures the amount of information the same - then the question is what is the consequence of this disagreement?

I think questions of these kinds are all questions that indirectly come out of Verlindes paper. Verlindes does not answer them, but I think they are good. So I don't like some of the reactions that somehow suggest that verlinde would be some crackpot because it's easy to see that he lacks all answers. That's a too simple reaction.

/Fredrik

 

[CHIKO-2010]

Dear Fredrik,

you are asking interesting questions concerning the original proposal by Verlinde. However, the points you have mentioned are in essence methaphysical. I think this is not how physics as an empirical science works. A given physical hypothesis (idea, model) must be able to describe certain observational facts. If it does, it becomes a theory which maybe subsequently refined, improved etc. If deductions made out of the basic idea fail to explain observations, the idea is declared false...there are many such false ideas in the history of physics much much more than the correct ones...

So the question in this particular discussion is that whether Motl's objections and the related arguments in the paper indeed show that Verlinde's idea on entropic gravity is false. I tend to think they do, because those objections are based on avery basic idea by Verlinde - gravity is due to the difference in entropy of a particle related with its position. The questions related with detailed physics of holographic screens, observers etc are irrelevant here, in my opinion.

 

[Fra]

So the question in this particular discussion is that whether Motl's objections and the related arguments in the paper indeed show that Verlinde's idea on entropic gravity is false. I tend to think they do

Ok that point is well taken. You're right that's the specific question here. And I admit that if we define the question as such, it's probably false. But my only point was that I feel that's somehow a too simple question & conclusion.

My point is that, although Verlindes idea seems imperfect, I think it's more interesting to try to discuss how it can be improved, rather than trying to just proove that it's false.

gravity is due to the difference in entropy of a particle related with its position. The questions related with detailed physics of holographic screens, observers etc are irrelevant here, in my opinion.

I disagree. I merely question that then notions of entropy is more complex than verlinde admits, but I think the underlying idea is not wrong. It may be wrong because the measures of information and the holographic principle is wrong. Both these things are IMHO related to the careless treatment of the observer, because the observer is also the home of the theory in my view.

So while it seems probably that the specific attempt of verlinde is flawed, there is something about the idea that I think is right. And since ideas in this direction are unfortunately rare, I show my support even though his first attempt is wrong. I see more than a flawed theory here - I see a way of reasoning that can generate theories.

/Fredrik

 

[MTd2]

I do not understand where did you get ~10billion light years (?!) from when you just need to reproduce local gravitational field of Earth

Well, now I see where why we were disagreeing. Sure, the neutron can be made with 0 entropy, but I never thought that you were positioning exactly or behind the holographic screen, since you were talking about an experiment actually done.

Sure, your equations are right then. All your calculations are OK, but the holographic screen, that z, that you based your eq. 11 from Verlinde's paper, is a cosmological idea. And that holographic screen is not his idea, he cites Wald for that. Raphael Bousso also cited and used that holographic screen, which he identifies with the apparent horizon of the FRW model. This model is by no means local, not even any of its ancestors, except for the thermodynamical gravity, because it uses input, at all points, from the past light cone, following Wald's idea.

There are 2 z's, actually, on your equation, but both sits at the same line. One that measures the distance from the center of the Earth and other to the holographic screen. Since you are doing a differential equation, it doesn't really matter that much where you put the ground state. It matters, though, in the end, where you put the actual values.

 

[MTd2]

My calculation above is wrong, it should be a exp(-5*10^38), much stronger than yours, but all intervals of integration should be changed anyway. So this value is moot anyway.

 

[czes]

Fredrik wrote above that gravity has to be derived from the quantum information. I think it is in the general Wheeler's idea that the future physics will be physics of the information.

Could we say the pure quantum state is something like a virtual particle ?
When I wrote the massive real particle alone has its environment the gravitational field, the virtual particles-antiparticles. The real massive particle absorbs and emits the virual p-a and therefore it oscillate. The virtual p-a hasn't an entropy and temperature but the massive real particle has. It is in a rest if the absorption=emission. it means the entropy and temperature remains on the same level.

If the absorption is faster than the emission the massive particle accelerate (negentropy) due tu Unruh effect.
If the emission is faster the massive particle gets slower (entropy increases) temperature goes down.

The gravitational field could be here as a field of the virtual plasma (virtual particles-antiparticles). The virtual p-a appear and disappear due inversely proportional to the squared distance from the massive particle. Therefore the location is important.
Verlinde's entropy is not like Boltzman's entropy in a gas. The virtual p-a due to Unruh are not real but the calculation is similar.

I have the calculations using the Compton wavelength and not the Boltzmann constant and have the same results.

 

[MTd2]

Given the cosmological aspect of this theory, we have to look for ways to find what went wrong in the above derivation. We know that the entropy of a particle at the particle horizon is 0. So, there must be a boundary condition for the U operator, otherwise, this problem will look like it was done in a universe a little bit larger than Earth, where the absober is a few micrometers away from the edge of the universe.

 

[CHIKO-2010]

Given the cosmological aspect of this theory, we have to look for ways to find what went wrong in the above derivation. We know that the entropy of a particle at the particle horizon is 0. So, there must be a boundary condition for the U operator, otherwise, this problem will look like it was done in a universe a little bit larger than Earth, where the absober is a few micrometers away from the edge of the universe.

There are no cosmological issues in deriving Newton's law within Verlinde's theory! Newton's force is a local force, which presumes that the Universe is static locally. Newton's law of gravity does not hold on cosmological scales. To describe gravitating system neurton-Earth, the relevant quantity is the entropy change due to the change in the position of a test particle (neutron). That is, the net result depends on the entropy gradient (see e.g. Verlinde's eq. 3.7 for the force) which, according to verlinde's eq. 3.6, is constant. Your objections/intepretations, therefore, are invalid.

 

[CHIKO-2010]

P.S. Note that Verlinde's holographic sreens have nothing to do with cosmological, black hole etc horizons

 

[CHIKO-2010]

Could we say the pure quantum state is something like a virtual particle ?
When I wrote the massive real particle alone has its environment the gravitational field, the virtual particles-antiparticles. The real massive particle absorbs and emits the virual p-a and therefore it oscillate. The virtual p-a hasn't an entropy and temperature but the massive real particle has. It is in a rest if the absorption=emission. it means the entropy and temperature remains on the same level.

For futher clarification: Pure state can describe both virtual and real particles. real massive (quantum-mechanical) particle which is described by pure state has no entropy and no temperature. The classical particle (or field) can be viewed as a incoherent collection of many (statistically large number of) quantum-mechanical states and only that object carries entropy!



The gravitational field could be here as a field of the virtual plasma (virtual particles-antiparticles). The virtual p-a appear and disappear due inversely proportional to the squared distance from the massive particle. Therefore the location is important.
Verlinde's entropy is not like Boltzman's entropy in a gas. The virtual p-a due to Unruh are not real but the calculation is similar.

In certain sense you are right. In field-theoretic approach Newton's law (potential) can be derived as an exchange of virtual gravitons bewteen two gravitating real particles (weak gravity approximation + take non-relativistic limit). however for isolated particle (take the second particle to infinity) the gravitons are essentially on-shell, they are real -- these describe gravitational radiation (waves)

All the above is the very standard view on gravity as well as other interactions, say electromagnetism etc...Verlinde's theory is radically different though. There are NO gravitons there neither virtual nor real

 

[qsa]

Given the cosmological aspect of this theory, we have to look for ways to find what went wrong in the above derivation. We know that the entropy of a particle at the particle horizon is 0. So, there must be a boundary condition for the U operator, otherwise, this problem will look like it was done in a universe a little bit larger than Earth, where the absober is a few micrometers away from the edge of the universe.

verlide has discussed this issue in the starting pragraphs in his
paper


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


""Gravity dominates at large distances, but is very weak at small scales. In fact, its
basic laws have only been tested up to distances of the order of a millimeter. Gravity is
also considerably harder to combine with quantum mechanics than all the other forces.
The quest for unifcation of gravity with these other forces of Nature, at a microscopic
level, may therefore not be the right approach. It is known to lead to many problems,
paradoxes and puzzles. String theory has to a certain extent solved some of these, but
not all. And we still have to figure out what the string theoretic solution teaches us""

I think this is related to my thread which seems nobody wants to touch.

https://www.physicsforums.com/showthread.php?t=432711

 

[MTd2]

There are no cosmological issues in deriving Newton's law within Verlinde's theory! Newton's force is a local force, which presumes that the Universe is static locally. Newton's law of gravity does not hold on cosmological scales.

Quantum Mechanics is not a local theory so you will have to look for boundary values and sum over all space if you want to do it right. For example,the operator U is expanded linearly as its border conditions at the holographic screen were irrelevant. In reality, U should be written like U=Aexp(...), where A is a function that makes U goes smoothly to 0 at the screen and to the maximum value of entropy at r=0.

How that is defined, is up to cosmological considerations. But given the smoothing conditions, it must be such that for local boosts, near the center, the value of the constant k, on eq. 13, is almost null. Otherwise, one is using "unfair" boundary conditions to rule out the theory.

 

[qsa]

Ok that point is well taken. You're right that's the specific question here. And I admit that if we define the question as such, it's probably false. But my only point was that I feel that's somehow a too simple question & conclusion.

My point is that, although Verlindes idea seems imperfect, I think it's more interesting to try to discuss how it can be improved, rather than trying to just proove that it's false.



I disagree. I merely question that then notions of entropy is more complex than verlinde admits, but I think the underlying idea is not wrong. It may be wrong because the measures of information and the holographic principle is wrong. Both these things are IMHO related to the careless treatment of the observer, because the observer is also the home of the theory in my view.

So while it seems probably that the specific attempt of verlinde is flawed, there is something about the idea that I think is right. And since ideas in this direction are unfortunately rare, I show my support even though his first attempt is wrong. I see more than a flawed theory here - I see a way of reasoning that can generate theories.

/Fredrik

the paper by joakim helps to clarify many issues, it should be a very interesting read. It has been listed before in PF but I think a close examination is warranted here.

http://arxiv.org/PS_cache/arxiv/pdf/1003/1003.1262v3.pdf

 

[CHIKO-2010]

Quantum Mechanics is not a local theory so you will have to look for boundary values and sum over all space if you want to do it right. For example,the operator U is expanded linearly as its border conditions at the holographic screen were irrelevant. In reality, U should be written like U=Aexp(...), where A is a function that makes U goes smoothly to 0 at the screen and to the maximum value of entropy at r=0.

What are you talking about? U as it is defined in the paper is the standard operator of translations which is valid in quantum mechanics! acting on a state \psi(z) it transforms the state into a new state \psi(z+\delta z) (\delta z can be counted as infinitesimally small). Again, it is completely irrelevant what are the entropies of these two states \psi(z) and \psi(z+\delta z). what is relevant is an entropy difference. QM is nonlocal theory in ceratin sense but in this particular case we are talking about gravitational potential energy of neutron-Earth system which depends only on local coordinate z.

 

[CHIKO-2010]

P.S. The only physical boundary condition on the wave-function of neutron is \psi(z=0)=0 (if the mirror is treated as a perfect reflector)

 

[MTd2]

U as it is defined in the paper is the standard operato

A standard operator that is non unitary...

 

[CHIKO-2010]

A standard operator that is non unitary...

It is not unitary only for the specific problem: neutron-earth system where the gravitational interactions are treated according to Verlinde. This follows from the entropic description of gravity and falsified by the observations. your heroic attempts to modify U on cosmological scales etc are wrong. well, you could try to modify quantum mechanics also -- the claim in the paper is only valid if the usual quantum mechanical description of the given problem is correct, of course.

 

[MTd2]

well, you could try to modify quantum mechanics also -- the claim in the paper is only valid if the usual quantum mechanical description of the given problem is correct, of course.

This is what is done in the paper too, it is set up its own axioms to modify quantum mechanics. There are other ways too, like, I sugested, setting up a kappa modification dependent on space and/or time.

 

[MTd2]

An holographic surface is a remain from a very limited attempt to see common aspects of GR and QM, which is the surface of a black hole quantized by a discreet quantity. This was later generalized by Wald (past cone only) and Bousso to any surface that represented causal discontinuity.

So, if one is claiming an holographic surface within a Newtonian context, that person is imposing quite artificially a causal discontinuity. For low gravitational fields, that's ok, but if one is trying to get a quantization in this same context, some GR low energy limit must be taken or considered. The reason for this it is that the integration will include places where gravitational distortions are very important, like the particle horizon or apparent horizon.

 

[CHIKO-2010]

This is what is done in the paper too, it is set up its own axioms to modify quantum mechanics. There are other ways too, like, I sugested, setting up a kappa modification dependent on space and/or time.

Your suggestion of nullifing kappa in eq. 13 would mean that the gradient of entropy for free falling neutron in the gravitational field of Earth is zero. According to Verlinde this means that neutron does not gravitate at all (i.e. free fall acceleration g=0) -- you will end up contradicting the experiments anyway

 

[CHIKO-2010]

P.S. The parameter kappa is entirely determined by the entropy gradient which is a constant (see e.q. 3.6 of Verlinde) i.e. it does not depend on space and time

 

[MTd2]

What I meant it is that the deficit in the non unitarity does not have to be equal to the gradient of entropy.