Gravity is not entropic force ?

In summary, Motl claims that the gravity cannot be an entropic force because the states of the holographic sheet related with the diffraction are not entangled with those of the mass, that is, they are orthogonal subspaes. But the above article, not only entangles them, but also considers that mass can be set as 0 entropy.
  • #1
czes
222
1
MTd2 already spotted this one! Looks interesting:
http://arxiv.org/abs/1009.5414
Gravity is not an entropic force - 27 Sep 2010
Archil Kobakhidze claims that if a slit of 15 microns (bilion times the Compton wavelength of neutron) is opaque for a slowly neutrons because of a quantum state formed in the slit, the gravity is not an entropic force.
Could you explain me what the quantum states formed in the potential well between the gravitational field on top and a horizontal mirror on bottom have to do with the origin of the gravity ?
The experiment was performed in 2003
arXiv:hep-ph/0306198
 
Physics news on Phys.org
  • #2
Although I am not always happy with Motl's style, I think his comments on Verlinde's approach have been fair and useful. The paper you point out is essentially the same argument as Motl posted in january
Why gravity can't be entropic
 
  • #3
humanino said:
Although I am not always happy with Motl's style, I think his comments on Verlinde's approach have been fair and useful. The paper you point out is essentially the same argument as Motl posted in january
Why gravity can't be entropic
Thank you Humanino
Motl's arguments are better understandable for me.
But what this experiment with neutrons has to do with a theory of Verlinde ?
 
  • #4
czes said:
Thank you Humanino
Motl's arguments are better understandable for me.
But what this experiment with neutrons has to do with a theory of Verlinde ?

Neutron interferometry shows that the quantum wavefunctions are influenced by gravity, in a predictable manner (you can observe phase shifts in the interference as the gravitational field changes)

Any theory of gravity has to explain that, Motl claims Verlinde's approach fails to do so.
 
  • #5
Verlnde`s fails because it is right. He finds Newtonian gravity and any deviation of it would be an error, there is no equivalence principle, speed of light as a constant does not make sense, etc.
 
  • #6
MTd2 said:
Verlnde`s fails because it is right. He finds Newtonian gravity and any deviation of it would be an error, there is no equivalence principle, speed of light as a constant does not make sense, etc.
What are you talking about? In the same paper, Verlinde also develops a general-relativistic variant of entropic gravity.
 
  • #7
Lubos Motl wrote:
"Holography leads us to imagine that the microstates of any system can be embedded into the quantum bits of a holographic screen that resembles a black hole horizon. However, any viable interpretation must be able to explain that the vacuum is a unique state and there are unique states for any position of the neutron in the gravitational field, and so on. String theory in general and the AdS/CFT correspondence in particular satisfy these constraints while Erik's framework does not."
He is not against the Holographic Principle, I think. He is against the Vacuum-aether made of particles like a ordinary molecules of a gas.
 
  • #8
Demystifier said:
What are you talking about? In the same paper, Verlinde also develops a general-relativistic variant of entropic gravity.

Yes, thanks. But the argument still does not hold because it supposes that the neutron cannot start with a zero entropy. On the top of page one:

"The above equations (10) and (11) tell us that even if we start with a pure neutron state
rN (z) at some z, i.e., SN (z) = 0, it ”evolves” into a mixed state rN (z + Dz) under the
translation along z".

We are talking about the holographic screen, so there will always be an information about the existence of the neutron on the screen. Mass has gravity and so entropy, starting without any entropy whatsoever, means starting without mass. This is why he finds a term of shift on (16), proportional to the rest mass: a neutron was created! There should be none of such term since it should cancel out.
 
  • #9
I am unable to see how just a difference in the number of states between the slits, would lead to break down of interference pattern (as Motl says).
 
  • #10
crackjack said:
I am unable to see how just a difference in the number of states between the slits, would lead to break down of interference pattern (as Motl says).

The states of the holographic sheet related with the diffraction are not entangled with those of the mass, that is, they are orthogonal subspaes. But the above article, not only entangles them, but also considers that mass can be set as 0 entropy.


Here is a case analog to that of the neutron experiment:

Suppose you want to calculate the entropy of a steam machine from its microstates. You can safely ignore the entropy of the quantum numbers of subatomic particles of the atoms, like those related to spin, color etc. There is no nuclear reaction involved, the entropy is already constant an maximal, what comes is comes out unchanged for practical purposes. Repeat the argument for the "quantum numbers of subatomic particles of the atoms" with "mass" and "steam machine entropy" with slit experiment, and there you have it.
 
  • #11
MTd2 said:
Yes, thanks. But the argument still does not hold because it supposes that the neutron cannot start with a zero entropy. On the top of page one:

"The above equations (10) and (11) tell us that even if we start with a pure neutron state
rN (z) at some z, i.e., SN (z) = 0, it ”evolves” into a mixed state rN (z + Dz) under the
translation along z".

We are talking about the holographic screen, so there will always be an information about the existence of the neutron on the screen. Mass has gravity and so entropy, starting without any entropy whatsoever, means starting without mass. This is why he finds a term of shift on (16), proportional to the rest mass: a neutron was created! There should be none of such term since it should cancel out.


All what matters is the entropy difference, eq. (11). SN (z) = 0 is not used to derive eq. (16). Also, your claim about mass and entropy is awkward - do you claim that massive pure states does not exist?
 
  • #12
Yes, you got a point. I never thought about that. In verlinde's gravity, mass cannot have entropy = 0, so no, there isn't a pure state of mass, because mass is a measure of disorder.

You see, temperature is a kind of measure of desorder, and in nuclear physics, it is commonly equated with temperature. Not that this is the same case here, but it follows along as a concept of disorder. And, as you know m=E, c=1. So, it makes sense that m= temperature!
 
Last edited:
  • #13
MTd2 said:
Yes, you got a point. I never thought about that. In verlinde's gravity, mass cannot have entropy = 0, so no, there isn't a pure state of mass, because mass is a measure of disorder. Hmm, good idea :)

You see, temperature is a kind of measure of desorder, and in nuclear physics, it is commonly equated with temperature. Not that this is the same case here, but it follows along as a concept of disorder. And, as you know m=E, c=1. So, it makes sense that m= temperature! :)


you are very wrong on this. within the special relativity there is an unambiguous definition of (inertial) mass. The inertial mass, according to experiments, is equal to the gravitational mass with very high accuracy. the concept of mass has certainly nothing to do with temperature and disorder.
 
  • #14
Verlinde is proposing a different theory of gravity, so SR and GR holds as diffusion equations. And certainly the mean deviation is very small. Just look at the size of the observable universe, and use that as more or less of the same magnitude of the holographic screen, which is much larger than 10^110plank^2.

And given that we are not exactly talking about GR or SR, but thermodynamical analogs, some things will cease to be states and rather, become statistical measures of other states, and in this case, it seems for me, that mass will become something like the temperature of a particle whereas the curvature will become something like the entropy.
 
  • #15
MTd2 said:
Verlinde is proposing a different theory of gravity, so SR and GR holds as diffusion equations. And certainly the mean deviation is very small. Just look at the size of the observable universe, and use that as more or less of the same magnitude of the holographic screen, which is much larger than 10^110plank^2.

And given that we are not exactly talking about GR or SR, but thermodynamical analogs, some things will cease to be states and rather, become statistical measures of other states, and in this case, it seems for me, that mass will become something like the temperature of a particle whereas the curvature will become something like the entropy.

Firstly, I do not think Verlinde has ever proposed the relation between mass and temperature as you put it here. Secondly, I think that precisely the fact that in Verlinde's theory there are nor pure states contradicts experiments, as it is pointed out in Mottl's essay and in the above paper
 
  • #16
Remember, if a neutron had a pure state, as you say, with S=0, it wouldn't be endowed with a gravitational field, so it wouldn't move anyway, or maybe it wouldn't exist, since it wouldn't be contained in an holographic screen. Note also that these are very rough analogies. A temperature does not need to be the same of that of a thermometer. It can be negative, if it is defined, for example, if it is defined in terms of spin of a lattice of ferromagnetic material.
 
  • #17
MTd2 said:
Remember, if a neutron had a pure state, as you say, with S=0, it wouldn't be endowed with a gravitational field, so it wouldn't move anyway, or maybe it wouldn't exist, since it wouldn't be contained in an holographic screen. Note also that these are very rough analogies. A temperature does not need to be the same of that of a thermometer. It can be negative, if it is defined, for example, if it is defined in terms of spin of a lattice of ferromagnetic material.


Well, again what you are saying follows from Verlinde's theory, and it looks like that its key predictions are not supported by observations
 
  • #18
One could formulate the statement. A particle with a 0 entropy of state is not represented on an holographic screen, so it doesn't exist. But that particle exist, thus Verlinde's gravity is false.

The contradiction can be solved, I think, like this: a particle exist, thus it has, at least, a relativistic mass. The minimum condition for existence of something, in Verlinde's gravity, is a bit of data on the holographic screen. The common denominator for the existence of both it is that a bit of data must have a relativistic mass. Thus, you cannot attribute, if you are working with this theory, with a particle without entropy.
 
  • #19
MTd2 said:
One could formulate the statement. A particle with a 0 entropy of state is not represented on an holographic screen, so it doesn't exist. But that particle exist, thus Verlinde's gravity is false.

The contradiction can be solved, I think, like this: a particle exist, thus it has, at least, a relativistic mass. The minimum condition for existence of something, in Verlinde's gravity, is a bit of data on the holographic screen. The common denominator for the existence of both it is that a bit of data must have a relativistic mass. Thus, you cannot attribute, if you are working with this theory, with a particle without entropy.

Why do you think that a particle in pure state cannot be represented on an holographic screen? This is certainly not correct! Think about of an ensemble of microstates, each individually having 0 entropy. I think the key point in Verlinde's theory is a difference in entropy between two holographic screens, that is, a state describing a neutron in the gravitational potential changes its entropy with the position. The case against Verlinde's theory is based on the arguments that precisely this basic feature of the theory is not supported by the quantum-mechanical experiments with neutrons.
 
  • #20
If you want to calculate the entropy, as defined by Verlinde, as it is the count of the very fundamental states, one cannot ignore the mass, by default, as a carrier of entropy by itself. If you ignore that, the theory will not work, as pointed out it the paper.

For me, the merit of that paper, if slightly changed, is to show that mass cannot be anything other than a measure of entropy. This is a great deal, not made clear by the author himself.
 
  • #21
MTd2 said:
If you want to calculate the entropy, as defined by Verlinde, as it is the count of the very fundamental states, one cannot ignore the mass, by default, as a carrier of entropy by itself. If you ignore that, the theory will not work, as pointed out it the paper.

For me, the merit of that paper, if slightly changed, is to show that mass cannot be anything other than a measure of entropy. This is a great deal, not made clear by the author himself.


I certainly cannot agree on this, and looks like we can't get a consensus...Anyway, thanks for the discussion. Cheers!
 
  • #22
MTd2 said:
The states of the holographic sheet related with the diffraction are not entangled with those of the mass, that is, they are orthogonal subspaes. But the above article, not only entangles them, but also considers that mass can be set as 0 entropy.


Here is a case analog to that of the neutron experiment:

Suppose you want to calculate the entropy of a steam machine from its microstates. You can safely ignore the entropy of the quantum numbers of subatomic particles of the atoms, like those related to spin, color etc. There is no nuclear reaction involved, the entropy is already constant an maximal, what comes is comes out unchanged for practical purposes. Repeat the argument for the "quantum numbers of subatomic particles of the atoms" with "mass" and "steam machine entropy" with slit experiment, and there you have it.

I have not read Verlinde's paper (just read Motl's critique). So, sorry, I could not make much out of your comment.
I will rephrase my question: Interference slowly starts breaking down if we differ the width of the slits etc, but how can I translate this (even heuristically) into a quantifiable statement based on differing number of states at the slits? In other words, how do we see that exp(10^15) (the value that Motl uses in his example) is huge enough for experimental detection of the pattern change?
 
  • #23
Motl's criticism is different from the paper of this thread. In the case of the paper, what is argued is the inadequacy of Verlinde's gravity to account for wave function evolution of massive because there is weird phase shift proportional to its relativistic mass. For me, this shift should not happen because the so called phase shift would be there from the beginning, yielding no difference at the end.

On Motl's article, he argues that since gravity is like a heat wave propagating on a solid, or aether, and if gravity disturbs a particle's path, the wave function would get phase random contributions from agitation of the "atoms" of this aether-like medium. This means that after a particles pass through a double slit, the two possible paths will interfere, on the projection screen, with random interference patterns.

That number he chose is the partition function a linear projection screen where we would count the states by a grid whose spacings would have the classical size of a neutron. He could had chose any other value, except infinite, but this is convenient due its intuitive experimental nature. When the "aether" is activated, that is, by running through different non inertial referentials(GR means being equivalent to apply a gravitational field), we clearly not expect to see anything.
 
  • #24
My own take on the latter criticism it is that if we are looking the problem from the space time point of view, we have to look at the minimum possible length. So, it shouldn't be a problem anyway, since there are 16 orders of magnitude to hide weird stuff.
 
  • #25
crackjack said:
I have not read Verlinde's paper (just read Motl's critique). So, sorry, I could not make much out of your comment.
I will rephrase my question: Interference slowly starts breaking down if we differ the width of the slits etc, but how can I translate this (even heuristically) into a quantifiable statement based on differing number of states at the slits? In other words, how do we see that exp(10^15) (the value that Motl uses in his example) is huge enough for experimental detection of the pattern change?



As far as I can understand Mottl's argument, the interference pattern is destroyed due to the decoherence which is inevitable in Verlinde's theory. The initial neutron state (which supposed to be a mixed state with an entropy) sent towards the slits and the neutron states in two slits are described by the different number of microstates, they carry different entropy...in such situation neutron wave-functions decohere, and the factor exp(10^15) "measures" this decoherence. You simply do not see the interference pattern, which in contradiction with observations.

The paper of the thread is essentially based on the same argument. loosely speaking the bound states in the gravitational potential well can be viewed as standing waves formed due to the interference between incident neutron waves and the ones reflected from the mirror. These states are affected due to the entropy difference in the Verlinde's gravity, and the result contradicts experiments.

MTd2 is wrong supposing that the results of the paper are based on the assumption on the existence pure neutron states (with zero entropy). this is not true, as i wrote previously what is important is the entropy difference. Besides, the claim of non-existence of massive pure states is completely bizarre.
 
  • #26
P.S. According to MTd2 arguments photons as massless particle do not gravitate, which is certainly not correct -- light bends in the gravitational field.
 
  • #27
In http://online.itp.ucsb.edu/online/joint98/verlinde2/" , he essentially acknowledges that entropy can't be the whole story in his derivation of gravity, and in the second half starts looking for a property of the individual pure state which can play an analogous role.
 
Last edited by a moderator:
  • #28
CHIKO-2010 said:
P.S. According to MTd2 arguments photons as massless particle do not gravitate, which is certainly not correct -- light bends in the gravitational field.

A photon bends space time and its path bended because it has mass. The type of mass is relativistic, as I said before. The rest mass is 0, though.
 
  • #29
MTd2 said:
A photon bends space time and its path bended because it has mass. The type of mass is relativistic, as I said before. The rest mass is 0, though.


What do you mean under the notion of "relativistic mass"? One of the key differences between GR and the Newtonian gravity is that the latter does not predict gravitation of a massless particles -- but this is NOT because the "relativistic mass" of photon approach to its rest mass = 0 in the non-relativistic limit! The photon gravitates in GR because of the tensorial nature of the gravitational field, while Newtonian gravitational field (potential) is a scalar and it simply cannot couple to a massless vector field (photon). Again, gravitation of photon in no way depends on relativity, you may write perfectly relativistic theory of gravitation where photon does not gravitate (scalar theory of gravitation)
 
  • #30
Relativistic mass is the same as gravitational mass locally and in inertial frame, by the equivalence principle. And I am not referring anywhere to a non relativistic limit.
 
  • #31
Sorry, but photon mass is 0 in any (locally) inertial reference frame! That how the mass is defined -- in group theory language mass is an eigenvalue of the second casimir operator of the relativistic Lorentz group
 
  • #32
Alright, let's say that if a particle does not hold any energy, it doesn't exist.
 
  • #33
MTd2 said:
Alright, let's say that if a particle does not hold any energy, it doesn't exist.

Very funny statement -- can you imagine a material object which has no energy? in fact it is true statement - a quantum state without energy describes (again locally) vacuum states which has no real particles! But what this statement has to do with the holographic sreens and Verlinde's theory?
 
  • #34
George F.Smoot wrote in arXiv:1003.5952v1[hep-th] 31 Mar 2010 :
Chapter IV. It seems that having the temperature proportional to the curvature of space-time is a strange thing. We have entropy proportional to the inverse of the curvature (S~R^2) and temperature proportional to the curvature (T~1/R^2). The product is a constant for a fixed enclosed energy.
The Unruh temperature is T=h a /2 pi c k
here Smoot wrote:
T=hc/2 pi k R

It means a massive object curves the space-time and will perceive the same temperature as an accelerated object accordingly. If there is one particle with a rest mass it creates also a gravitational field around and there has to be a temperature. It will be also if the particle is alone in a Void. It means that it absorbs and emits quantum states from its space-time and oscillate due to Planck's black body radiation.

Does it mean each massive particle absorbs and emits quantum states due to its Compton wavelength in rest and shifted due to its relativistic energy ?
In the experiment there are neutrons in a motion and absorbed neutron in rest. Their wave functions has to interfere, I think.
 
  • #35
An isolated particle ("particle in a Void") does indeed emits and absorbs quantum states constantly, but these are the so-called virtual states, they are part of the vacuum, and as such do not create neither entropy nor temperature
 

Similar threads

  • Beyond the Standard Models
Replies
2
Views
2K
Replies
8
Views
4K
Replies
2
Views
2K
  • Beyond the Standard Models
Replies
6
Views
8K
  • Quantum Physics
Replies
12
Views
3K
  • Beyond the Standard Models
Replies
13
Views
4K
  • Beyond the Standard Models
Replies
2
Views
2K
  • Beyond the Standard Models
Replies
11
Views
3K
  • Beyond the Standard Models
2
Replies
39
Views
5K
  • Quantum Physics
2
Replies
45
Views
10K
Back
Top