1 January 2011

The ups and downs of entropic gravity

Electromagnetism is due to exchange of photons. Gravity is due to an exchange of gravitons.

Large numbers of photons have entropy. So do large numbers of gravitons.

Thus the electromagnetic field has entropy. Therefore, so does gravity and space-time.

Thus electromagnetism is entropic. Therefore, so is gravity and space-time.

There is nothing really new in all this. Nor is there anything to be questioned about it.

Why do people like Lubos Motl and Peter Woit get so upset about entropic gravity? This is a good case for studying prejudices.

Motl is upset because the argument makes it clear that an explanation of gravity with gravitons does not need superstrings. And he is upset because the argument makes clear that gravity is a purely three-dimensional effect. This does not fit his prejudice that the world is 11-dimensional.

Woit is upset because the argument is so simple. Woit, despite his fight against many prejudices, also promotes some of his own prejudices. And one that he promotes is: nature is complicated and needs complicated mathematics for its description.

We see that the most disparate scientists can criticize obviously true statements because of their prejudices. For both of them, prejudices count more than facts.


  1. Photons and "virtual photons" are very different in their properties. Photon flux or gas may be in equilibrium or not and it is described with giving occupation numbers of photon states.

    "Virtual photons" is a sloppy name for interaction between charges. Factually, it is something like 1/|r1 - r2|. It is quite certain. I do not see how one can infer any entropy from it.

  2. Real and virtual photons have almost the same properties.

    The virtual photons that create 1/r^2 also create field uncertainties which contain entropy. (This entropy is a pure quantum effect that does not exist classically.)

  3. If the real photons are observed in many experiments and are an experimental notion, virtual photons stand for direct interaction of charges that had been known well before inventing the term "virtual photon". Moreover, it is a "perturbative" approach notion, and we know that the perturbation theory in QED fails badly and needs immediately repairing.

  4. Te argument above is independent of the issue of perturbation theory. Electromagnetic fields have uncertainties, and these uncertainties lead to an entropy of electromagnetic fields.

  5. Yes, a photon subsystem may have entropy but the virtual photons belong to particles and the particle subsystem entropy is calculated somewhat differently.

  6. Ok, but we are very far from the original posting now. I just wanted to say that gravity is obviously entropic, no doubt about it.

  7. I came across this post by browsing the internet, and can not resist to put a remark. The author seems has complete misunderstanding of very simple things such as classical Coulomb and Newtonian forces. Surely, transition from quantum electrodynamics to classical electrodynamics involves decoherence and thus the classical field carries the entropy. However, it is the gradient of the potential (not the entropy!) which produces the Coulomb force. The same presumably applies to the gravitational force, unless you believe Verlinde. I do not.

  8. Anonymous,

    Not believing that gravity is entropic means denying black hole entropy.

    Everybody has his own prejudices.

  9. "Not believing that gravity is entropic means denying black hole entropy."

    Sorry, but the above remark is nonsense

  10. Gravity has an entropic side: after all it leads to black hole entropy. If you believe that gravity has no entropic side, you deny black hole entropy. The Verlinde argument just takes shows that this entropic aspect, when applied to all systems, not only black holes, leads to Newton's law.

    Feel free to believe that gravity is not entropic, but then you live in another universe.

  11. Dear Nemo, you are totally confused - any classical configuration (such as black hole) carry entropy, but that does not mean that entropic phenomena are fundamental. For example, a bunch of charged particles carry entropy, but at fundamental level the dynamics of charged particles is govern by quantum electrodynamics.

    Feel free to believe that also electromagnetism (and perhaps everything else)is entropic, but then I am sorry for you.

    P.S. This paper http://prd.aps.org/abstract/PRD/v83/i2/e021502 shows that Verlinde's idea is wrong. If you are familiar with basics of QM you will be able to understand why.

  12. The paper you mention is pure nonsense. I quote:

    "Thus, if E. Verlinde’s idea is correct, the neutrons would have to travel through the slit without substantial losses even if λ < h <<< z1."

    The author was obviously drunk when he wrote this.

    It is not clear why you feel as you do, but you are free to feel as you like. Physics is not about feelings, and even less about *your* feelings.

    Here we have a male physicist that uses feelings as an argument! The way you write you are neither a male nor a physicist.

  13. Dear Clara, alias Nemo,

    What did you find so nonsensical in that quote? Before criticizing someone I would kindly suggest to educate yourself at least at the level of Wikipedia.

    The paper is published in one of the leading physics journal and obviously went through the referring process. Be more rational and a little bit more humble in expressing your views.

  14. Dear Anonymous,

    nonsense is nonsense, even if peer-reviewed. In physics truth is determined by facts, not by peer-reviewing. My own education also has nothing to do with whether this is true or false.

    You sound as if you were a frustrated man who believes more in authority than in facts. In fact, you sound like the author of that paper. Be happy that you were able to screw the system in getting nonsense into PRD - it remains nonsense nevertheless.

    Like many journals, also PRD publishes nonsense from time to time. If you think that "being in PRD" is a reason that I or others should conclude that an idea is true, then your place is not in physics research, but in an asylum.

  15. The weird thing about the Kobakhidze article is that it is so fundamentally flawed that a high school physics teacher should be able to tell what's wrong with its assumptions... and if a first semester physics student couldn't tell in a couple of minutes why the claimed decoherence effect would not be measurable with current cryogenics experiments on ultra-cold neutrons, even if it existed, I would send him to the liberal arts department, because he/she will clearly not make a career out of physics.

    One can only conclude that publishing in PRD is not worth a dime these days, the reviewers are clearly sleeping while reviewing papers.