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Gravity and Thermodynamics

  1. Sep 8, 2012 #1
    We know about dye molecules diffusing in solution, or solvation, as entropic forces; they're not real forces but driven by thermodynamics.

    The increasing entropy gives us a direction of time. Dye molecules diffusing in water will never re-concentrate as a drop of dye in pure water.

    And yet, gravity can affect time, according to general relativity. Most importantly, it can slow the rate of time passage. This seems to be a very strong connection between thermodynamics and gravitation.

    I think then that the gravitational waves are an entropic process: stable orbits would violate thermodynamics as that's a closed system with a spontaneous process yet is not increasing in entropy; orbits must decay (as we see with gravitational wave emission) simply due to entropic considerations.

    So how does time dilation, black holes, etc. factor into a possible thermodynamic view of the universe? Can heat affect time, as RMS velocities approach relativistic speeds?

    Are there any more examples of interesting phenomena in gravitation fully explainable by a thermodynamic view?
  2. jcsd
  3. Sep 8, 2012 #2


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    Why? All physical theories* use time for the evolution of systems.

    *well, there are a few exceptions, but they do not matter here
    That two parts of physics share the concept of time does not mean that they are the same. The evolution of stars uses time, so does the description of a car. That does not mean that stars are similar to cars.

    Quantum mechanical ground-state orbits are stable.

    There is no example. While thermodynamics is used to derive Hawking radiation at black holes, this still needs gravity.
  4. Sep 8, 2012 #3
    However there have been papers written about gravity as a non-fundamental, entropic force.


    There are also popular articles written about this.

    Quantum mechanical ground state orbits are not actual orbits. They are orbitals, which is just a state of a particle denoted by an energy quantum number and has an angular momentum.

    In fact, if you don't take into account perturbations from the zero-energy all around, non-ground state orbitals are stable too! This is usually covered in a later chapter of a quantum mechanics course in time dependent perturbation theory. So I don't see the significance of a quantum mechanical particle in a certain state being stable.

    Yes, all physical theories use time, but, I believe that significant insights into relativity were made through the use of light as a toy model, and extrapolating things that occurred to light to other things. I believe the same thing can be done for gravity and thermodynamics.
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