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Disclaimer : Im not a physicist!

I have heard Loschmidt's paradox described as "the laws of thermodynamics are time asymmetric because entropy always increases, but the underlying laws of physics are symmetric under time reversal."

Could someone explain the main difference between thermodynamics (is this the same as Kinetic Theory or Statistical Mechanics?) and the 'underlying laws of physics' - which I assume they mean Molecular Dynamics.

Maybe reference to Figure 1 on page 2 of this http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.97.2226 could help me. Some main differences :

  • What size scale do they work on?
  • How many bodies (or particles) are there usually in the systems they describe?
  • Do they describe different physical phenomena?
  • Can one pass between the two, i.e do the theories agree?
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  • $\begingroup$ You mean statistical physics rather than molecular dynamics, which is usually understood as the name of a specific computational method. $\endgroup$
    – Roger V.
    Commented Nov 26, 2020 at 11:03
  • $\begingroup$ Hi! There is a lot of literature on this. You say you're not a physicist, are you asking because you did not find a reference that was accessible to non-physicists? What is your backround? $\endgroup$
    – user2723984
    Commented Nov 26, 2020 at 11:04
  • $\begingroup$ I do math. Yes there is almost too much literature felt like I was drowning in terms and references I didn't know the meaning of. $\endgroup$
    – Monty
    Commented Nov 26, 2020 at 11:23

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I would say thermodynamics is a coarse-grained view where one disregards that the components of a system are constituted by myriads of particles which obey the fundamental laws of physics. The fundamental principles of thermodynamics are mainly of phenomenological origins (they have been found by induction from experimental results). They emerge from the coarse-graining view which involves loss of information about the molecular state of the system.

Statistical mechanics (for which the kinetic theory of gases can be seen as a sort of a prequel) tends, among other things, to demonstrate that indeed the principles of thermodynamics emerge from the more fundamental laws of physics through the coarse-graining process.

My understanding is that the apparent paradox comes from the asymmetric nature of the coarse-grainig process itself, because it involves some loss of information.

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