How entropy (thermodynamics) related to time flow?

Question

In Tenet (2020), Laura speaks about inverted bullets:

Laura: It's inverted. Its entropy runs backward. So, to our eyes, its movement is reversed. We think it's a type of inverse radiation triggered by nuclear fission.

use a movie scene to explain a movie scene p.s: Don't ask me if this physics is accurate LOL — tryst with freedom, Dec 11 '20 at 18:12
Btw I think you made a typo, it should be entropy (In title and in body) — tryst with freedom, Dec 11 '20 at 18:13
https://physics.stackexchange.com/q/430073/274807 check this post. — David, Dec 11 '20 at 19:43
What is time, does it flow, and if so what defines its direction? — mmesser314, Dec 11 '20 at 23:48

score 3 · Accepted Answer · answered Dec 11 '20 at 19:41

Strictly speaking, thermodynamic entropy is not function of time, but function of thermodynamic state. 2nd law implies only that when adiabatic system goes from equilibrium state 1 to equilibrium state 2, thermodynamic entropy of state 2 cannot be lower than thermodynamic entropy of state 1.

We can introduce more definition of entropy for non-equilibrium processes that is consistent with the previous one for equilibrium states and then 2nd law implies(with caveats) that such entropy cannot sustainably decrease in time, it can only fluctuate - it can spontaneously decrease by small amount for a small time with small probability.

Measuring time flow by entropy increase is a very inaccurate and pointless thing to do. Mechanical or electronic clocks are much better.

How entropy (thermodynamics) related to time flow?

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