my question is about the nature of time across classical/macro and quantum scales. I understand that the 2nd law of thermodynamics and entropy increase has a lot to do with our understanding of time and what it is in classical physics; manifested in the irreversibility of certain physical processes, linear cause and effect etc.

However, I have been reading about the difficulties in determining the nature of time in particular at the quantum level. I have read it variously stated that quantum mechanics is time symmetric, that time evolves from thermodynamics only at the macro-scale and that in effect, time is not perceived at the quantum level.

I understand that energy dissipation itself, and therefore entropy may not be conceived in the same way as it is on macro scales, but the probabilistic nature of quantum events surely meets the definition of irreversibility. i.e.: even taking all output parameters from an event and running the event in reverse there is only a probability of the initial state arising, not a certainty. Therefore cause and effect, are also linear at the quantum scale. Indeed due to this probability element, there is no way to accurately reverse events.

Is there not a correlation between this underlying objective indeterminism of quantum mechanics and the fact that time exists at all? If quantum mechanics were deterministic and not based on probability then I DO see an issue in that events would in fact be reversible. Otherwise I don't see any time symmetry problem... am I mistaken?

Sorry if this has been asked before; I couldn't see anything precisely of this nature but perhaps that is a result from my poor comprehension of some of the more technical question formulations.

  • $\begingroup$ Do you mean "time exists at all" in the sense that if the universe was at thermal equilibrium, nothing macroscopic would change? Are you thinking that if quantum mechanics was reversible, it would be deterministic? Such a universe at equilibrium could not take a microscopic step away from a max entropy state? Or if it did, it would step right back? $\endgroup$ – mmesser314 Oct 19 '18 at 13:52
  • $\begingroup$ Im struggling to understand where “the problem of time” arises in quantum mechanics and why it is viewed as time symmetrical. Surely the aspect representing the one way nature of events (time) in quantum mechanics is the probabilistic nature of quantum events. Perfect thermodynamic equilibrium would entail no time but this also makes sense at the quantum scale since there would be no quantum events occurring so time would not arise from those events. $\endgroup$ – user43685 Oct 19 '18 at 16:54
  • $\begingroup$ Ps: I think a universe in perfect thermodynamic equilibrium would not have time, matter or much that we could consider “real” about it. I lack (any) knowledge on the topic but I don’t see how entropy would even have a meaning in such a universe. $\endgroup$ – user43685 Oct 19 '18 at 18:29

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