Does the second law of thermodynamics still work without wavefunction collapse?
I received these contradicting answers on Quora:
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The strict answer to the question in the title is: no, isolated systems evolving unitarily do not increase their entropy (if they are isolated, where would the information go?). The subtleties around this issue have to do with the question of "what is entropy anyway". Entropy is always defined by postulating some ignorance about the complete knowledge of the state of the system. In the statistical mechanics formalism the ignorance is (roughly) about the microscopic degrees of freedom. In quantum mechanics it is common to associate ignorance with inaccessible environment. Entropy appears when you discard ("trace out") the information about the state that you are ignorant about. Wave function collapse is the result of discarding all information except for one observable (i.e. position). How the second law of thermodynamics works in quantum mechanics is an area of active research and there is much more to say here.
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