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I am taking the second law of thermodynamics in the 'useful energy minimized' sense, although I am not sure if that is necessarily the most correct.

Take the example of two electrons: electron A and electron B. Electron B has a wavefunction. Can we say that electron B's wavefunction will not have high probabilities close to electron A because that would lead to there being more useful energy than otherwise?

Otherwise, is there a similar type of 'wasted energy maximized' or 'entropy maximized' for singular wavefunctions rather than groups of particles?

Instead of treating entropy as an outcome of different particles, perhaps the wavefunction's different possible positions of particles can do a similar job?

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    $\begingroup$ Thermodynamics applies to (large) systems of particles with emergent behavior. It is not useful to apply to one or two isolated particles. $\endgroup$ Commented Sep 1, 2023 at 11:15
  • $\begingroup$ If that is so, why do small-scale things involving very few particles show the same behaviour? Things like nuclear fission, or individual chemical reactions, all involve some form of loss of 'useful energy'. I feel as if I am missing some key concept! $\endgroup$ Commented Sep 1, 2023 at 11:19
  • $\begingroup$ The quora answer quora.com/… seems to suggest otherwise. This physics.stackexchange.com/q/171777 also maybe suggests that there is some quantum equivalent that isn't necessarily emergent, although i may be misinterpreting. $\endgroup$ Commented Sep 1, 2023 at 11:20
  • $\begingroup$ "This definition implies that the von Neumann entropy of any pure state is zero" en.wikipedia.org/wiki/Density_matrix $\endgroup$ Commented Sep 7, 2023 at 7:46
  • $\begingroup$ Would you elaborate on what you might mean specifically by this "I am taking the second law of thermodynamics in the 'useful energy minimized' sense"? $\endgroup$ Commented Sep 8, 2023 at 20:17

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No. The laws of thermodynamics and statistical physics^1 applied only to large collections of particles, in so-called thermodynamic limit, where the number of particles $N$ and the volume $V$ of the system go to infinity, but some parameters, like density $n=N/V$ are kept constant. Typically, one takes Avogadro number of particles $N_A\approx 10^{24}$ as the case where the corrections to the quantities of interest (aka fluctuations) are negligibly small. However with appropriate care one can apply thermodynamic laws to much smaller systems, keeping in mind that the fluctuations decrease as $\propto \frac{1}{\sqrt{N}}$.

Remark: Not sure if this is behind the question: randomness inherent in quantum mechanics should not be confused with the statistical (but classical) randomness in statistical physics. Obviously, both are combined when we deal with large collections of quantum objects.

Update in response to comments
As pointed above, statistical mechanics deals with large collections of particles, and the statistical averaging is understood as averaging over all the possible configurations of particles (e.g., their positions, momenta, spin, etc.)

In traditional interpretations of quantum mechanics (like the Copenhagen interpretation) averaging is understood as averaging over many identical experiments on identical particles/systems prepared in the same state. Mathematically, it is also statistical averaging, but over a different ensemble. Such an averaging is necessitate by the fact that the measurement collapses the wave function (unless it is in the eigenstate of the measurement operator), and thus cannot be repeated on the same system. Note however, that this ensemble is meaningful even for a single particle (where we cannot talk about thermodynamic properties.)

States of a collection of quantum particles are described by multiparticle wave functions, which often cannot be factorized into the product of wave functions of single particles (due to interactions, and due to the indistinguishability of particles, which imposes (anti)symmetry on the many-particle wave function.) Different configurations of the many-particle system then refer to different eigenstates (wave functions) of the many-particle system, and statistical averaging implies averaging over these states (i.e., "averaging of wave functions".) As the result, in *quantum statistical physics one has to resort to using the density matrix - a construction more general than wave function.


^1 Statistical physics is probably more correct term here, since we talk about microscopic description rather than phenomenology)

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  • $\begingroup$ Ballentine views the wavefunction $\psi$ as characterizing a statistical ensemble of systems not an individual system. Aside from the controversy around Ballentine being right or wrong in light of Bell's theorem, does this statistical interpretation have any entropic relevance in the sense of the 2nd law to OdinOblivion's question? $\endgroup$ Commented Sep 13, 2023 at 16:12
  • $\begingroup$ @hyportnex most canonical formulations of QM deal with statistical ensembles, but these are not the same as the statistical ensembles in statistical physics. This is an interesting direction however+ perhaps I will expand my post tomorrow to discuss it from this angle. $\endgroup$ Commented Sep 13, 2023 at 19:51
  • $\begingroup$ @hyportnex I posted an update in response to your comment. $\endgroup$ Commented Sep 14, 2023 at 8:42

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