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I am looking for a book/paper which derives classical mechanics starting from quantum mechanics, to better understand the transition. Expected level of mathematical rigour is equivalent to graduate level physics texts.

Any recommendations are welcome.

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    $\begingroup$ Are you familiar with the path integral formulation of QM? $\endgroup$ Commented Jul 7, 2023 at 4:05
  • $\begingroup$ Familiar, but not very comfortable. I will slog through it if necessary $\endgroup$
    – junfan02
    Commented Jul 7, 2023 at 4:06
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    $\begingroup$ Wikipedia: “the mathematical operation involved in classical limits is a group contraction, approximating physical systems where the relevant action is much larger than the reduced Planck constant ħ, so the "deformation parameter" ħ/S can be effectively taken to be zero (cf. Weyl quantization.) Thus typically, quantum commutators (equivalently, Moyal brackets) reduce to Poisson brackets, in a group contraction.” $\endgroup$
    – Ghoster
    Commented Jul 7, 2023 at 4:19
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    $\begingroup$ Some of us find comfort in monitoring the classical limit of (quantum) Moyal brackets to Poisson brackets, in phase space; are you at peace with all of classical mechanics following from Poisson brackets? If so, all you need is study the phase-space formulation of QM, to eliminate Hilbert space in its cultural mismatch to classical mechanics.... Regardless of bloviations and platitudes, in this formulation you must work out concrete, specific problems to appreciate the point... $\endgroup$ Commented Jul 7, 2023 at 15:55
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    $\begingroup$ Linked, & linked, & linked. $\endgroup$ Commented Jul 7, 2023 at 17:34

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There many different ways to discuss this transition. The most straightforward one uses the path integral formulation of quantum mechanics, in which the transition amplitude from some state $\left| \psi \right\rangle$ to another state $\left| \phi \right\rangle$ is found by adding up all of the possible classical trajectories (in phase space) that could in principle interpolate between these two states, each one weighed by $\exp(iS/\hbar)$, where $S$ is the action for the trajectory in question. \begin{equation} \left\langle \phi,t_f|\psi,t_i \right\rangle = \sum_{\text{all histories}} e^{iS/\hbar}. \end{equation} For a macroscopic system, the action for most histories is an enormous number in units of $\hbar$, whose value changes appreciably even between phase space paths which are very similar. The result is that the complex exponentials tend to cancel each other for most of the paths in phase space, due to the rapid oscillations causing a sort of ''destructive interference'' in the sum. The only exceptions to this are the histories which happen to sit in a minimum of the action. For these, the action will have a lower value that does not change so rapidly between neighboring phase space trajectories around this minimum, so their phases do actually add constructively. The end result is that for macroscopic systems, the transition amplitude is dominated by the contribution of the histories that lie on the minima of the action, which are the classical trajectories. Most graduate level QM books discuss this in detail. One undergraduate book that has a nice explanation of this is A Modern Approach to Quantum Mechanics, by Townsend.

Another, completely different way of looking at how a quantum system becomes classical is by thinking in terms of decoherence. This a more complicated subject, which explains how small perturbations to a quantum system due to interactions with the environment end up destroying its quantum properties and causing it to ''collapse'' (or decohere) to its classical configurations. Since it is impossible to isolate a macroscopic system from the environment, this is a way to understand why such a system effectively obeys classical mechanics, despite its fundamental quantum nature. An excellent pedagogical introduction to this is https://arxiv.org/pdf/1508.04101.pdf, in which this phenomenon is worked out explicitly for a toy model. The original papers by Zurek on decoherence are also recommended, as well as his talks, which you can find on Youtube. Here is one of them https://www.youtube.com/watch?v=7Sn63t3BeMc.

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There is an enormous literature on why classical equations of motion are a good approximation on the scale of everyday life. When information is copied out of a quantum system that prevents interference and working out the implications in enough detail leads to equations of motion that look like those of classical mechanics:

Quantum Decoherence

Macroscopic Superpositions, Decoherent Histories and the Emergence of Hydrodynamic Behaviour

Relative States and the Environment: Einselection, Envariance, Quantum Darwinism, and the Existential Interpretation

Decoherence and Ontology, or: How I Learned To Stop Worrying And Love FAPP

Decoherence by Maximilian A. Schlosshauer

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    $\begingroup$ This is really a link-only answer; each link should be expanded to give title, authors etc and provide some details on the contents and the level of the work, in accordance with the guidelines on resource recommendations. $\endgroup$ Commented Jul 7, 2023 at 12:22

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