# Why is quantum mechanics called quantum *mechanics*? [closed]

At least a few books have "Quantum Mechanics" in their title, e.g., Sakurai's Modern Quantum Mechanics. However, I don't think measuring spin, which is "the" way of introducing quantum mechanics, concerns itself with mechanics, the study of motion. It seems to me that these books ought to have "Quantum Physics" in their titles as opposed to "Quantum Mechanics". "Quantum Physics" better encapsulates what these books cover. What's the deal with this?

## closed as primarily opinion-based by ZeroTheHero, Alfred Centauri, John Rennie, Aaron Stevens, user191954 Oct 6 '18 at 4:36

Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise. If this question can be reworded to fit the rules in the help center, please edit the question.

• Have you considered asking this in our chat room? I've voted to close this question because it is primarily opinion based. – Alfred Centauri Oct 6 '18 at 3:28
• There's a lot more to mechanics than just motion. – Anyon Oct 6 '18 at 3:32
• QM was developed from quantizing the most advanced forms of classical mechanics at the time: action, Poisson brackets, and Hamilton Jacobi Equations, etc. These are generally studied at the graduate level, long after undergrads take their 1st crack at QM. – JEB Oct 6 '18 at 3:45
• Something for History of Science and Mathematics SE? – Avantgarde Oct 6 '18 at 3:48
• Couldn't you argue that "Quanum Physics" is too broad? – Aaron Stevens Oct 6 '18 at 4:09

Furthermore, standard quantum mechanics, and quantum dynamics, is fundamentally similar to classical mechanics in terms of algebraic properties (of course the Hilbert space of quantum mechanics is very unclassical). As Sakuri remarks on page 48, Dirac's correspondence principle is that you take the classical mechanical Poisson bracket, replace it with a commutator and divide by $$i\hbar$$ and now you have the quantum mechanical structure. This is true since the Poisson bracket and the commutator produce similar algebras. This fact is illuminated in the Heisenberg Picture, where the Heisenberg equation of motion is used (which is reminiscent of the classical mechanical analogue), trajectories are found, and commutators are everywhere!