Ehrenfest's Theorem Revisited

Abstract

Historically, Ehrenfest's theorem (1927) is the first one which shows that classical physics can emerge from quantum physics as a kind of approximation. We recall the theorem in its original form. Next, we highlight its generalizations to the relativistic Dirac particle, and to a particle with spin and izospin. We argue that apparent classicality of the macroscopic world can probably be explained within the framework of standard quantum mechanics.

Article based on talk given at the XXII Kraków Methodological Conference "Emergence of the Classical". Cracow, 11-12 October 2018. To appear in a special issue of "Philosophical Problems in Science"