# Ehrenfest's Theorem Revisited

## TL;DR

This paper revisits Ehrenfest's theorem, discussing its original form and extensions to relativistic and spinning particles, and argues that classicality emerges naturally from quantum mechanics.

## Contribution

It provides a comprehensive review of Ehrenfest's theorem and its generalizations, emphasizing the potential explanation of classicality within standard quantum mechanics.

## Key findings

- Ehrenfest's theorem connects quantum and classical physics.
- Generalizations include relativistic and spin considerations.
- Classicality may arise from quantum mechanics without additional assumptions.

## 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.