What is the relativistic spin operator?

Understand

Although the spin is regarded as a fundamental property of the electron, there is no universally accepted spin operator within the framework of relativistic quantum mechanics.

• We investigate the properties of different proposals for a relativistic spin perator.
• It is shown that most candidates are lacking essential features of proper angular momentum operators, leading to spurious Zitterbewegung (quivering motion) or violating the angular momentum algebra.
• Only the Foldy-Wouthuysen operator and the Pryce operator qualify as proper relativistic spin operators.