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When I studied a spin, the textbook said spin is an intrinsic quantity like mass. However, while we can calculate just expectation values $ \langle \textrm{S}^2\rangle $ or $ \langle S_z\rangle $, the mass $m$ seems to be fixed everywhere in quantum mechanics. Is it right? If so, why isn't there the uncertainty about the mass of particles?

Qmechanic
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Jungmin Kim
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1 Answers1

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The mass is given by the Casimir operator $\hat{P^2}$, the square of the momentum, generated by translation invariance. This will give you the actual mass of the particle, as opposed to the bare mass that you will find in the theory, as the actual mass can be affected by the particle's interactions.

Slereah
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