If an operator commutes with the Hamiltonian, then, the eigenvalues are said to be good quantum numbers. For example, the helicity. But then, helicity is not an invariant for a massive particle. I can always go to another Lorentz frame such that the helicity is now reversed. How then, can it be a good quantum number?(adsbygoogle = window.adsbygoogle || []).push({});

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# Good quantum number

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