Good quantum numbers, such as helicity, are defined by their commutation with the Hamiltonian operator. While helicity serves as a good quantum number for massless particles, it is not invariant for massive particles, as it can change when transitioning between different Lorentz frames. This discussion highlights the importance of measuring good quantum numbers without altering the particle's energy, despite their frame-dependent nature. The conversation also raises questions about the validity of helicity commuting in the quantum Hamiltonian, referencing Dirac's cautionary notes on the subject.
PREREQUISITESPhysicists, particularly those specializing in quantum mechanics and particle physics, as well as students seeking to deepen their understanding of quantum numbers and their applications in various frames of reference.