Theorems in quantum mechanics

Kramers' theorem

In quantum mechanics, the Kramers' degeneracy theorem states that for every energy eigenstate of a time-reversal symmetric system with half-integer total spin, there is another eigenstate with the same energy related by time-reversal. In other words, the degeneracy of every energy level is an even number if it has half-integer spin. The theorem is named after Dutch physicist H. A. Kramers. In theoretical physics, the time reversal symmetry is the symmetry of physical laws under a time reversal transformation: If the Hamiltonian operator commutes with the time-reversal operator, that is then, for every energy eigenstate , the time reversed state is also an eigenstate with the same energy. These two states are sometimes called a Kramers pair. In general, this time-reversed state may be identical to the original one, but that is not possible in a half-integer spin system: since time reversal reverses all angular momenta, reversing a half-integer spin cannot yield the same state (the magnetic quantum number is never zero). (Wikipedia).

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Related pages

Antiunitary operator | Hilbert space | Deuterium | Magnetic quantum number | Proton | Hamiltonian (quantum mechanics) | Fermion | Electron | Angular momentum | Spin–statistics theorem | Spin (physics) | Half-integer | T-symmetry | Neutron