A quantum-mechanical symmetry of a system under time reversal, i.e. the transformation of the time t to −t. Time reversal is concerned with relations between quantum states rather than being a property of a state. The operation T transforms a wavefunction into its complex conjugate. The concept of time reversal in quantum mechanics was proposed by Eugene Wigner in 1932 and enabled him to derive Kramers’ theorem as a consequence of time-reversal symmetry.