 Because of the specific properties of the quantum description, the discrete symmetries of spatial reflection symmetry or parity (P) and time reversal (T) were “rediscovered” in the quantum context, taking on a new significance. Parity was introduced in quantum physics in 1927 in a paper by Wigner, where important spectroscopic results were explained for the first time on the basis of a group-theoretic treatment of permutation, rotation and reflection symmetries. Time reversal invariance appeared in the quantum context, again due to Wigner, in a 1932  paper.[13]  To these was added the new quantum particle-antiparticle symmetry or charge conjugation (C). Charge conjugation was introduced in Dirac’s famous 1931 paper “Quantized singularities in the electromagnetic field”.