CPT Theorem

In quantum field theory the CPT theorem states that any canonical (that is, local and Lorentz-covariant) quantum field theory is invariant under the operation, which is a combination of three discrete transformations: charge conjugation, parity transformation, and time reversal . It was first proved by G.Lüders, W.Pauli and J.Bell in the framework of Lagrangian field theory.

At present, is the sole combination of, observed as an exact symmetry of nature at the fundamental level.