The CPT Theorem

The CPT theorem says that any Lorentz invariant quantum field theory must also be invariant under the combined operation of charge conjugation C, parity P, and time reversal T, even though none of those individual invariances need hold. It is quite strange. Why should a quantum field theory be invariant under the combination of two spatiotemporal discrete transformations, and then a quite different type of transformation (matter–anti-matter transformation)? In one of the first attacks on these and related questions by a philosopher, this chapter argues that CPT symmetry is better understood as PT symmetry. If the author is right, CPT symmetry is really saying that quantum field theory does not care about temporal orientation or spatial handedness.