On the CPT theorem

•We provide a rigorous proof of the CPT theorem within the framework of ‘mainstream’ (as opposed to axiomatic) quantum field theory.•We include a careful treatment of symmetries in classical and quantum field theories.•We explain the geometric origin of CPT symmetries in any spacetime dimension above 2.•No detailed features of the representation theory of the Lorentz group are used.

We provide a careful development and rigorous proof of the CPT theorem within the framework of mainstream (Lagrangian) quantum field theory. This is in contrast to the usual rigorous proofs in purely axiomatic frameworks, and non-rigorous proof-sketches in the mainstream approach. We construct the CPT transformation for a general field directly, without appealing to the enumerative classification of representations, and in a manner that is clearly related to the requirements of our proof. Our approach applies equally in Minkowski spacetimes of any dimension at least three, and is in principle neutral between classical and quantum field theories: the quantum CPT theorem has a natural classical analogue. The key mathematical tool is that of complexification; this tool is central to the existing axiomatic proofs, but plays no overt role in the usual mainstream approaches to CPT.