25.4 Charge conjugation, parity, and time reversal

The operation that replaces every particle by its antiparticle is referred to
as C (which stands for charge conjugation). A physical interaction that is
invariant under the replacement of particles by their antiparticles (and
vice-versa) is called C-invariant.
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This operation of spatial reflection (reflec-
tion in a mirror) is referred to as P (which stands for parity).
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In accordance with the above discussion, in §25.3, ordinary weak interactions are not
invariant under either P or C separately, but it turns out that they are
invariant under the combined operation CP (= PC). We may regard CP as
the operation performed by an unusual mirror, in which each particle is
reflected as its antiparticle. We see that CP sends a particle’s zig into its
antiparticle’s zag, and vice versa.
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There is one further operation that is normally discussed in relation to these, which is that of time reversal, referred to as T. An interaction is invariant under T if it unaltered if we view it from the prespective of a time direction that is the reverse of
normal.
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There is a famous theorem in quantum Weld theory, referred to
as the CPT theorem which asserts every physical interaction is invariant if
all three of the operations C, P, and T are applied to it at once.
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