# CPT theorem

The CPT theorem is a basic principle in particle physics which states that the combined operation of charge conjugation (C), time reversal (T), and parity (P) in any order is an exact symmetry of any interaction and that under the three transformations all physical laws must be invariant. This means that if any particle is replaced with its corresponding antiparticle, and the space coordinate and time are reversed, the physical laws are unchanged.

Charge conjugation symmetry refers to the symmetry between particles and their corresponding antiparticles. Parity inversion symmetry refers to the symmetry between left and right handedness, or the symmetry between a physical system and its mirror image. Time reversal symmetry refers to the symmetry between the forward and backward evolution of a physical system.

According to the mathematics of the theorem, CPT may be invariant if each of the independent parameters C, P, and T is invariant. If one of the parameters is not conserved then there must also be some compensating violation in one of the other parameters in order for the theorem to be sound.

Like CP, the combined operation of CPT converts particles at rest to antiparticles at rest, and invariance under this operation requires particles and antiparticles to have the same masses and lifetimes. This is in accord with experiment and is tested by observations on so-called "flavor oscillations" in neutral meson systems. See also CP violation.