CPT in physics usually means the CPT theorem or CPT symmetry. It says that the laws of physics stay the same if you apply three transformations together: charge conjugation CCC, parity PPP, and time reversal TTT.

What each letter means

  • C — Charge conjugation: swap particles with antiparticles.
  • P — Parity: flip spatial coordinates, like looking in a mirror.
  • T — Time reversal: run time backward.

Why it matters

In quantum field theory, CPT symmetry is a deep result: if a theory is local, Lorentz-invariant, and has a Hermitian Hamiltonian, then it should obey CPT symmetry. This is important because it implies particles and antiparticles have the same mass and lifetime, among other matching properties.

Simple intuition

A good way to think about CPT is: if you built a universe made of antimatter, mirrored in space, and played backward in time, the fundamental laws should look the same. Individual symmetries like C, P, or T can be violated in some interactions, but the combined CPT symmetry is expected to hold in known fundamental physics.

Mini example

For example, weak interactions can violate parity and even CP symmetry, but the full CPT combination remains the key symmetry expected by the theorem. That is why CPT is considered one of the most important symmetry principles in modern physics.

TL;DR: CPT in physics means charge, parity, and time together, and the CPT theorem says the laws of nature should be unchanged under all three transformations applied at once.