CPT theorem

We have met three separate flips: C swaps particles for antiparticles, P holds the world up to a mirror, and T runs time backwards. Each one can be broken by the weak force. The CPT theorem makes a striking promise: do all three at once — turn matter to antimatter, mirror-reflect it, and reverse time — and the result must obey exactly the same laws as the original. Not approximately, but precisely. CPT together is believed to be an unbreakable symmetry of nature.

What makes this more than a hopeful guess is that CPT is a theorem — it can be proved, not just assumed. Any theory built on a few very basic and well-tested foundations (special relativity, quantum mechanics, and the reasonable requirement that cause precedes effect) is forced to have CPT symmetry. You do not get to choose; the structure of relativistic quantum field theory hands it to you. This is why physicists treat a violation of CPT as essentially unthinkable within the current framework: finding one would mean one of those bedrock assumptions is wrong.

CPT has powerful, testable consequences. The most important is that a particle and its antiparticle must have exactly the same mass and exactly the same lifetime (and opposite charges). This is why experiments measure the mass of the antiproton against the proton, or trap antihydrogen and compare its light to ordinary hydrogen's, to dizzying precision — any difference would shatter CPT and overturn the foundations of modern physics. So far, every test agrees: matter and antimatter match perfectly under the combined CPT flip, even though the individual flips C, P, CP, and T can each be broken.