There are 3 very basic and related symmetries of nature, Charge, Parity and Time. Charge symmetry (C) implies that the laws of physics are unchanged if every particle is replaced by its anti-particle (charge conjugation). Parity (P) refers to a mirror image of a process. Parity symmetry implies that the physics of any process should be the same for the mirror image of that process. Finally, Time symmetry (T) implies that for dynamic systems, the fundamental laws of physics are the same for the forwards system and the Time reversed system. Symmetry plays a dramatic role in physics. One of the most amazing results of modern physics is the discovery of CPT symmetry. Almost every experiment in physics verifies the rule of T symmetry; that the laws of physics are the same if the direction of time is complemented. The same is true for P symmetry. During the mid 50’s the results from two new experiments violated the principles of Charge symmetry and of Parity symmetry. What was then thought was that CP was still a symmetry of physics. This meant that both Charge and Parity had to be changed at the same time for the laws of physics to remain exactly the same. It was in 1964 that we learned that the decay of the Kaon violated CP symmetry. That experimental result trashed CP symmetry and convinced the world of physics that the fundamental symmetry of nature is CPT. It is crucial to understand that CPT does not mean that C, P, T or other such symmetries (involving combinations of Charge, Parity and Time) are inexact or approximate under most circumstances. CP symmetry is essentially correct for all of physics except for situations where K0 (and perhaps B0) decays play a role.
The conclusion that physics has CPT symmetry is, philosophically, very important. For some questions we mustn’t tally the number of diverse experiments like votes in an election. If we always did so, we wouldn’t believe in CPT symmetry. Similarly, the question of whether a quantity of physics is ultimately continuous or discrete cannot be answered by noting all the experimental evidence that supports the continuous hypothesis since one good experiment that shows the quantity to be discrete is enough to decide the issue. It doesn’t matter how many experiments verify translational symmetry and angular isotropy. What is interesting is how few scientists seem to understand these principles. Ironically, when backed to the wall, the defenders of physical continuity cite the success of the calculus as evidence against discreteness.
