Abstract
One of the most fundamental symmetries in physics is CPT invariance. This article reviews the conditions under which CPT symmetry holds by recalling two proofs of the CPT theorem: The original Lagrangian-based analysis and the more rigorous one in the context of axiomatic quantum field theory. The presentation of the proofs is followed by a discussion of the major physical implications that arise from CPT symmetry. Motivated by recent theoretical and experimental interest in CPT tests, various approaches to the violation of CPT symmetry are mentioned, and it is briefly discussed how they evade the CPT theorem. An attempt has been made to keep this work self-contained and at a level suitable for a wider readership by excising as many technical aspects as possible.
Highlights
In the last century, the concept of symmetry in the laws of nature has played an increasingly important role
The established discrete spacetime symmetry is given by CPT invariance, where
CPT is the sole combination of C, P and T observed as an exact symmetry of nature at the level of known fundamental physics
Summary
The concept of symmetry in the laws of nature has played an increasingly important role. CPT is the sole combination of C, P and T observed as an exact symmetry of nature at the level of known fundamental physics. On the other hand, studying as to whether the transformed dynamical variables continue to satisfy the same equation is more meaningful Note that this corresponds to the actual experimental situation, in which measurements before and after the transformation of the apparatus (and not the observer) are compared. Some theoretical approaches to physics beyond the Standard Model, such as those mentioned, contain a violation of CPT invariance as a key prediction. For these reasons, CPT tests represent an interesting and timely avenue for experimental research in fundamental physics.
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