We live in a universe that is dominated by matter and contains very little antimatter. The laws of physics, however, include an almost exact symmetry between matter and antimatter. That symmetry is not the simple charge conjugation (C) that relates a particle and its corresponding antiparticle. Rather, it is CP , the product of C with coordinate inversion, or parity (P).

Before the Dirac equation was introduced in 1928, there was no concept of a symmetry relating matter and antimatter; indeed, antimatter had not been conceived. There was simply a conservation law, the conservation of matter. All the matter in the universe—all the stuff with mass—must always have been there. The prevailing view of Western science, and of Western religions and philosophy, was that we live in a static, unchanging universe. The constancy of matter in such a universe is no puzzle.

Hubble’s law, the linear...

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