Editing C-symmetry (section)

{{Short description|Symmetry of physical laws under a charge-conjugation transformation}}
{{Refimprove|date=December 2008}}

In [[physics]], '''charge conjugation''' is a [[transformation (mathematics)|transformation]] that switches all [[particle]]s with their corresponding [[antiparticle]]s, thus changing the sign of all [[charge (physics)|charges]]: not only [[electric charge]] but also the charges relevant to other forces.  The term '''C-symmetry''' is an abbreviation of the phrase "charge conjugation symmetry", and is used in discussions of the symmetry of physical laws under charge-conjugation. Other important discrete symmetries are [[P-symmetry]] (parity) and [[T-symmetry]] (time reversal).

These discrete symmetries, C, P and T, are symmetries of the equations that describe the known [[fundamental force]]s of nature: [[electromagnetism]], [[gravity]], the [[strong interaction|strong]] and the [[weak interaction]]s. Verifying whether some given mathematical equation correctly models [[nature]] requires giving physical interpretation not only to [[continuous symmetry|continuous symmetries]], such as [[motion]] in time, but also to its [[discrete symmetries]], and then determining whether nature adheres to these symmetries. Unlike the continuous symmetries, the interpretation of the discrete symmetries is a bit more intellectually demanding and confusing. An early surprise appeared in the 1950s, when [[Chien Shiung Wu]] demonstrated that the weak interaction violated P-symmetry. For several decades, it appeared that the combined symmetry CP was preserved, until [[CP violation|CP-violating]] interactions were discovered. Both discoveries led to [[Nobel Prize]]s.

The C-symmetry is particularly troublesome, physically, as the universe is primarily filled with [[matter]], not [[anti-matter]], whereas the naive C-symmetry of the physical laws suggests that there should be equal amounts of both. It is currently believed that CP-violation during the early universe can account for the "excess" matter, although the debate is not settled. Earlier textbooks on [[cosmology]], predating the 1970s,{{which|date=November 2020}} routinely suggested that perhaps distant galaxies were made entirely of anti-matter, thus maintaining a net balance of zero in the universe.

This article focuses on exposing and articulating the C-symmetry of various important equations and theoretical systems, including the [[Dirac equation]] and the structure of [[quantum field theory]]. The various [[fundamental particles]] can be classified according to behavior under charge conjugation; this is described in the article on [[C-parity]].