P-symmetry is simply the spatial
symmetry exhibited during a
reflection. P stands for
parity. Apart from orientation reflection would mathematically mean a reversal of either one or all three axes of reference in a three-dimensional
Cartesian coordinate system. It was taken for granted that all physical laws preserved P-symmetry, and nobody ever doubted the possible falsity of the idea, until the 1950s. In fact, all of
electromagnetism and
gravity (both
quantum and
relativistic versions) preserved such a symmetry. It would mean that, 'left' or 'right', a 'positive' or a 'negative'
electric charge, 'north' or 'south' poles of a magnet, and all such things are mere naming conventions, and there is no absoluteness in the definition of a choice for such dualities.
Feynman explains it interestingly as how we would be able to communicate to an intelligent alien as to what we mean by 'right' or 'left' (so as to shake hands with the 'right' hand, when we meet them first), without using any objects (stars, for instance) for reference, and solely through an experiment conducted by the aliens that would in principle show them what 'right' or 'left' is. The preservation of P-symmetry by all physical phenomena, means that there is no such experiment that would in principle distinguish 'left' from 'right'.
It came as a shock to most physicists in the 1950s that P-symmetry was violated in some radioactive phenomena which involved weak forces. In fact, an experiment by Lee and Yang using radio-cobalt exhibited a bias towards a spatial direction (ie., not conserved under a reflection and simply put, in a mirror image experiment would have a bias different from the mirror image of the direction stated). This lead to a search of stronger symmetries that would be preserved. In fact, the violation of the C-symmetry (charge inversion symmetry) and T-symmetry (time reversal symmetry) also was evident. For some time from then, it was believed that a combination of the two, CP-symmetry was preserved, which was again found to be wrong. Finally, for now, we have a theorem that maintains the preservation of a CPT-symmetry. All known laws of physics preserve the CPT-symmetry.
The well known statement, "God is weakly left-handed", stands to explain this P-violation.
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