parity (P)
/ PAIR-ih-tee /
Hold a clock up to a mirror and the reflection runs backwards; your right hand becomes a left hand. Parity, written with the letter P, is the physics version of this mirror flip — it reverses left and right (technically it inverts all three directions of space, turning every position into its exact opposite through the origin). The question parity asks is simple: does the mirror image of a process obey the same laws as the original?
For most of nature, the answer had always seemed obviously yes. A bouncing ball, a colliding pair of protons under the strong force, a beam of light bending through glass — film any of these, then watch the mirror-flipped version, and you cannot tell which is the 'real' one. Gravity, electromagnetism, and the strong force all treat left and right even-handedly, so parity is a symmetry for them. This felt so natural that for a long time everyone simply assumed it must hold everywhere.
Then in 1956 came a shock: the weak force does not respect parity. In a famous experiment, radioactive cobalt nuclei were lined up and their beta-decay electrons came out preferentially in one direction — the mirror-image version, which should have been equally likely, did not happen the same way. Nature, through the weak force, can tell its left hand from its right. This discovery of parity violation overturned a cherished assumption and reshaped how physicists think about symmetry: a symmetry can be exact for some forces and badly broken by another.
Neutrinos are nature's clearest proof that parity is broken: every neutrino ever observed spins like a left-handed screw relative to its motion, and right-handed neutrinos are never seen. A perfect mirror would turn a left-handed neutrino into a right-handed one — but that mirror-image particle simply does not appear, so the weak force is visibly not parity-symmetric.
Only left-handed neutrinos exist, so the mirror world is missing a particle.
Parity is not a universal law: the weak force violates it maximally. Before 1956 physicists assumed it always held, a reminder that even an apparently obvious symmetry must be tested, not taken for granted.