The parity law of physics states that for any atomic or nuclear system no new physical consequence or law should result from the construction of a new system, differing from the original by being a mirror twin.

Consider Particle” spinning about a direction M’.

Now construct or find Particle 2, which is chosen to be identical to the mirror image of 1.

The parity law says that there should be no observable difference between the two particles, 1, and 2, which may be detected by measurements made along direction M’.

This law permits one to make predictions: suppose 1 is radioactive, disintegrating into electrons.

The parity law predicts that equal numbers will be emitted towards A, and A’.

Why? Consider the alternative.

If 1 emits more electrons towards A, 2 must emit more towards A’ since 1 becomes identical to 2 simply by turning it upside down.

But now 2 is no longer the same as the mirror image of 1.

The physicist observing would make one decision about the relation between the direction of favored electron emission and the spin sense; the physicist in the mirror world would obtain a different answer.

Parity law would have been violated.

For the past thirty years, the special conditions predicted by the philosophically pleasing idea of mirror symmetry have borne fruit, consistently making successful predictions about atomic and nuclear processes.

However a general theory of the structure of matter eluded us.

Then, in the new subject of “strange particles,” the K-mesons studied at Brookhaven and Berkeley, the first parity puzzle appeared.

This led to the Lee Yang proposal.

The preferential emission of electrons towards one direction of its spin is the observation that disproved the parity law. ~Wolfgang Pauli, Atom and Archetype, Pages 225-226