Two theorists guessed nature might not be perfectly mirror-symmetric; it was Chien-Shiung Wu who PROVED it, cooling cobalt-60 to near absolute zero and watching its decay. Her experiment showed the electrons fly out preferring one direction relative to the spin — so a mirror-image universe is DISTINGUISHABLE from ours. The men she guided won the Nobel; she did not. Slide the mirror and watch it break.
Parity symmetry says the mirror-image of any physics is equally valid physics. Wu measured beta electrons from aligned cobalt-60 nuclei: emission probability ∝ 1 + α(σ·p̂), the correlation between nuclear SPIN and electron MOMENTUM. Under a mirror, momentum (a vector) flips but spin (an axial vector) does not, so if α≠0 the mirror world differs — parity is VIOLATED. Wu found a large asymmetry (electrons preferring against the spin). A fail-loud self-check throws unless the along-spin and against-spin rates differ (α≠0), so mirror ≠ original.
A toy 1+αcosθ model of the real angular distribution; α here is illustrative of Wu’s large measured asymmetry (1957). That parity is violated in the weak interaction — and that the mirror universe is distinguishable — is the exact, Nobel-winning result (Lee & Yang got the prize; Wu was passed over).