Landmarks: Breaking the Mirror

Symmetry is a cherished principle in theoretical physics. For example, the mirror reflection of any physical process appears to operate according to the same laws as its real-world counterpart. So it was a huge surprise in the 1950s when researchers showed in the Physical Review that mirror symmetry doesn’t always apply. They found that reactions involving the weak nuclear force, such as the radioactive decay of cobalt-60, can violate parity conservation, as the mirror symmetry is known. The discovery marked the abrupt introduction of symmetry breaking into fundamental physics.