Symmetry is broken
Before Chien-Shiung Wu and her ingenious experiment, people believed that the laws of physics would not change if we mirrored the coordinates of our universe. But the weak interaction breaks this symmetry.
Wu and her colleagues cooled down cobalt atoms and used a magnetic field to align their spins. Cobalt atoms decay into nickel atoms through the weak interaction, emitting an electron and an anti-neutrino. The electrons were not homogenously distributed in space; instead, most were found in the direction opposite to the spin of the cobalt atoms. This surprising result proved that we can distinguish our universe from its mirror image.
What does radioactivity have to do with it?
Chien-Shiung Wu and the experiment that defied our intuition.