When it was discovered in 1957 that the weak interactions of elementary particles are not symmetric under the parity operation P, theorists retreated to the seemingly safe presumption that the combined operation CP remained an inviolate symmetry. (The charge-conjugation operation C replaces all particles by their antiparticles.) But seven years later came a second rude awakening: The decay of neutral K mesons revealed a minuscule but undeniable violation of CP symmetry. So particles viewed in a mirror don’t behave exactly like their antiparticles.

The violation of CP symmetry strongly implies that the weak interactions must also be asymmetric under time reversal T. That’s because inviolate CPT symmetry was, and still remains, a bedrock theorem of particle theory. In 1999 experimenters thought they had found the first direct evidence of T violation in the observed difference in the forward and backward rates for the oscillatory metamorphosis K0 ↔...

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