Hunting neutron–antineutron oscillation

When Murray Gell‐Mann and Abraham Pais predicted in 1955 that the $K0$ meson should exhibit an oscillating probability for metamorphosis into its antiparticle, the $K̄0,$ they cited the neutron as a counterexample. The conservation of baryon number, they pointed out, would prevent the neutron–antineutron analog of neutral‐kaon oscillation. But nowadays, with grand unified theories of the elementary particles very much in favor, all bets based on baryon‐number conservation are off. A prodigious experimental effort (PHYSICS TODAY, January 1980, page 17) attests to the widespread expectation that proton decay will be seen with a lifetime of about $1031$ years.