That leap in understanding arrived in 1957, when three physicists from the University of Illinois—John Bardeen, Leon Neil Cooper, and John Robert Schrieffe—developed the BCS theory (named after their last initials) to describe all the quantum goings-on when a metal becomes a superconductor at critical temperature, or Tc (which varies by material).
The theory goes that negatively charged electrons, which would usually repel each other under normal circumstances, actually form pairs (known as “cooper pairs”) when cooled down to Tc. This pairing is effectively created by electron interactions with vibrations, or “phonons,” in the atomic crystalline structure. The result? Zero electrical resistance.