M is for

M-theory was born at another Witten talk, nearly two decades earlier. At the time, physicists had hit a wall with their best hope for uniting quantum mechanics and general relativity: string theory. The theory showed that when people zoom in closely enough on an elementary particle like an electron or a top quark, they find that it's actually a tiny string wiggling around in 10 dimensions. Different particles play like musical notes on a vibrating string. But physicists had five equally consistent but fundamentally different string theories--and when it comes to theories of everything, no one wants five. Here, Gefter cracks the case of M-theory