M theory (the theory formerly known as strings)

Superunification underwent a major paradigm shift in 1984 when eleven-dimensional supergravity was knocked off its pedestal by ten-dimensional superstrings. This last year has witnessed a new shift of equal proportions: perturbative ten-dimensional superstrings have in their turn been superseded by a new nonperturbative theory called M theory, which describes supermembranes and superfivebranes, which subsumes all five consistent string theories and whose low energy limit is, ironically, eleven-dimensional supergravity. In particular, six-dimensional string/string duality follows from membrane/fivebrane duality by compactifying M theory on S-1/Z(2) x K3 (heterotic/heterotic duality) or S-1 x K3 (Type IIA/heterotic duality) or S-1/Z(2) x T-4 (heterotic/Type IIA duality) or S-1 x T-4 (Type IIA/Type IIA duality).