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year has witnessed a new shift of equal proportions: perturbative ten-dimensional superstrings have in their turn been superseded by a new nonperturbative theory called M theory, which describes supermembranes and superfivebranes, which subsumes all five consistent string theories and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 × K3 (heterotic/heterotic duality) or S ¹ × K3 (Type IIA/heterotic duality) or S ¹ /Z ₂ × T ⁴ (heterotic/Type IIA duality) or S ¹ × T ⁴ (Type IIA/Type IIA duality).