M-THEORY (THE THEORY FORMERLY KNOWN AS STRINGS)

Superunification underwent a major paradigm shift in 1984 when eleven-dimensional supergravity was knocked off its pedastal 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year has witnessed a new shift of equal proportions: perturbative ten-dimensional superstrings have in their turn been superseded by a new non-perturbative theory called M-theory, which describes supermembranes and superfivebranes, which subsumes all five consistent string theories and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 S1 /Z2×K3 (heterotic/heterotic duality) or S1 ×K3 (Type IIA/heterotic duality) or S1 /Z2 × T 4 (heterotic/Type IIA duality) or S1 × T 4 (Type IIA/Type IIA duality).