The central quandary facing 21st-century physics is that the two
main pillars of 20th-century physics – quantum mechanics and
Einstein's general theory of relativity, are mutually
incompatible. Quantum theory deals with the very small: atoms,
subatomic particles and the forces between them. General
relativity deals with the very large: stars, galaxies and
gravity, the driving force of the cosmos as a whole. The dilemma
is that on the microscopic scale, Einstein's theory fails to
comply with the quantum rules that govern the behaviour of the
elementary particles. On the macroscopic scale, black holes are
threatening the very foundations of quantum mechanics. Something
big has to give. This augurs a new scientific revolution.

Some believe that this revolution is already under way because
of "superstrings". As their name suggests, superstrings are
one-dimensional string-like objects. Just like violin strings,
they can vibrate, and each mode of vibration, each note if you
like, corresponds to a different elementary particle. This note
is an electron, that one a quark, a Higgs boson, and so on. The
strange feature of superstrings is that they live in a universe
with nine space dimensions and one time dimension. Since the
world around us seems to have three space dimensions, the extra
six would have to be curled up to an unobservably small size if
the theory is to be at all realistic. Fortunately, in the
equations this actually happens. Unfortunately, there are not
one but five mathematically consistent superstring theories,
each competing for the title "Theory of Everything" – an
embarrassment of riches.

This problem is cured by M-theory, a unique all-embracing theory
which subsumes the five superstring theories by requiring 11
space-time dimensions and incorporating higher-dimensional
extended objects called branes. M stands for "Magic, Mystery or
Membrane", according to taste.

Theorists are so excited about string theory and M-theory
because they seem at last to provide the long-dreamed-of
consistent quantum theory of gravity and hold promise of
incorporating and extending standard models of particle physics
and cosmology.

*Michael Duff is Abdus Salam Professor of Theoretical Physics
at Imperial College London, where he was speaking on Wednesday*