Definition
String theory is a theoretical framework in which the elementary constituents of nature are not point particles but tiny, vibrating one-dimensional objects — strings — whose different modes of vibration correspond to different particles. The theory automatically contains a massless spin-2 mode that behaves like the graviton, so quantum gravity is built in.
Consistency requires the strings to live in 10 spacetime dimensions (11 for M-theory), with the extra spatial dimensions assumed to be curled up too small to observe.
Why it matters
How it works
In ordinary quantum field theory, an electron is a point. In string theory it is a tiny loop or segment of string, a Planck length or so in size. As the string moves through spacetime it sweeps out a two-dimensional worldsheet, and string interactions are smooth topological events: two worldsheets joining into one. There is no instantaneous point of interaction where infinities can arise.
The string can vibrate in many modes, each with a definite mass and spin. One mode is automatically massless with spin 2 — the graviton, the long-sought quantum of gravity. Others correspond to gauge bosons, fermions, and (in the superstring versions) supersymmetric partners. The hope is that the entire Standard Model spectrum can emerge as low-energy vibrations of strings on the correct background geometry.
Consistency forces strict requirements. Bosonic string theory needs 26 dimensions; superstring theory needs 10. The extra six spatial dimensions are imagined to be compactified into a small Calabi-Yau shape whose geometric details select the four-dimensional particle content and couplings. Five different consistent superstring theories were discovered in the 1980s — Type I, Type IIA, Type IIB, Heterotic SO(32), Heterotic E8×E8 — and in 1995 Edward Witten showed they are all limits of a single 11-dimensional theory, M-theory.
The empirical situation is hard. The natural string energy scale is the Planck scale, ~10¹⁹ GeV — fifteen orders of magnitude beyond the LHC. Any low-energy footprint would come from chosen background geometry, and the choices form a vast "landscape" estimated at 10⁵⁰⁰ or more vacua. Some physicists view this as evidence that the theory is unconstrained; others see it as the universe's actual configuration space.