String theory and M-theory
← Theories of Everything · Glossary
Author: Many; M-theory synthesis by Edward Witten, 1995
Published: Peer-reviewed physics literature, 1970s onward
Reference: String theory
Point particles are replaced by one-dimensional strings whose vibrational modes are the particles. Requires extra spatial dimensions, and produces a graviton without being asked to.
Standing: Active mainstream research; not experimentally confirmed.
The strongest argument for it
String theory was not built to explain gravity. A massless spin-2 particle — exactly what a graviton must be — falls out of the mathematics unavoidably. A framework aimed at the strong force turned out to contain gravity whether or not anyone wanted it there. That is the kind of unforced result that makes physicists take a programme seriously.
It also requires ten or eleven dimensions, with the extra ones compactified below observable scale. Witten’s 1995 insight was that the five competing string theories are limits of a single deeper structure, M-theory.
Why it is a programme and not established
It has produced no confirmed experimental prediction in fifty years. The energies at which strings would be directly visible are around 1019 GeV — roughly a quadrillion times what the LHC reaches.
The sharper criticism is the landscape: compactification admits an estimated 10500 vacua, each with different physical constants. A framework that can accommodate almost any observation risks forbidding none, which is what critics mean when they question whether it is falsifiable at all. Defenders answer that the theory is one structure and the landscape a selection problem, not a free parameter.
Filed as a research programme: mainstream, serious, extensively developed, unconfirmed.