String Theory & M-Theory

String theory is an ambitious framework in which the fundamental building blocks of nature are not point-like particles but tiny, vibrating strings. Different vibrations of the string would correspond to different particles, much as different notes come from one violin string. Its great appeal is that it naturally includes gravity alongside the other forces, making it a leading candidate for the long-sought theory of quantum gravity that would unite general relativity with quantum mechanics.

M-theory is a later development that unites several competing versions of string theory into a single framework, at the cost of requiring eleven dimensions of spacetime rather than the familiar four.

Why it mattered to Hawking

Hawking was not primarily a string theorist, but he engaged seriously with these ideas, especially late in his career. In The Grand Design, written with Leonard Mlodinow, he argued that M-theory is the best current candidate for a complete theory of the universe, and used it to suggest that the laws of physics might allow a multitude of different universes.

String theory also produced the holographic principle, one of the most promising routes to resolving the information paradox that Hawking raised, meaning his black hole work and string theory remain deeply intertwined in the search for quantum gravity.