The Multiverse & M-Theory

In his later years Hawking increasingly entertained a startling possibility: that physics describes not a single universe but a vast collection of them, a multiverse. This idea, tied to M-theory, is laid out most fully in his book The Grand Design, written with Leonard Mlodinow.

What M-theory proposes

M-theory is an attempt to unify the several competing versions of string theory into one framework, at the cost of requiring eleven dimensions of spacetime. One of its features is that it does not seem to predict a single, unique set of physical laws. Instead, it allows for an enormous number of possible solutions, each of which could correspond to a universe with its own particles, forces and constants of nature.

From M-theory to the multiverse

Combine this with cosmic inflation, which in many models keeps generating new regions of space endlessly, and you arrive at a picture in which countless universes are created, each potentially with different laws. Hawking argued that this could resolve one of the deepest puzzles in physics: why our universe seems so finely tuned for the existence of life. If there are vast numbers of universes with different properties, then it is no surprise that we find ourselves in one of the rare ones whose laws happen to permit observers, the kind of anthropic reasoning Brandon Carter pioneered.

A characteristic boldness

In The Grand Design Hawking went further, arguing that M-theory and this multiverse picture mean the universe could create itself from the laws of physics alone, with no need for a creator. It was a typically bold claim, and a controversial one. The multiverse remains speculative and, by its nature, extraordinarily hard to test, but it shows Hawking doing to the end what he always did: chasing the largest question of all, why there is a universe, with whatever tools the physics of his day provided.