Everett was a doctoral pupil of John Wheeler in the 1950s at Princeton. In 1957 he published a famous paper on the foundations of quantum mechanics describing what has become known as the ‘many worlds’ interpretation. The paper was entitled Relative State Formulation of Quantum Mechanics.

The traditional Copenhagen interpretation of quantum mechanics applied only to the submicroscopic world. Everett broke away from this tradition and attempted to apply quantum mechanics to the universe. He established a universal wave function that could be applied to both microscopic entities and macroscopic observers. As a consequence, there is no collapse of the wave function and quantum paradoxes, such as Schrödinger's cat, are avoided.

This approach, however, is not without paradoxical conclusions of its own. In Everett's formulation, the result of a measurement is to split the universe into as many ways as to allow all possible outcomes of the measurement. Thus if an observer were to check the outcome of a die throw, the universe would split into six copies with each one containing one of the six possible outcomes of the throw. Everett proposed that each outcome is realized in a number of parallel universes between which there is no communication.

While Everett's work has inevitably been taken up by many science fiction writers, it has also been taken seriously by other scientists. Gell-Mann, for example, has tried to develop a version of quantum theory that eliminates the role of the observer, in the manner of Everett, but reduces the idea of ‘many worlds’ to one of possible histories of the universe to which a probability value can be assigned.