Many Worlds Interpretation of Quantum Mechanics

Hugh Everett III developed his relative-state formulation of quantum mechanics while a graduate student in physics at Princeton University [5–7]. It was a reaction to his belief that the standard von Neumann—Dirac collapse formulation of quantum mechanics could not be consistently applied to systems which, like the universe, contained observers. Everett proposed solving the quantum measurement problem by dropping the collapse postulate from the standard formulation of quantum mechanics then deducing the empirical predictions of the standard collapse theory as the subjective experiences of observers who were themselves treated as physical systems described by the theory. While it remains unclear precisely how Everett intended for this to work, the relative-state formulation of quantum mechanics is often taken to be identical to Bryce DeWitt's popular many-worlds interpretation of Everett [1,2,4]. (See also ► Bohmian mechanics; Measurement theory; Metaphysics in Quantum Mechanics; Modal Interpretation; Objectification; Projection Postulate).

On Everett's relative state formulation of quantum mechanics observers were to be thought of as automatically functioning machines possessing recording devices that could be correlated with their environments. Everett's goal then was to deduce the appearance of the statistical predictions of quantum mechanics with the collapse postulate, as physical records in the memory of the observer, from pure wave mechanics without the collapse postulate: “We are then led to the novel situation in which the formal theory is objectively continuous and causal, while subjectively discontinuous and probabilistic. While this point of view thus shall ultimately justify our use of the statistical assertions of the orthodox view, it enables us to do so in a logically consistent manner, allowing for the existence of other observers” [7, p. 9].