Many-Worlds Interpretation

The many-worlds interpretation (MWI), proposed by Hugh Everett III in 1957, resolves quantum mechanics' measurement problem by the most radical possible means: denying that collapse ever occurs. The Schrödinger equation is always right; at every measurement, the universe branches into all possible outcomes, each branch containing observers who see only one result.

MWI restores the determinism that Copenhagen abandoned: the total quantum state of the universe evolves unitarily, continuously, and predictably — the branching is deterministic in the sense that all branches occur. But it purchases this determinism at the price of an immensely proliferating ontology: there are as many copies of every observer as there are possible measurement outcomes, continuously multiplying.

The interpretation's deepest problem is not proliferation but probability: if all branches exist with certainty, in what sense does any branch have probability 1/3 rather than 1/2? The Born rule — which tells us the probabilities of measurement outcomes — does not emerge naturally from the branching structure alone. Multiple attempts have been made to derive it (Deutsch, Wallace), but they remain contested. If MWI cannot explain why some branches seem more probable than others, it explains quantum mechanics' predictions only by assuming them.

As a picture of reality, MWI is the closest modern physics has come to Laplace's demon — a fully deterministic universe with no hidden variables. But it is a demon that can never recognize itself in the mirror, because each branch-observer sees only one face.