"Relative State" Formulation of Quantum Mechanics

Everett's thesis is the founding text of the many-worlds interpretation — the single most influential multiverse proposal in physics.

"From the viewpoint of the theory, all elements of a superposition are equally 'real.'" (Everett, 1957, Reviews of Modern Physics)

Everett takes the wave function to be physically real — not merely a calculational device. The universal wave function is the fundamental ontology.

"We shall assume the wave function to be the basic physical entity." (Everett, 1957)

In Everett's formulation, the universal wave function evolves deterministically — all apparent randomness is a feature of the observer's relative state, not of the fundamental dynamics.

"The wave equation is satisfied at all times for the complete system — observer plus object." (Everett, 1957)

The thesis is motivated by realist dissatisfaction with the Copenhagen interpretation's instrumentalism — Everett wants a theory of what physically exists, not just what we can predict.

"The task of describing the theory is that of describing the nature of the physical reality." (Everett, long thesis, 1956)

Everett's approach treats the observer as a physical system within nature — no special observer-collapse postulate, no consciousness exception.

"The observer is treated as a physical system; there is no separate postulate for observation." (Everett, 1957)

The thesis raises foundational questions about theory choice, ontological parsimony, and the interpretation of formalism that have driven philosophy of physics for decades.

"The formalism is capable of yielding its own interpretation." (Everett, 1957)