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Many-worlds interpretation

The many-worlds interpretation is a postulate of quantum mechanics that asserts the objective reality of the universal wavefunction, but denies the actuality of wavefunction collapse, which implies that all possible alternative histories and futures are real—each representing an actual "world" (or "universe"). It is also referred to as MWI, the relative state formulation, the Everett interpretation, the theory of the universal wavefunction, many-universes interpretation, or just many worlds.

Many-worlds claims to reconcile how we can perceive non-deterministic events, such as the random decay of a radioactive atom, with the deterministic equations of quantum physics. Prior to many-worlds, reality had been viewed as a single unfolding history. Many-worlds, rather, views reality as a many-branched tree, wherein every possible quantum outcome is realised.

In many-worlds, the subjective appearance of wavefunction collapse is explained by the mechanism of quantum decoherence. By decoherence, many-worlds claims to resolve all of the correlation paradoxes of quantum theory, such as the EPR paradox and Schrödinger's cat, since every possible outcome of every event defines or exists in its own "history" or "world". In layman's terms, there is a very large—perhaps infinite—number of universes, and everything that could possibly have happened in our past, but didn't, has occurred in the past of some other universe or universes.

Source: Wikipedia - Many-worlds interpretation