The manyworlds interpretation is an interpretation of quantum mechanics that asserts the objective reality of the universal wavefunction, but denies the actuality of wavefunction collapse. Manyworlds 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, manyuniverses interpretation, or just manyworlds.
The original relative state formulation is due to Hugh Everett in 1957. Later, this formulation was popularized and renamed manyworlds by Bryce Seligman DeWitt in the 1960s and 1970s. The decoherence approaches to interpreting quantum theory have been further explored and developed, becoming quite popular. MWI is one of many multiverse hypotheses in physics and philosophy. It is currently considered a mainstream interpretation along with the other decoherence interpretations and the Copenhagen interpretation.
Before manyworlds, reality had always been viewed as a single unfolding history. Manyworlds, however, views reality as a manybranched tree, wherein every possible quantum outcome is realised. Manyworlds claims to reconcile the observation of nondeterministic events, such as the random radioactive decay, with the fully deterministic equations of quantum physics.
In manyworlds, the subjective appearance of wavefunction collapse is explained by the mechanism of quantum decoherence, which resolves 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 lay terms, there is a very large—perhaps infinite—number of universes, and everything that could possibly have happened in our past, but did not, has occurred in the past of some other universe or universes.
Quantum mechanics 

Read more about Manyworlds Interpretation: Outline, Interpreting Wavefunction Collapse, Probability, Properties of The Theory, Common Objections, Brief Overview, Relative State, Comparative Properties and Possible Experimental Tests, Reception, Manyworlds in Literature and Science Fiction Main Site Subjects
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