Many-worlds interpretation
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The many-worlds interpretation (or MWI) is an interpretation of quantum mechanics that proposes the existence of multiple "parallel universes", all of which have the same physical laws and constants, but occupy different states. MWI was initially formulated by Hugh Everett as an alternative to wavefunction collapse used in the Copenhagen interpretation to explain non-deterministic processes (such as measurement) in quantum mechanics
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As with the other interpretations of quantum mechanics, the many-worlds interpretation is motivated by behavior that can be illustrated by the double-slit experiment. When particles of light (or anything else) are passed through the double slit, a calculation assuming wave-like behavior of light is needed to identify where the particles are likely to be observed. Yet when the particles are observed, they appear as particles and not as non-localized waves. The Copenhagen interpretation of quantum mechanics proposed a process of "collapse" from wave behavior to particle-like behavior to explain this phenomenon of observation.
By the time John von Neumann wrote his famous treatise Mathematische Grundlagen der Quantenmechanik in 1932, the phenomenon of "wavefunction collapse" was accommodated into the mathematical formulation of quantum mechanics by postulating that there were two processes of wavefunction change:
The discontinuous probabilistic change brought about by observation and measurement.
The deterministic time evolution of an isolated system that obeys Schrödinger's equation.
The phenomenon of wavefunction collapse for (1) proposed by the Copenhagen interpretation was widely regarded as artificial and ad-hoc, and consequently an alternative interpretation in which the behavior of measurement could be understood from more fundamental physical principles was considered desirable.
Everett's Ph. D. work was intended to provide such an alternative interpretation. Everett proposed that for a composite system (for example that formed by a particle interacting with a measuring apparatus) the statement that a subsystem has a well-defined state is meaningless. This led Everett to suggest the notion of relativity of states of one subsystem relative to another.
Everett's formalism for understanding the process of wavefunction collapse as a result of observation is mathematically equivalent to a quantum superposition of wavefunctions. Since Everett stopped doing research in theoretical physics shortly after obtaining his degree, much of the elaboration of his ideas was carried out by other researchers.