The Many-Worlds Interpretation of quantum mechanics was outlined in 1957 by Hugh Everett. Everett was a brilliant theoretician whose Many-Worlds theory failed to find support in his lifetime. His formulation is re-emerging as an important facet of contemporary thought in part because no less a thinker than Stephen Hawking is a strong proponent.
Despite a somewhat dysfunctional life (he was greatly overweight a died at 52 in part because of alcohol and tobacco addiction) Everett will always be remembered for his bold thinking and ability to communicate his ideas both within and outside the scientific/mathematical community. His paper, Wave Mechanics Without Probability, later retitled The Theory of the Universal Wave Function, laid the theoretical groundwork for the Many-Worlds Interpretation, which asserts that the universal waveform has objective reality and, for that reason, wave function collapse is illusory.
The picture that emerges is strange, but really no less tenable than some of the consequences of waveform collapse. What the Many-Worlds Interpretation asserts is that there are many universes (philosophers characteristically use the term “worlds”) approaching if not actually infinite in number. Everything that could have happened in the past of each item in our universe comprises one of these worlds, and each of them has an independent and objective reality. The idea seems fanciful, but there are definite mathematical and theoretical arguments that can be made in its favor.
Prior to the Many-Worlds Interpretation, waveform collapse served to explain paradoxes such as Schrödinger’s infamous cat. The cat was held to have both dead and alive existences dependent on a truly (non-quasi) random event such as atomic decay. In the Standard Interpretation the cat’s reality is undifferentiated until the box is opened, whereupon the probabilistic wave function collapses and one or more of the other alternatives at that time becomes true.
In the Many-Worlds Interpretation, no such wave function collapse arises. Instead, reality branches out to create alternative universes in which each narrative applies. In one world the cat lives, and in the other the cat does not live. The multiverse consists of countless diverging quantum worlds. All in all, the Many-Worlds Interpretation presents a grand picture that resembles a fractal, each branch graphically representing a different possible, therefore objectively real universe. (It also seems to have inspired several episodes of the Star Trek TV series franchise.)
The Many-Worlds Interpretation is not so much a theory that could be verified or demolished by some type of experiment as it is a frame of reference supplementing the various linear quantum theories.
The region between the Many-Worlds Interpretation and hard-core wave collapse ideology is populated by a dialectical profusion of carefully staked-out positions. Some physicists are noncommittal, regarding Many Worlds as a sort of metaphor, while others believe these discrete worlds are real even if unknowable.
Certainly we are in an age when quantum theory is a work in progress. Attempts to fuse it to Relativity Theory have yielded some interesting results. Notable among these is String Theory, a brilliant, bold synthesis still, however, lacking the slightest amount of experimental verification.
One might ask, if there are multiple worlds where all possible alternatives have objective reality, would not different, perhaps conflicting, versions of the Many-Worlds Interpretations be fervently held by inhabitants to be true? The Many Worlds-Interpretation is a decoherent formulation not encumbered with Copenhagen’s collapse baggage, and accordingly it gains credibility in the context of Occam’s Razor. For this reason it seems probable that Many Worlds will continue to gain traction in the fullness of time.
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