Editing Many-worlds interpretation (section)

=== Alternative to wavefunction collapse ===
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 [[photon|particles of light]] (or anything else) pass through the double slit, a calculation assuming wavelike behavior of light can be used to identify where the particles are likely to be observed. Yet when the particles are observed in this experiment, they appear as particles (i.e., at definite places) and not as non-localized waves.

Some versions of the Copenhagen interpretation of quantum mechanics proposed a process of "collapse" in which an indeterminate quantum system would probabilistically collapse onto, or select, just one determinate outcome to "explain" this phenomenon of observation. Wave function collapse was widely regarded as artificial and ''[[ad hoc]]'',<ref>{{Cite book |last=Wimmel |first=Hermann |url=https://books.google.com/books?id=I63sCgAAQBAJ |title=Quantum Physics And Observed Reality: A Critical Interpretation Of Quantum Mechanics |date=1992-05-26 |publisher=World Scientific |isbn=978-981-4505-46-8 |pages=45 |language=en}}</ref> so an alternative interpretation in which the behavior of measurement could be understood from more fundamental physical principles was considered desirable.

Everett's PhD work provided such an interpretation. He argued that for a composite system—such as a subject (the "observer" or measuring apparatus) observing an object (the "observed" system, such as a particle)—the claim that either the observer or the observed has a well-defined state is meaningless; in modern parlance, the observer and the observed have become entangled: we can only specify the state of one ''relative'' to the other, i.e., the state of the observer and the observed are correlated ''after'' the observation is made. This led Everett to derive from the unitary, deterministic dynamics alone (i.e., without assuming wave function collapse) the notion of a ''relativity of states''.

Everett noticed that the unitary, deterministic dynamics alone entailed that after an observation is made each element of the [[quantum superposition]] of the combined subject–object wave function contains two "relative states": a "collapsed" object state and an associated observer who has observed the same collapsed outcome; what the observer sees and the state of the object have become correlated by the act of measurement or observation. The subsequent evolution of each pair of relative subject–object states proceeds with complete indifference as to the presence or absence of the other elements, ''as if'' wave function collapse has occurred,<ref name=dewitt73/>{{rp|67,78}} which has the consequence that later observations are always consistent with the earlier observations. Thus the ''appearance'' of the object's wave function's collapse has emerged from the unitary, deterministic theory itself. (This answered Einstein's early criticism of quantum theory: that the theory should define what is observed, not for the observables to define the theory.){{efn|"Whether you can observe a thing or not depends on the theory which you use. It is the theory which decides what can be observed."—[[Albert Einstein]] to [[Werner Heisenberg]], objecting to placing observables at the heart of the new quantum mechanics, during Heisenberg's 1926 lecture at Berlin; related by Heisenberg in 1968.<ref name="einstein26">[[Abdus Salam]], ''Unification of Fundamental Forces'', Cambridge University Press (1990) {{ISBN|0-521-37140-6}}, pp 98–101</ref>}} Since the wave function ''appears'' to have collapsed then, Everett reasoned, there was no need to actually assume that it ''had'' collapsed. And so, invoking [[Occam's razor]], he removed the postulate of wave function collapse from the theory.<ref name=dewitt73/>{{rp|8}}