The Copenhagen interpretation introduced the concept of wave function collapse but failed to precisely define the conditions that cause a collapse or why it collapses. The Everett interpretation (many-worlds) simply removes the idea of collapse and the all the unresolved questions regarding collapse.

The Copenhagen interpretation postulates the spontaneous reduction of only one final observer. The experiment should be described from this observer's perspective. The reduction, like the velocity of the system, depends on the choice of the final observation system.

The many-worlds interpretation implies that there are most likely an uncountable number of universes. It is one of a number of multiverse hypotheses in physics and philosophy. MWI views time as a many-branched tree, wherein every possible quantum outcome is realized.

Einstein, however, persistently argued that the Copenhagen interpretation was incomplete. He conjectured that there might be hidden variables or processes underlying quantum phenomena; or perhaps 'pilot waves', proposed by de Broglie, govern the behaviour of particles.