Article: Quantum mechanics
Section: Philosophical implications

Question: According to what theory, "parallel universes" will never be accessible to us.

Since its inception, the many counter-intuitive aspects and results of quantum mechanics have provoked strong philosophical debates and many interpretations.
Even fundamental issues, such as Max Born's basic rules concerning probability amplitudes and probability distributions took decades to be appreciated by society and many leading scientists.
Richard Feynman once said, "I think I can safely say that nobody understands quantum mechanics."
According to Steven Weinberg, "There is now in my opinion no entirely satisfactory interpretation of quantum mechanics.
"The Copenhagen interpretation - due largely to the Danish theoretical physicist Niels Bohr - remains the quantum mechanical formalism that is currently most widely accepted amongst physicists, some 75 years after its enunciation.
According to this interpretation, the probabilistic nature of quantum mechanics is not a "temporary" feature which will eventually be replaced by a deterministic theory, but instead must be considered a "final" renunciation of the classical idea of "causality".
It is also believed therein that any well-defined application of the quantum mechanical formalism must always make reference to the experimental arrangement, due to the complementarity nature of evidence obtained under different experimental situations.
Albert Einstein, himself one of the founders of quantum theory, disliked this loss of determinism in measurement.
Einstein held that there should be a local hidden variable theory underlying quantum mechanics and, consequently, that the present theory was incomplete.
He produced a series of objections to the theory, the most famous of which has become known as the Einstein–Podolsky–Rosen paradox.
John Bell showed that this "EPR" paradox led to experimentally testable differences between quantum mechanics and local realistic theories.
Experiments have been performed confirming the accuracy of quantum mechanics, thereby demonstrating that the physical world cannot be described by any local realistic theory.
The "Bohr-Einstein debates" provide a vibrant critique of the Copenhagen Interpretation from an epistemological point of view.
The Everett many-worlds interpretation, formulated in 1956, holds that "all" the possibilities described by quantum theory "simultaneously" occur in a multiverse composed of mostly independent parallel universes.
This is not accomplished by introducing some "new axiom" to quantum mechanics, but on the contrary, by "removing" the axiom of the collapse of the wave packet.
"All" of the possible consistent states of the measured system and the measuring apparatus (including the observer) are present in a "real" physical - not just formally mathematical, as in other interpretations - quantum superposition.
Such a superposition of consistent state combinations of different systems is called an entangled state.
While the multiverse is deterministic, we perceive non-deterministic behavior governed by probabilities, because we can observe only the universe (i.e., the consistent state contribution to the aforementioned superposition) that we, as observers, inhabit.
Everett's interpretation is perfectly consistent with John Bell's experiments and makes them intuitively understandable.
However, according to the theory of quantum decoherence, these "parallel universes" will never be accessible to us.
The inaccessibility can be understood as follows: once a measurement is done, the measured system becomes entangled with "both" the physicist who measured it "and" a huge number of other particles, some of which are photons flying away at the speed of light towards the other end of the universe.
In order to prove that the wave function did not collapse, one would have to bring "all" these particles back and measure them again, together with the system that was originally measured.
Not only is this completely impractical, but even if one "could" theoretically do this, it would have to destroy any evidence that the original measurement took place (to include the physicist's memory!
); in light of these Bell tests, Cramer (1986) formulated his transactional interpretation.
Relational quantum mechanics appeared in the late 1990s as the modern derivative of the Copenhagen Interpretation.