Question: According to what theory, "parallel universes" will never be accessible to us.
About the article "Quantum mechanics", section "Philosophical implications".

Candidate passages:
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.
In the novel, the Biblical number of the beast turns out to be, not 666, but formula_1, or 10,314,424,798,490,535,546,171,949,056, which is the initial number of parallel universes accessible through the continua device.
It is later theorized by the character Jacob that the number may be merely the instantly accessible universes from a given location, and there is a larger structure that implies an infinite number of universes. 
From the back of the book:In this thrilling journey into the mysteries of our cosmos, bestselling author Michio Kaku takes us on a dizzying ride to explore black holes and time machines, multidimensional space and, most tantalizing of all, the possibility that parallel universes may lie alongside our own.
Kaku skillfully guides us through the latest innovations in string theory and its most recent iteration, M-theory, which posits that our universe may be just one in an endless multiverse, a singular bubble floating in a sea of infinite bubble universes.
If M-theory is proven correct, we may perhaps finally find an answer to the question, "What happened before the big bang?"
This is an exciting and unforgettable introduction to the cutting-edge theories of physics and cosmology from one of the preeminent voices in the field. 
The concept of multiple universes is mentioned many times in Hindu Puranic literature, such as in the Bhagavata Purana:"Every universe is covered by seven layers — earth, water, fire, air, sky, the total energy and false ego — each ten times greater than the previous one.
There are innumerable universes besides this one, and although they are unlimitedly large, they move about like atoms in You.
Therefore You are called unlimited" (Bhagavata Purana 6.16.37)"Lord Śiva said: "My dear son, I, Lord Brahmā and the other devas, who move within this universe under the misconception of our greatness, cannot exhibit any power to compete with the Supreme Personality of Godhead, for innumerable universes and their inhabitants come into existence and are annihilated by the simple direction of the Lord"" (Bhagavata Purana 9.4.56)"After separating the different universes, the gigantic universal form of the Lord, which came out of the causal ocean, the place of appearance for the first puruṣa-avatāra, entered into each of the separate universes, desiring to lie on the created transcendental water" (Bhagavata Purana 2.10.10) The number of universes seems to be uncountable according to the Puranic literature:"Even though over a period of time I might count all the atoms of the universe, I could not count all of My opulences which I manifest within innumerable universes " (Bhagavata Purana 11.16.39)Analogies to describe multiple universes also exist in the Puranic literature:"What am I, a small creature measuring seven spans of my own hand?
I am enclosed in a potlike universe composed of material nature, the total material energy, false ego, ether, air, water and earth.
And what is Your glory?
Unlimited universes pass through the pores of Your body just as particles of dust pass through the openings of a screened window" (Bhagavata Purana 10.14.11) "Because You are unlimited, neither the lords of heaven nor even You Yourself can ever reach the end of Your glories.
The countless universes, each enveloped in its shell, are compelled by the wheel of time to wander within You, like particles of dust blowing about in the sky.
The śrutis, following their method of eliminating everything separate from the Supreme, become successful by revealing You as their final conclusion" (Bhagavata Purana 10.87.41) "The layers or elements covering the universes are each ten times thicker than the one before, and all the universes clustered together appear like atoms in a huge combination" (Bhagavata Purana 3.11.41)The concept of parallel universes appears in the Brahma Vaivarta Purana:"And who will search through the wide infinities of space to count the universes side by side, each containing its Brahma, its Vishnu, its Shiva?
Who can count the Indras in them all--those Indras side by side, who reign at once in all the innumerable worlds; those others who passed away before them; or even the Indras who succeed each other in any given line, ascending to godly kingship, one by one, and, one by one, passing away?"
(Brahma Vaivarta Purana) 
The title refers to the fact that the story takes place in two parallel universes, a concept loosely based on multiverse theory.