The original formulation of the multivariate interpretation of parallel universes belongs to Hugh Everett (1957). History saluted him for this. Later, this formulation was popularized and renamed the world-wide interpretation of Everett by Bryce Seligman DeWitt in the 60-70s. Further, decoherence approaches to the interpretation of various theories from the same field were studied and developed.
Theory number 1
The multi-world interpretation of quantum mechanics is one of the many hypotheses for the existence of the multiverse in physics and philosophy. Currently, it is considered the main interpretation along with other interpretations of decoherence, collapse theories (including the historical Copenhagen hypothesis), and hidden variational theories such as Bomian mechanics.
Many worlds
Before the emergence of this theory, reality has always been considered as one unfolding story without the possibility of alternative development. However, many worlds see historical reality as a branched tree in which all possible quantum outcomes are realized. A multi-world interpretation reconciles the observation of non-deterministic events, such as random radioactive decay, with fully determinate equations of quantum physics.
In many worlds, the subjective appearance of the collapse of the wave function is explained by the mechanism of quantum decoherence, and this should resolve all the correlation paradoxes of quantum theory, such as the Schrödinger cat paradox, since each possible result of each event is determined or exists in its own history or world.
First mention
In Dublin in 1952, Erwin Schrödinger gave a lecture in which at some point he jokingly warned his listeners that what he was about to say might sound crazy. He went on to argue that the equation that allowed him to win the Nobel Prize describes several different stories that are not alternative, but occur simultaneously. This is the earliest known reference to the possibility of multiple worlds.
The essence of the theory
Although Hugh Everett proposed several versions of a multi-world interpretation at the beginning of the work, they all contain one key idea: the equations of physics simulating the time evolution of systems without built-in observers are sufficient to model systems that contain observers. In particular, in such equations there is no collapse of the wave function caused by observation, which is what the so-called Copenhagen interpretation offers. Provided that the theory is linear with respect to the wave function, the exact form of the simulated quantum dynamics, whether it be the non-relativistic Schrödinger equation, relativistic quantum field theory or some form of quantum gravity or string theory, does not change the veracity of the theory of multi-world interpretation. This is a metatheory applicable to all linear quantum theories, and there is no experimental data on any nonlinearity of the wave function in physics. The main conclusion of the theory is that the universe (or the multiverse in this context) consists of a quantum superposition of very many, perhaps even infinitely many, increasingly diverging, non-communicating parallel universes or quantum worlds.
The history of multiworld interpretation
This theory first appeared at Princeton University at Everett. The name “Theory of the universal wave function” was coined by his dissertation adviser John Archibald Wheeler, a shorter summary of which was published in 1957 under the title “Relative Formulation of Quantum Mechanics”. The authorship of the term “manyworlds” belongs to Bryce DeWitt, who was responsible for the wider popularization of the Everett theory, which was ignored during the first decade after publication. The name proposed by DeWitt - “a multi-world interpretation” - became so popular that the original names of the theory - “universal wave function” or “relative state formulation” - quickly went out of use even within the framework of academic science.
Similar theories
The interpretation of many worlds has much in common with later interpretations of quantum mechanics, which also use decoherence to explain the measurement process or the collapse of the wave function. Everett’s multi-world interpretation interprets other stories or worlds as real, since he recognizes the universal wave function as “the basic physical entity” or “fundamental entity that always obeys the deterministic wave equation”. Other decoherent interpretations, such as the theory of consecutive stories, existential interpretation, etc., either consider additional quantum worlds as metaphorical or agnostically relate to the very possibility of their existence. Sometimes it is difficult to distinguish between these theories. Everett’s theory of multi-world interpretation has two qualities: it assumes the realism that it assigns to the wave function and has a possible minimal formal structure, rejecting any hidden variables, quantum potential, any form of collapse postulates or mental postulates (such as the interpretation of many minds).
Non-gradient interpretations of many worlds using einsection to explain how a small number of classical states can arise from a huge Hilbert space of superpositions was proposed by Wojciech H. Zurek. His ideas complement a multi-world interpretation and bring it into line with our perception of reality.
Hypothesis Characterization
Many worlds are often called a theory, and not just an interpretation, by those who suggest that this theory can give verifiable predictions (David Deutsch) or those who believe that all other, “non-world-wide” interpretations are inconsistent, illogical, or unscientific when processed measurements. Hugh Everett claimed that his formulation is a metatheory, since it admits the possibility of the existence of other interpretations of quantum theory.
As in other interpretations, the multidimensional interpretation of quantum mechanics is motivated by behavior that can be illustrated by a double-gap experiment. When particles of light (or anything else) pass through a double slit, a calculation assuming a wave-like behavior of light can be used to determine where the particles are. However, when particles are observed in this experiment, they appear precisely as particles (i.e., in certain places), and not as non-localized waves.
Theory advantage
In some versions of the Copenhagen interpretation of quantum mechanics, a collapse process was proposed in which an indefinite quantum system would curl down or determine only one specific result to explain this observation phenomenon. The collapse of the wave function was widely regarded as an artificial process, so an alternative interpretation was proposed in which the behavior of the measurement could be understood from more fundamental physical principles.
Everett's candidacy provided such an alternative interpretation. Everett said that the assertion that for the observed system its subsystem is in a certain state is meaningless. This allowed him to come to that state of systems in relation to each other relative.
Wave function rejection
Everett noted that unitary deterministic dynamics confirmed that after observation is made, each element of the quantum superposition of the combined subject-object wave function contains two relative states: the state of the “collapsed” object and the associated observer (who observed the same minimized outcome) and the state in which the object begins to correlate with the action of measurement or observation. The subsequent evolution of each pair of relative subjective-subject states proceeds with complete indifference regarding the presence or absence of other elements, as if a collapse of the wave function occurred, which leads to the fact that subsequent observations are always consistent with earlier ones. Thus, the collapse of the wave function of an object arose from a unitary, deterministic theory. This corresponded to an early criticism of Einstein's quantum theory, according to which the theory should determine what is observed, and not observations determine the theory. Since the wave function simply collapsed (i.e., became optional), then, as Everett reasoned, there is no need to actually assume that it collapsed. And therefore, attracting Occam's razor, he removed the postulate of the collapse of the wave function from theory.
According to Martin Gardner, other worlds, according to a multi-world interpretation, have two different interpretations: real or unreal. He also claims that Stephen Hawking and Stephen Weinberg both advocate an unrealistic interpretation of this theory. Gardner later added that most physicists prefer an unrealistic interpretation, while "realistic" opinion is supported only by experts in the field of multivariate interpretation of Everett, such as Deutsch and Bruce DeWitt. Hawking said that according to Feynman’s idea, all other worlds are as real as our own, and Martin Gardner answers Hawking that this theory is generally true. In 1983, Hawking also said that he took the theory of many worlds for granted, but neglected to interpret quantum mechanics, saying: "When I hear about Schrödinger's cat, I reach for my gun."
In the same interview, he also said: “But, look: all that needs to be done is to calculate conditional probabilities - in other words, the probability that A will happen, given B. I think that’s all. Some people put a lot on it mysticism due to the fact that the wave function is split into different parts, but everything that you calculate is conditional probability. " In another place, Hawking contrasted his attitude to the “reality” of physical theories with the opinion of his colleagues, such as Roger Penrose, saying about him: “He is a Platonist, and I am a positivist. He worries that Schrödinger's cat is in a quantum state, where he is half alive and half dead. He feels that he cannot be true. This does not bother me. I do not require the theory to be true, because I do not know what it is. Reality is not a quality that you can experience with litmus test. All that bothers me is that theory should predict measurement results. Because quantum theory is very successful. "
Criticism
There is a wide range of claims to the theory of multi-world interpretations. Some, for example, believe that Everett himself was not completely sure of his theory. However, adherents of the multi-world concept of the universe (such as Devitt, Tegmark, Deutsch and others) believe that they fully understand Everett's theory as implying the literal existence of other worlds. In addition, recent biographical sources make it clear that Everett believed in the literal reality of other quantum worlds. Everett's son said that Hugh Everett never hesitated in his belief in the theory of many worlds. It was also reported that Everett believes that the theory of his many worlds guaranteed him immortality.
Asher Perez was an ardent critic of this theory. For example, a section in his 1993 textbook was titled "The World-Wide Interpretation of Everett and Other Strange Theories." Perez not only questions whether it really is an “interpretation”, but rather, he is not at all sure of the need for such hypotheses. Interpretation can be seen as a purely formal transformation that adds nothing to the rules of quantum mechanics. Peres seems to suggest that the theory of the existence of an infinite number of parallel universes is very suspicious for those who interpret this as a violation of Occam's razor, since it does not minimize the number of hypothetical entities. Perez has many supporters who likewise criticize such quantum theories in terms of pragmatism, utilitarianism, and Occam's razor. The consequences of a world-wide interpretation for science, according to such critics, are not the most positive, since it can indirectly confirm many marginal hypotheses.
The theory of many worlds is considered by some as not subject to analysis and, therefore, unscientific, since multiple parallel universes are not communicated with each other, in the sense that no information can be transmitted between them. Others argue that this theory can still be verified directly. Everett thought his interpretation was completely falsified, since any test that falsifies ordinary quantum theory also applies to it.
David Deutsch
One of the strongest proponents of multi-world interpretation (parallel universes) is David Deutsch. According to Deutsch, the only photon interference pattern observed in the double-slit experiment can be explained by the interference of photons in many universes. Thus, an experiment with a single photon interference is indistinguishable from an experiment with the interference of several photons. In a more practical sense, in one of his earliest computational work, he suggested that parallelism stemming from the reality of a multi-world interpretation could lead to a method by which some probabilistic tasks can be completed faster thanks to a universal supercomputer. Here it is not only the abstract calculations of physicists, but also cybernetics. Who knows what new opportunities it will open before us?
Deutsch also suggested that when such computers become conscious, the theory of a multitude of worlds will be tested and confirmed, and the gluing together of a multi-world interpretation will be combined into a single consistent theory.