Many-worlds interpretation Totally Explained

Everything about the Many-worlds Interpretation totally explained

The many-worlds interpretation or MWI (also known as relative state formulation, theory of the universal wavefunction, parallel universes, many-universes interpretation or many worlds), is an interpretation of quantum mechanics. Many-worlds denies the objective reality of wavefunction collapse. Many-worlds then explains the subjective appearance of wavefunction collapse with the mechanism of quantum decoherence. Consequently, many-worlds claims this resolves all the "paradoxes" of quantum theory since every possible outcome to every event defines or exists in its own "history" or "world". In layman's terms, this means that, in some sense, there are an infinite number of universes and that everything that could possibly happen in our universe (but doesn't) does happen in another.
   Proponents argue that MWI reconciles how we can perceive non-deterministic events (such as the random decay of a radioactive atom) with the deterministic equations of quantum physics. Prior to many worlds this had been viewed as a single "world-line". Many-worlds rather views it as a many-branched tree where every possible branch of history is realised.
   The relative state formulation is due to Hugh Everett who formulated it in 1957. Later, this formulation was popularised and renamed many worlds by Bryce Seligman DeWitt in the 1960s and '70s. The decoherence approach to interpreting quantum theory has been further explored and developed becoming quite popular, taken as a class overall. MWI is one of many Multiverse hypotheses in physics and philosophy. It is currently considered a mainstream interpretation along with the other decoherence interpretations and the Copenhagen interpretation.
   The many worlds interpretation offers the possibility of deriving the probability interpretation of quantum mechanics from other assumptions. In fact, this was first done by Everett and DeWitt in the 1950's, but the old argument was criticized on philosophical grounds. In a September 2007 conference David Wallace reports on a proof by Deutsch and himself of the Born Rule starting from Everettian assumptions and this has been reported in the press as support for parallel universes.

Outline

Although several versions of MWI have been proposed since Hugh Everett's original work, Everett originally called his approach the "Correlation Interpretation", although in Everett's usage the term correlation is what is now called quantum entanglement). The phrase "many worlds" is due to Bryce DeWitt, or "the fundamental entity, obeying at all times a deterministic wave equation". The other decoherent interpretations, such as many histories, consistent histories, the Existential Interpretation etc, either regard the extra quantum worlds as metaphorical in some sense, or are agnostic about their reality; it's sometimes hard to distinguish between the different varieties. MWI is distinguished by two qualities: it assumes realism .)
Since Everett stopped doing research in theoretical physics shortly after obtaining his Ph.D., much of the elaboration of his ideas was carried out by other researchers and forms the basis of much of the decoherent approach to quantum measurement.

Advantages

MWI removes the observer-dependent role in the quantum measurement process by replacing wavefunction collapse with quantum decoherence. Since the role of the observer lies at the heart of most, if not all, "quantum paradoxes" this automatically resolves a number of problems; see for example Schrödinger's cat thought-experiment, the EPR paradox, von Neumann's "boundary problem" and even wave-particle duality. Quantum cosmology also becomes intelligible, since there's no need anymore for an observer outside of the universe.
MWI allows quantum mechanics to become a realist, deterministic, local theory making it more akin to classical physics (including the theory of relativity), at the expense of losing counterfactual definiteness.

MWI (or other, broader multiverse considerations) provides a context for the anthropic principle which may provide an explanation for the fine-tuned universe.

MWI, being a decoherent formulation, is axiomatically more streamlined than the Copenhagen and other collapse interpretations; and thus favoured under certain interpretations of Ockham's razor. Of course there are other decoherent interpretations that also possess this advantage with respect to the collapse interpretations.

Objections

The many worlds interpretation is very vague about the ways to determine when splitting happens, and nowadays usually the criterion is that the two branches have decohered. However, present day understanding of decoherence doesn't allow a completely precise, self contained way to say when the two branches have decohered/"do not interact", and hence many worlds interpretation remains arbitrary. This is the main objection opponents of this interpretation raise, saying that it isn't clear what is precisely meant by branching, and point to lack of self contained criterion specifying branching to be described. � :MWI response: the decoherence or "splitting" or "branching" is complete when the measurement is complete. In Dirac notation a measurement is complete when:

:

lang O[i]|O[j]
ang = delta_.

This also shows that in between the measurements given by proper (that is, non-unitary) quantum operations, one can interpolate arbitrary unitary evolution.

Quantum probabilities explained by continuous branching

Dr. David Deutsch along with Oxford colleagues have demonstrated mathematically that the bush-like branching structure created by the universe splitting into parallel versions of itself can explain the probabilistic nature of quantum outcomes. In the New Scientist article on the discovery, Andy Albrecht, a physicist at the University of California at Davis, is quoted as saying "This work will go down as one of the most important developments in the history of science." Deutsch and his Oxford colleaques are thus seen to apparently bolster March - May '07 internet postings of Dr. David Anacker (to physics cognoscenti including Lisa Randall, Lee Smolin, David Deutsch, G. T'Hooft, S. Glashow, S. Weinberg, M. Kaku, L. Susskind, et.al.) via internet archive

earlier establishing agreement between predictive statistics of the Everett and Copenhagen interpretations. that Everett himself wasn't entirely clear as to what he meant; however MWI adherents believe they fully understand Everett's meaning, pointing to his stated belief in quantum immortality (which requires absolute belief in the reality of all the many worlds) and the reality of all components the uncollapsed universal wavefunction.
"Many worlds"-like interpretations are now considered fairly mainstream within the quantum physics community. For example, a poll of 72 leading physicists conducted by the American researcher David Raub in 1995 and published in the French periodical Sciences et Avenir in January 1998 recorded that nearly 60% thought many worlds interpretation was "true". Max Tegmark (see reference to his web page below) also reports the result of a poll taken at a 1997 quantum mechanics workshop. According to Tegmark, "The many worlds interpretation (MWI) scored second, comfortably ahead of the consistent histories and Bohm interpretations." Other such unscientific polls have been taken at other conferences: see for instance Michael Nielsen's blog (External Link

) report on one such poll. Nielsen remarks that it appeared most of the conference attendees "thought the poll was a waste of time". MWI sceptics (for instance Asher Peres) argue that polls regarding the acceptance of a particular interpretation within the scientific community, such as those mentioned above, cannot be used as evidence supporting a specific interpretation's validity. However, others note that science is a group activity (for instance, peer review) and that polls are a systematic way of revealing the thinking of the scientific community.
A 2005 minor poll on the Interpretation of Quantum Mechanics workshop at the Institute for Quantum Computing University of Waterloo produced contrary results, with the MWI as the least favored.(External Link

) One of MWI's strongest advocates is David Deutsch. According to Deutsch the single photon interference pattern observed in the double slit experiment, can be explained by interference of photons in multiple universes. Viewed in this way, the single photon interference experiment is indistinguishable from the multiple photon interference experiment. In a more practical vein, in one of the earliest papers on quantum computing, he suggested that parallelism that results from the validity of MWI could lead to "a method by which certain probabilistic tasks can be performed faster by a universal quantum computer than by any classical restriction of it". Deutsch has also proposed that when reversible computers become conscious that MWI will be testable (at least against "naive" Copenhagenism) via the reversible observation of spin. Asher Peres was an outspoken critic of MWI, for example in a section in his 1993 textbook with the title Everett's interpretation and other bizarre theories. In fact, Peres questioned whether MWI is really an "interpretation" or even if interpretations of quantum mechanics are needed at all. Indeed, the many-worlds interpretation can be regarded as a purely formal transformation, which adds nothing to the instrumentalist (for example statistical) rules of the quantum mechanics. Perhaps more significantly, Peres seems to suggest that positing the existence of an infinite number of non-communicating parallel universes is highly suspect as it violates those interpretations of Occam's Razor that seek to minimize the number of hypothesized entities. Proponents of MWI argue precisely the opposite, by applying Occam's Razor to the set of assumptions rather than multiplicity of universes. In Max Tegmark's formulation, the alternative to many worlds is the undesirable "many words", an allusion to the complexity of von Neumann's collapse postulate).
MWI is considered by some to be unfalsifiable and hence unscientific because the multiple parallel universes are non-communicating, in the sense that no information can be passed between them. Others Gardner also claims that the interpretation favoured by the majority of physicists is that the other worlds are not real in the same way as our world is real, whereas the "realist" view is supported by MWI experts David Deutsch and Bryce DeWitt. However Stephen Hawking is on record as a saying that the other worlds are as real as ours and Tipler reports Hawking saying that MWI is "trivially true" (scientific jargon for "obviously true", which Gardner seems not to realise) if quantum theory applies to all reality. Roger Penrose agrees with Hawking that QM applied to the universe implies MW, although he considers the current lack of a successful theory of quantum gravity negates the claimed universality of conventional QM.

Speculative implications

Speculative physics deals with questions also discussed in science fiction.

Choice and travel

Under the Many-Worlds interpretation, it's theoretically possible that every choice a person makes results in the creation of two or more 'new' universes: one for each 'option' in a given choice. Price gives evidence for both sides to the speculation. On the one hand he says that quantum effects rarely or never affect human decisions. On the other hand he says that all possible decisions are realized in some worlds. It is further speculated that it might be possible to move 'between' these universes, of which there would be an infinite number or a very large finite number. Price believes that travel between worlds is impossible.

Quantum suicide

It has been claimed that there's an experiment that would clearly differentiate between the many-worlds interpretation and other interpretations of quantum mechanics. It involves a quantum suicide machine and an experimenter willing to risk death. However, at best, this would only decide the issue for the experimenter; bystanders would learn nothing. The flip side of quantum suicide is quantum immortality.
Another speculation is that the separate worlds remain weakly coupled (for example by gravity) permitting "communication between parallel universes". This requires that gravity be a classical force and not quantized.
The many-worlds interpretation has some similarity to modal realism in philosophy, which is the view that the possible worlds used to interpret modal claims actually exist. Unlike philosophy, however, in quantum mechanics counterfactual alternatives can influence the results of experiments, as in the Elitzur-Vaidman bomb-testing problem or the Quantum Zeno effect.

Time travel

The many-worlds interpretation could be one possible way to resolve the paradoxes that one would expect to arise if time travel turns out to be permitted by physics (permitting closed timelike curves and thus violating causality). Entering the past would itself be a quantum event causing branching, and therefore the timeline accessed by the time traveller simply would be another timeline of many. In that sense, it would make the Novikov self-consistency principle unnecessary.

Many worlds in literature and science fiction

The many-worlds interpretation (and the somewhat related concept of possible worlds) have been associated to numerous themes in literature, art and science fiction.
Some of these stories or films violate fundamental principles of causality and relativity, and are extremely misleading since the information-theoretic structure of the path space of multiple universes (that is information flow between different paths) is very likely extraordinarily complex. Also see Michael Clive Price's FAQ referenced in the external links section below where these issues (and other similar ones) are dealt with more decisively.
Another kind of popular illustration of many worlds splittings, which doesn't involve information flow between paths, or information flow backwards in time considers alternate outcomes of historical events. According to many worlds, most of the historical speculations entertained within the alternate history genre are realised in parallel universes.

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