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Physique. L'hypothèse dite du multimonde remise en cause

Ce texte s'appuie sur un article de Philip Ball que vient de publier le site Quantamagazine dont nous ne manquerons pas de conseiller la lecture.

https://www.quantamagazine.org/why-the-many-worlds-interpretation-of-quantum-mechanics-has-many-problems-20181018

Cette hypothèse du multimonde, plus exactement dite “interprétation “ des résultats de la mécanique quantique, MQ, a été suggérée par certains physiciens pour prendre en compte l'un des résultats surprenant de cette approche, tenant au fait que la MQ ne permet pas de préciser dans quel monde « réel » s'appliquent les « indéterminations » de cette théorie. La « fonction d'onde » décrivant selon elle une particule quantique  est une fonction mathématique qui définit tous les états possibles observables de cette particule, sans indiquer l'état qu'un observateur doit prendre en compte dans tel cas précis, par exemple pour utiliser cette particule dans un calculateur quantique. Or celui-ci perd toute utilité s'il ne peut fournir de résultats objectifs, ou s'il envisage une infinité de résultats.

Pour lever cette incertitude, selon la MQ, il faut procéder à une « observation ». Mais celle-ci ne propose qu'un résultat parmi d'autres permettant de définir objectivement  la particule,  sans indiquer où se trouvent  ou ce que sont les différents autres états de celle-ci.

L'interprétation dite des multi-mondes (dite MWI en anglais) suggère que la MQ fournit des résultats précis dans un monde ou univers déterminé, les autres résultats s'appliquant également avec précision, mais dans une infinité d'autres univers. Ceux -ci coexistent avec le nôtre sans que nous le sachions.

Aussi étrange que soit cette interprétation pour le sens commun, cette étrangeté ne l'est pas plus que les autres proposées par la MQ, telle que l'indétermination concernant l'espace et le temps. Le problème est qu'aucun humain ne peut se rendre dans un de ces univers, puisqu'il reste indéfiniment prisonnier du sien, autrement dit du nôtre.

La MWI a été proposée en 1957 par le jeune physicien Hugh Everett alors qu'il écrivait une thèse de doctorat sous le contrôle de John Weeler. Everett suggère de remettre en cause, non la MWI mais notre conception de la réalité, autrement dit toute la physique telle qu'envisagée actuellement. En allant plus loin, il suggère que c'est notre esprit qui devrait être remis en cause.

Hugh Everett considère que l'univers entier peut être décrit comme une fonction d'onde gigantesque qui contient toute les réalités possibles. En évoluant, l'univers fait disparaître certaines de ces superpositions. Chaque « observation » fait appraitre l'un des univers superposés restant.

L'hypothèse du multi-monde doit être distinguée de celle avec laquelle elle est souvent confondue, dite des universe multiples ou multivers. Ceux-ci résulteraient d'une infinité de Big Bangs créant une infinité de mondes comparables ou différents du nôtre.

La MWI avait été largement ignorée, avant d'être reprise en 1970 par le physicien Brice DeWitt dans la revue  Physics Today

Elle a suscité de nombreuses objections, car elle remplaçait la complexité de la fonction d'onde par une complexité aussi grande concernant l'univers. De plus elle suggérait que chacune de nos « interprétations « donnait naissance à un nouvel univers crée par notre observation. Néanmoins elle a rencontré un grand succès, non seulement chez certains physiciens mais au sein du  grand public. La raison en est sans doute qu'elle correspond à des conceptions largement répandues par de nombreuses religions et croyances pour lesquelles le monde que nous observons n'est pas le seul à prendre en compte.

Remises en cause récentes de la MWI

On trouvera ici la suite de l'article, que nous n'avons pas le temps de traduire, simplifier et éventuellement commenter. Nous y reviendrons  si possible

There are, of course, some questions to be asked.

For starters, about this business of bifurcating worlds. How does a split actually happen?

That is now seen to hinge on the issue of how a microscopic quantum event gives rise to macroscopic, classical behavior through a process called “decoherence,” in which the wavelike states of a quantum system become uncoordinated and scrambled by their interactions with their environment. Parallel quantum worlds have split once they have decohered, for by definition decohered wave functions can have no direct, causal influence on one another. For this reason, the theory of decoherence developed in the 1970s and '80s helped to revitalize the MWI by supplying a clear rationale for what previously seemed a rather vague contingency.

In this view, splitting is not an abrupt event. It evolves through decoherence and is only complete when decoherence has removed all possibility of interference between universes. While it's popular to regard the appearance of distinct worlds as akin to the bifurcation of futures in Jorge Luis Borges' story “The Garden of Forking Paths,” a better analogy might therefore be something like the gradual separation of shaken salad dressing into layers of oil and vinegar. It's then meaningless to ask how many worlds there are — as the philosopher of physics David Wallace aptly puts it, the question is rather like asking, “How many experiences did you have yesterday?” You can identify some of them, but you can't enumerate them.

What we can say a little more precisely is what kind of phenomenon causes splitting. In short, it must happen with dizzying profusion. Just within our own bodies, there must be at least as many splitting events affecting each of us every second as there are encounters between our molecules in the same space of time. Those numbers are astronomical.

The main scientific attraction of the MWI is that it requires no changes or additions to the standard mathematical representation of quantum mechanics. There is no mysterious, ad hoc and abrupt collapse of the wave function. And virtually by definition it predicts experimental outcomes that are fully consistent with what we observe.

But if we take what it says seriously, it soon becomes clear that the conceptual and metaphysical problems with quantum mechanics aren't banished by virtue of this apparent parsimony of assumptions and consistency of predictions. Far from it.

......The MWI is surely the most polarizing of interpretations. Some physicists consider it almost self-evidently absurd; “Everettians,” meanwhile, are often unshakable in their conviction that this is the most logical, consistent way to think about quantum mechanics. Some of them insist that it is the only plausible interpretation — for the arch-Everettian David Deutsch, it is not in fact an “interpretation” of quantum theory at all, any more than dinosaurs are an “interpretation” of the fossil record. It is simply what quantum mechanics is. “The only astonishing thing is that that's still controversial,” Deutsch says.

My own view is that the problems with the MWI are overwhelming — not because they show it must be wrong, but because they render it incoherent. It simply cannot be articulated meaningfully.

I'll attempt to summarize the problems, but first, let's dispense with a wrong objection. Some criticize the MWI on aesthetic grounds: People object to all those countless other universes, multiplying by the trillion every nanosecond, because it just doesn't seem proper. Other copies of me? Other world histories? Worlds where I never existed? Honestly, whatever next! This objection is rightly dismissed by saying that an affront to one's sense of propriety is no grounds for rejecting a theory. Who are we to say how the world should behave?

The very familiarity of the centuries-old doppelgänger trope prepares us to accept the notion of multiple versions of ourselves rather casually, and the level of the discourse about our alleged replica selves is often shockingly shallow.

A stronger objection to the proliferation of worlds is not so much all this extra stuff you're making, but the insouciance with which it is made. Roland Omnès says the idea that every little quantum “measurement” spawns a world “gives an undue importance to the little differences generated by quantum events, as if each of them were vital to the universe.” This, he says, is contrary to what we generally learn from physics: that most of the fine details make no difference at all to what happens at larger scales.

But one of the most serious difficulties with the MWI is what it does to the notion of self. What can it mean to say that splittings generate copies of me? In what sense are those other copies “me?”

Brian Greene, a well-known physics popularizer with Everettian inclinations, insists simply that “each copy is you.” You just need to broaden your mind beyond your parochial idea of what “you” means. Each of these individuals has its own consciousness, and so each believes he or she is “you” — but the real “you” is their sum total.

There's an enticing frisson to this idea. But in fact the very familiarity of the centuries-old doppelgänger trope prepares us to accept it rather casually, and as a result the level of the discourse about our alleged replica selves is often shockingly shallow — as if all we need contemplate is something like teleportation gone awry in an episode of “Star Trek.” We are not being astonished but, rather, flattered by these images. They sound transgressively exciting while being easily recognizable as plotlines from novels and movies.

Tegmark waxes lyrical about his copies: “I feel a strong kinship with parallel Maxes, even though I never get to meet them. They share my values, my feelings, my memories — they're closer to me than brothers.” But this romantic picture has, in truth, rather little to do with the realities of the MWI. The “quantum brothers” are an infinitesimally small sample cherry-picked for congruence with our popular fantasies. What about all those “copies” differing in details graduating from the trivial to the utterly transformative?Beyond Weird book cover

The physicist Lev Vaidman has thought rather carefully about this matter of quantum youness. “At the present moment there are many different 'Levs' in different worlds,” he says, “but it is meaningless to say that now there is another 'I.' There are, in other words, beings identical to me (at the time of splitting) in these other worlds, and all of us came from the same source — which is 'me' right now.”

The “I” at each moment of time, he says, is defined by a complete classical description of the state of his body and brain. But such an “I” could never be conscious of its existence.

Consciousness relies on experience, and experience is not an instantaneous property: It takes time, not least because the brain's neurons themselves take a few milliseconds to fire. You can't “locate” consciousness in a universe that is frantically splitting countless times every nanosecond, any more than you can fit a summer into a day.

One might reply that this doesn't matter, so long as there's a perception of continuity threading through all those splittings. But in what can that perception reside, if not in a conscious entity?

And if consciousness — or mind, call it what you will — were somehow able to snake along just one path in the quantum multiverse, then we'd have to regard it as some nonphysical entity immune to the laws of (quantum) physics. For how can it do that when nothing else does?

David Wallace, one of the most ingenious Everettians, has argued that purely in linguistic terms the notion of “I” can make sense only if identity/consciousness/mind is confined to a single branch of the quantum multiverse. Since it is not clear how that can possibly happen, Wallace might then have inadvertently demonstrated that the MWI is not after all proposing a conceit of “multiple selves.” On the contrary, it is dismantling the whole notion of selfhood. It is denying any real meaning of “you.”

I shouldn't wish anyone to think that I feel affronted by this. But if the MWI sacrifices the possibility of thinking meaningfully about selfhood, we should at least acknowledge that this is so, and not paper over it with images of “quantum brothers and sisters.”

19/10/2018
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