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In the Many-Worlds Interpretation of the Mach-Zehnder Interferometer experiment, one interprets the universe as splitting into two, with photons traveling in different directions in each universe. Thus, a semi-silvered mirror can split a universe into two distinct futures. This interpretation suggests the existence of parallel universes where anything conceivable exists, provided it doesn't violate the laws of physics.

However, how can a single photon traveling in one direction or another have such a significant impact? Could it be because there are astronomical numbers of photons striking silver mirrors and going in different directions, and their interactions with the environment ensure that every physically possible scenario (e.g. a parallel universe where I invented computers) will be actualized in some parallel universe?

While the Many-Worlds Interpretation of quantum physics implies the existence of a multiverse, but my query is: what ensures that every physically feasible outcome materializes within one of the parallel universes?

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  • $\begingroup$ The universe is governed by quantum mechanics, which is inherently probabilistic. The standard interpretation suggests that, when a measurement is made, the system collapses into a single possibility among all possibilities (which has a certain probability amplitude of occurring) and the rest of them "vanish". The MWI suggests instead that the rest of possibilities are materialised. There is no proposed mechanism for causing this, is just an interpretation of the observed results. The main assumptions are that the wavefunction is real and does not really collapse $\endgroup$
    – vengaq
    Commented Apr 4 at 12:53
  • $\begingroup$ If every outcome materialises then couldn't there be just as many universes where semi-silvered mirrors do not split universes into distinct futures? $\endgroup$
    – Wookie
    Commented Apr 4 at 13:37
  • $\begingroup$ "what ensures that every physically feasible outcome materializes within one of the parallel universes?" - surely this is one of the axioms of MWI ? I don't think MWI postulates a mechanism that ensures this - it just is what happens. $\endgroup$
    – gandalf61
    Commented Apr 4 at 15:29
  • $\begingroup$ @gandalf61 I don't remember such an axiom in MWI. It somehow goes from "decoherence can lead to splitting of an universe into two (or more)" to "such decoherence can gradually lead to larger number of systems existing in multiple versions" to "every physically feasible outcome materializes within one of the parallel universes" -- I don't understand how it infers the last statement. $\endgroup$ Commented Apr 4 at 15:47
  • $\begingroup$ Bear in mind that "physically feasible" implies that all branches must be consistent with the (unitary) evolution of the state of the universe before the split. $\endgroup$
    – PM 2Ring
    Commented Apr 4 at 16:29

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Saying the universe splits in two is a high level description of a messier process. Any measurable property of a quantum system is represented by an operator called an observable that describes how all of the possible values of that property are evolving. A measurement is an interaction that makes a record of some observable. The interaction changes some observable $\hat{A}_M$ of a measuring system so that it depends on the measured observable of the measured system $\hat{A}_S$. So then if some other system measures $\hat{A}_M$ then it has information about $\hat{A}_S$ too. Processes that spread information in this way prevent interference between different values of the measured observable: this effect is called decoherence. So the different values evolve autonomously of one another and since this process spreads decoherence from one system to another as time goes on you have a larger number of systems existing in multiple versions, one for each possible measurement outcome:

https://arxiv.org/abs/1111.2189

https://arxiv.org/abs/2008.02328

So splitting or whatever you want to call it spreads by local interactions from one system to another. So the multiverse as described by the Everett interpretation is a bit more complicated than just a collection of parallel universes because of interference and the locality of splitting.

So if a particular measurement outcome is possible it happens in some universe. There may be a universe in which somebody who resembles you in some respects invented computers.

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  • $\begingroup$ I understood that via decoherence information would spread from one system to another, but how can you say that there exists a decoherence instance via which someone like me invented computers? I understood all what you explained but not the last part: "So if a particular measurement outcome is possible it happens in some universe" -- a decoherence experiment which could cause this might be possible, but maybe did not happen? What guarantees that it would happen? $\endgroup$ Commented Apr 4 at 14:25
  • $\begingroup$ "So the different values evolve autonomously of one another and since this process spreads decoherence from one system to another as time goes on you have a larger number of systems" --> How does one guarantee that in this "large number of systems" there exists a system where someone resembling me invented computers? $\endgroup$ Commented Apr 4 at 14:39
  • $\begingroup$ @TarunGupta In a presumed infinite universe there are an infinite number of material configurations. That means that in some other region there must be someone resembling you who invented computers there. What is the difference between that and interpretations of quantum mechanics that say all outcomes must happen somewhere? $\endgroup$
    – Wookie
    Commented Apr 4 at 15:00
  • $\begingroup$ @Wookie perhaps "someone resembling me invented computers" is not a good example. Take another example: "Right at this moment of writing this comment, someone came and gave me a billion dollars". Since this is physically possible scenario, then as per MWI it happened in some parallel universe. $\endgroup$ Commented Apr 4 at 15:09
  • $\begingroup$ @TarunGupta I think both examples are equivalent. If, yesterday, you winked at some billionaire as you helped them with their bags onto the train, you might possibly find yourself in that situation. And, if the wink wasn't voluntary but caused by a few photons reflecting from the billionaire's watch, then? How can you attribute so little power to a photon's direction when it's impossible for us to trace outcomes backwards? MWI is one attempt to account for a perceived division. But remember, in MWI, that other you isn't a copy, like a blood cell dividing; it'd be two fractions of one cell. $\endgroup$
    – Wookie
    Commented Apr 4 at 15:50
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In the Many-Worlds Interpretation of the Mach-Zehnder Interferometer experiment, one interprets the universe as splitting into two, with photons traveling in different directions in each universe.

Not so. What happens is that the photon wavefunction after passing through a half-silvered mirror has two peaks, one transmitted, one reflected. When an observer interacts with the photon, the superposition property spreads to the observer who enters a superposition of states, one seeing a transmitted photon, the other seeing a reflected photon. Each part of the observer superposition is orthogonal to the other, cannot interact with it, and hence cannot perceive it. From the observer's point of view, it is as if the universe split into two separate worlds, with each possible outcome happening in one of the worlds.

The 'as if' is important. It's a pop-science way of explaining why we see a classical world where only one thing happens, selected at random, when quantum mechanics (without wavefunction collapse) predicts that both outcomes do, in superposition.

However, how can a single photon traveling in one direction or another have such a significant impact?

Because of what's called the 'butterfly effect'. Chaotic dynamics in fluid flow has a property of sensitive dependence on initial conditions. Any tiny perturbation gets magnified exponentially with time. It is said that the flap of a butterfly's wings in Brazil can set off a tornado in Texas.

It's then simply a matter of imagining how the weather could affect human affairs - maybe Charles Babbage died young in storm when a tree fell on him, the world followed a different path, all the other people thinking about it died in similar weather incidents, and eventually you were the first to survive. (Although whether you would/could have existed in a world without Babbage is another question.)

While the Many-Worlds Interpretation of quantum physics implies the existence of a multiverse, but my query is: what ensures that every physically feasible outcome materializes within one of the parallel universes?

It depends how you interpret "physically feasible". In a trivial sense, every physically feasible outcome must happen because that's what 'physically feasible' means. The collapse-free evolution of the wavefunction is deterministic. There is only one possible outcome. That outcome is what happens. The only thing allowed to happen by the laws of physics is what does happen. (It's sometimes called the Gell-Mann Totalitarian Principle: "Everything not forbidden is compulsory.")

But if instead you mean "conceivable", then we have to consider the possibility that we can conceive of things that are physically impossible. The initial state of the universe is unknown. All the possibilities that diverged into 'other universes' before we started observing are unknown. Our understanding of what is possible is approximate, and almost certainly wrong.

So while it seems very likely that all the usual 'alternative histories' we typically imagine are possible - if it's just a matter of the weather, or which sperm won the race, or some similar 'random' event - physics offers no guarantee.

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  • $\begingroup$ "In a trivial sense, every physically feasible outcome must happen because that's what 'physically feasible' means" -- So its possible that "someone like me invents computers" is not physically feasible because initial configuration of universe disallows it physically. If that's the case, then even millions of butterfly effects cannot ensure that in some parallel universe "someone like me invents computers". Hence, its impossible to tell for sure that "someone like me invents computers" in some parallel universe -- although it may be quite probable. Am I making sense? $\endgroup$ Commented Apr 5 at 3:47

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