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I've been dabbling in physics/QM for just a few years, and was reading "The One: How an ancient idea holds the future of physics" - my interest was piqued because I've read quite a bit about Neo-Platonism, and this philosophy, extremely influential historically, holds to the concept of a transcendent first principle or absolute "One." Okay, to the point - the author Heinrich Pas, who holds a pretty serious academic position in Germany and the book is praised on the cover by Hossenfelder - insists that the primary idea with Everett's interpretation of QM, popularized as "Many Worlds," is that what underlies our universe is a universal quantum function.

He says Everett himself did not like the emphasis that others put on the possibility of parallel universes. Everett is quoted (p.79) in an interview as saying "The question is one of terminology: to my opinion there is but a single (quantum) world, with its universal wave function. There are not "many worlds," no "branching," etc., except as an artifact due to insisting once more on a classical picture of the world. Pas says that in the Everettian view, held by Zeh and others, the most fundamental reality is the universal quantum wave function, and that our empirical physical universe, leaving aside the question of other worlds, is only derived, less fundamental, less "real."

The physicist Leblond, who interviewed Everett, was quoted "To me, the deep meaning of Everett's ideas is not the existence of many worlds, but to the contrary, the existence of a single quantum one" and "the 'many worlds' idea again is a left-over of classical conceptions in obvious contradiction of Everett's original intent." Pas says "what is typically overlooked is that Everett's multiverse is not fundamental, but rather apparent or 'emergent.'"

So my question is, does the idea of many alternative physical worlds follow necessarily from Everett's interpretation, especially if you are willing to leave aside classical assumptions and entertain the idea that the real fundamental reality is not our empirical world, but a universal quantum reality that lies our experienced world? Personally I have trouble buying the idea that every time my dog lifts his leg to relieve himself, he's creating billions of universes in the process.

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    $\begingroup$ I think this is opinion-based, and thus off-topic. It also doesn’t seem empirically testable, since all interpretations of QM lead to the same predictions. Picking your preferred QM interpretation is like picking your preferred religion, and I have never seen the point. $\endgroup$
    – Ghoster
    Nov 8, 2023 at 0:11
  • $\begingroup$ Here are a couple good videos on it. Veritasium - Parallel Worlds Probably Exist. Here’s Why. Sean Caroll, 5 minute version - Sean Carroll explains: what is the many-worlds interpretation?. 1 hour version The Many Worlds of Quantum Mechanics $\endgroup$
    – mmesser314
    Nov 8, 2023 at 1:30
  • $\begingroup$ @Ghoster The Everettian interpretation and the Copenhagen interpretation make different predictions about the state of the wavefunction after Wigner's friend type experiments. If you're someone who is bothered by the word "interpretation" being used to describe something that gives different predictions then replace the word "interpretation" above with the word "theory". $\endgroup$
    – Jagerber48
    Nov 8, 2023 at 6:15
  • $\begingroup$ @Ghoster also this question isn't asking about a preferred QM interpretation.. it's asking a specific question about a specific interpretation.. $\endgroup$
    – Jagerber48
    Nov 8, 2023 at 6:16
  • $\begingroup$ Related: physics.stackexchange.com/q/10140/2451 and links therein. $\endgroup$
    – Qmechanic
    Nov 8, 2023 at 7:14

3 Answers 3

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The physicist Leblond, who interviewed Everett, was quoted "To me, the deep meaning of Everett's ideas is not the existence of many worlds, but to the contrary, the existence of a single quantum one" and "the 'many worlds' idea again is a left-over of classical conceptions in obvious contradiction of Everett's original intent." Pas says "what is typically overlooked is that Everett's multiverse is not fundamental, but rather apparent or 'emergent.'"

In quantum theory without collapse the whole of reality is a structure that is more complex than the universe you see around you and looks a bit like a collection of parallel universes when decoherence takes place:

https://arxiv.org/abs/1111.2189

The sense in which there are multiple universes is similar to the sense in which rivers diverge. You can't point to a single point in space where the rivers diverge and there are places where it would be unclear what river a particular water molecule would be in but there are multiple rivers.

So my question is, does the idea of many alternative physical worlds follow necessarily from Everett's interpretation, especially if you are willing to leave aside classical assumptions and entertain the idea that the real fundamental reality is not our empirical world, but a universal quantum reality that lies our experienced world? Personally I have trouble buying the idea that every time my dog lifts his leg to relieve himself, he's creating billions of universes in the process.

There are times when the parallel universe view of quantum mechanics is misleading, but this is mostly because that approximation can break down drastically even for macroscropic objects. For example, the correlations in EPR experiments can be explained by locally inaccessible quantum information being carried in decoherent channels:

https://arxiv.org/abs/quant-ph/9906007

See also

https://arxiv.org/abs/2008.02328

You say that you have trouble with the idea that when a dog takes a wizz this results in the emergence of large numbers of parallel universes. A dog taking a wizz is a complex process involving a very large number of atoms, fluid dynamics, electrical signals in the dog's nervous system and so on. You should accept the existence of atoms and electrical signals in dog's nerves etc because that is the only available explanation of how a dog takes a wizz. That explanation also happens to involve multiple versions of the dog and there is no known way to explain the dog taking a wizz without those multiple versions. Unless you can find some other explanation of what is happening in reality when the dog takes a wizz you should get used to thinking that it involves multiple versions of the dog.

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No!! Everett's interpretation does not imply many world or parallel universes at all! Everett's interpretation says that there is a single universe with a single wavefunction $|\psi\rangle$. This wavefunction evolves unitarily under a universal time evolution operator $\hat{U}$. That is IT. That is all the Everettian interpretation says.

The surprising thing about the interpretation is that it implies macroscopic systems, especially humans bodies, ESPECIALLY human brains, can be in superpositions of classically contradictory states. For example, suppose a human plans to put a particle in a superposition of spin up and down and then perform a measurement. Ahead of time they decide that if they see spin up they will eat pizza for lunch, and if they see spin down they will eat salad for lunch. The Everett interpretation says that the measurement results in entanglement between the human and the spin of the particle such that the wavefunction has one term with the particle spin up and the human eating pizza, and one term with the particle spin down and the human eating salad.

These two terms of the wavefunction are what many people call two "worlds". But there are MANY issues with what it means for there to be a separate "world". These issues are often pointed at as criticisms of many worlds, e.g. critics will ask "how many worlds are there". However, the Everettian interpretation makes NO CLAIM about multiple worlds. Rather, it says there is one quantum world whose state happens to be in a particular entangled/superposition state.

This is not to say the Everettian interpretation is not without it's problem. The Everettian interpretation has the MAJOR problem that it gives NO PRESCRIPTION for how to relate the objective physical state of the universe (described as $|\psi\rangle$) to the subjective experience of people who live in that universe. That is.. if your brain is in a superposition/entangled state of both eating pizza and eating salad, then what is it you experience? Unfortunately, the Everettian interpretation has not answer to this question.

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    $\begingroup$ Your answer starts with "Everett's interpretation does not imply many worlds", but really it's just about terminology. You aren't saying Everett's theory lacks a physical feature, but that that physical feature shouldn't be called "worlds" because the word confuses people. Everett may have agreed, but it's still just an opinion. Also, I don't understand the last paragraph. No one knows how to connect physical states to subjective experiences, and the problem is no worse in Everett's theory. Seemingly impossible isn't harder than seemingly impossible. $\endgroup$
    – benrg
    Nov 8, 2023 at 23:09
  • $\begingroup$ @benrg To your first point: Sure... I guess I would say the thing that Everett's theory has (a wavefunction that contains "macroscopic entanglement and superposition") should not be called many worlds. I agree that this is my opinion about terminology. $\endgroup$
    – Jagerber48
    Nov 9, 2023 at 0:03
  • $\begingroup$ @benrg to your second point: Yes, I agree that no one knows how to connect physical states to subjective experiences. This is in fact a defense of the Everett theory when you say it the way you have. However, I'd say the task is easier for say the Copenhagen theory. In the Copenhagen theory the human brain is in a well defined physical state (no macroscopic superposition/entanglement allowed in that theory), so you need only postulate that subjective experiences are 1:1 with physical states of the human body/brain. What the 1:1 correspondence exactly IS is a major open question in (1/3) $\endgroup$
    – Jagerber48
    Nov 9, 2023 at 0:05
  • $\begingroup$ neuroscience and the philosophy of the mind. But, to someone used to classical laws of nature, it's not a hard pill to swallow that subjective experiences might be 1:1 with physical states. But for the Everett theory, you lose the ability to make a 1:1 correspondence between physical states and mental states. If my brain is in a 75%/25% superposition of eating pizza and eating salad then what is my mental state? What does "my" even mean? What if it's 50%/50%? Is the mental state different? Honestly tackling subjective experience is a mess in many worlds. (2/3) $\endgroup$
    – Jagerber48
    Nov 9, 2023 at 0:07
  • $\begingroup$ This is why I think the Copenhagen theory was invented so rapidly. It was patently obvious that you don't want to deal with the philosophical mind/body implications of humans being in superposition states. That said, I personally think this philosophical line of investigation is interesting and worthwhile to study. (3/3) $\endgroup$
    – Jagerber48
    Nov 9, 2023 at 0:09
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So my question is, does the idea of many alternative physical worlds follow necessarily from Everett's interpretation...?

Sort of. Everett's interpretation says that quantum mechanics without wavefunction collapse predicts that when systems interact, they evolve into a linear superposition of orthogonal states, each corresponding to one possible outcome. Because they are orthogonal, they don't interact with one another. They cannot see one another. (It's like water ripples on a pond passing through one another, each as if the other wasn't there.) To an observer in such a superposition, it is as if each outcome happened in its own separate world, each orthogonal component of the observer superposition sees one orthogonal component of the observed system. In Everett's words, the wavefunctions of observer and observed become correlated.

There is only one world, one wavefunction, but parts of it don't interact. All possible outcomes of a quantum observation occur, simultaneously, but they are separated from one another because their component wavefunctions are orthogonal. This gives each orthogonal component of an observer seeing the world from the inside the illusion that only one thing happens, picked at random.

Pure quantum mechanics, without any additional assumptions or speculative mechanisms, predicts that we will see a classical world. It explains why we don't see things in superposition. The moment we look, we enter into the same superposition, each version of ourselves seeing only one part.

Everett's criticism is that the "many worlds" being spoken of are the worlds of this false classical picture. And the big problem with it is that it misleads people into thinking that the universe actually splits; that there are new universes being created by some unspecified process. This causes all the same problems that wavefunction collapse does. What triggers the split? How fast does the split propagate? How can a whole universe of matter appear out of nowhere? What's the mechanism? The entire point of his thesis was to reject all that. There is no collapse/split. There is no collapse/split change that propagates faster than light. There is no collapse/split mechanism to explain. It's an illusion, because we didn't have the imagination to understand what we would see if we were inside a superposition. We don't see ghostly multiple images of cats both dead and alive. Each part of us sees one cat, one part sees it in a definite dead state, the other part sees it definitely alive.

And the particular beauty of it is that it doesn't require one to believe in anything extra that the Copenhagen interpretation doesn't already include. It just takes the unitary evolution part of the existing theory, and shows that it already explains the classical appearances we observe.

Because the other components of the observer superposition are forever unobservable, we have no way of telling whether they are really there. The simple linear equations that we all accept apply when we're not looking predict they are. But if some mysterious, unobservable, unexplained mechanism was to make all but one of them magically disappear at a speed faster than light and backwards in time, we'd never know it.

It's not a question of science, because there is no way to experimentally test it. Whether you find it aesthetically preferable to have zillions of alternate selves you can't see, or unexplained, causally bizarre, equally unobservable mechanisms making them all disappear, is purely a matter of taste.

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