Ask Lemmy

Nah, they're right, it is fantasy. I think some people have in their heads that particles spread out like waves in 3D space and Many Worlds is just like an objective collapse model where it collapses back into a particle when you look at it, but where all outcomes happen in a different branch of the multiverse rather than just having one outcome.

The reality is that it is only actually possible to consistently map quantum waves to 3D space when you have a single particle. The moment you introduce two or three, it quickly breaks down because the number of quantum waves grows exponentially. If you have 3 spin-1/2 particles then you would describe their state with 8 waves. You cannot consistently break apart 8 into 3. You end up quickly finding that it is actually impossible to assign the waves to any location at all in space or time, so you cannot think of them as something like a propagating field mode or anything like that.

These are waves made of nothing that do not exist anywhere and nobody can see them. One of the weirdest things about quantum mechanics I do not think people appreciate enough is how you evolve something that seems to have no relationship at all to the real-world system and yet it can predict its behavior statistically.

Most other interpretations see the waves as playing some role in determining where the particle in 3D space actually shows up. This is where MWI begins to make no sense: it denies that there ever even is a particle at all and physical reality is just the invisible waves. It does not actually posit that when an observation is made, the wave is reduced to an eigenstate on two different branches of a multiverse. It denies that there is ever a reduction at all.

Imagine a photon hits a beam splitter and has a 50%/50% chance of being reflected/transmitted, and you have two detectors on either side. At the end of the day, you will detect one or the other. But MWI denies that you will detect one or the other. It does not actually posit that the universe literally splits into two branches where you detect one or the other, because if all that exists is the quantum state and the quantum state also never reduces to anything, then neither detectors actually ever enter into an eigenstate where you can say a detection was made.

If you take MWI seriously, then what it is literally doing is denying the entirety of the reality that we observe. Everything we observe is just a lie, and true reality merely consists of a single giant infinite-dimensional wave that exists nowhere, is made of nothing, and nobody can ever see it. But clearly that is not what we perceive in the real-world, so MWI proponents have to claim what we perceive is an "illusion" created by "consciousness," and then will just kick the can down the road and say that the mystery of why what we perceive is nothing like "true" reality is caught up in the "mystery of consciousness" and when we solve that then we will also understand how the "illusion" is created. It doesn't really "solve" anything but just shifts one loaded topic under the umbrella of another.

Tim Maudlin has a good lecture on this problem in particular.

MWI proponents also constantly misrepresents the state of MWI to make it seem more "proven" than it actually is, such as repeatedly making the false claim that it is "simpler" because it deletes the Born rule. The Born rule was not added because it is funny, it was added because it is necessary rule to actually make predictions with the theory, to tie the quantum waves back to what we actually observe. If you delete it, you are left without any ability to derive probabilities, at least without adding another assumption.

Lev Vaidman did a survey of all the attempts to derive the Born rule in the literature and found every single one of them ends up introducing some additional assumption somewhere. They always at some point need to take on an assumption as arbitrary as the Born rule itself. Sean himself published a paper where he tries to develop a "quantum epistemic separability principle" to derive it which is based on doing a partial trace on the universal wave function and treating the diagonal entries in the reduced density matrix as probabilities, yet Richard Dawid and Simon Friederich pointed out in a response paper that there is no coherent justification for his ESP-QM other than it simply being proposed for the purpose of deriving the Born rule, and there is no justification that the diagonals of a reduced density matrix even tell you anything about probabilities unless you're already assuming the Born rule.

You can derive the Born rule through Gleason's theorem, but Gleason's theorem relies on one of its assumptions the idea that the quantum state actually translates to classical probabilities across classical measurement devices. This is obviously something denied in MWI as there are no classical measurement devices, and so Gleason's theorem cannot be used to justify the Born rule for MWI.

There is also an issue with locality. The EPR paper is basically a no-go theorem against local psi-complete interpretations of quantum mechanics. You cannot have a local psi-complete interpretation. MWI proponents may try to say it is "local" in Hilbert space, but this is rather meaningless as locality refers to position in 3D space. Something that is nonlocal is superluminal, it moves through space faster than light, but quantum waves do not "move." They have no position. The concept of locality is hardly relevant to them. If you actually look at the behavior of particles in 3D space, then MWI is manifestly nonlocal. I am not even claiming it being nonlocal is inherently a flaw, but more that they always claim it is local when you just look at the mathematics and it is not meaningfully local in any sense.

Sean also likes to say misleading statements like MWI is just "taking the Schrodinger equation seriously." This plays into a myth pushed by David Deutsch, which I constantly see this fallacy repeated by MWI believers, which is that the only two interpretations are MWI, which says things always evolve according to the Schrodinger equation, or objective collapse models, which say they do not, and since it's trivial to prove that objective collapse models are not mathematically consistent with quantum mechanics, therefore if you just "take the Schrodinger equation seriously" then you must believe in MWI.

But this is fallacious because objective collapse models are incredibly niche and hardly anyone buys into objective collapse models anyways, except maybe Penrose and his crew these days, but it's literally like <1% of academics. No interpretation is an objective collapse model, because objective collapse models necessarily make different predictions, so they fall under the category of a whole different theoretical model. There are like a couple dozen interpretations in the literature and they all "take the Schrodinger equation seriously." Even Copenhagen does not claim that there is literally a physical collapse but treats it as merely epistemic.

Indeed, all interpretations treat the "collapse" as an epistemic measurement update in some way, including even MWI (as you are merely "realizing what branch you're on"). When it actually comes to interpretations, MWI's competition is other interpretations, not objective collapse theories. Poking holes in objective collapse theories doesn't somehow provide evidence that MWI is correct.

Pathetic troll