I don’t think it works the other way either, I don’t see why anything in quantum mechanics specifically would give any more credence to determinism than classical mechanics which is also deterministic.

Bob is a scientist, they have hooked a computer to the R vs T experiment and when R occurs the screen flashes red. When the screen flashes red, red photons collide with Bob’s skin and eyes, signals enter their brain and they observe a red screen, and they remember it. So, given the state of redbob, I think it’s reasonable to say that perceived R.

These are all classical assumptions which Many Worlds denies. Again, you keep repeating classical descriptions to explain Many Worlds. I do not know how I can explain it as I’ve already went over this several times. You do not get discrete events out of the Schrodinger equation, there is no “redbob,” there is no photons colliding with Bob’s skin and eyes. None of this happens if all you have is the Schrodinger equation.

If R or T never happen from an external perspective doesn’t really matter to us though.

I am not sure what you mean by “an external perspective.” The point is that R or T never occur in physical reality, so you have to then explain how it is that we actually perceive R or T if it’s literally something that doesn’t occur in the real world.

If we accept many worlds as true for a second, then it follows that the total quantum states describes quite a lot, your exact configuration is somewhere within the total state.

There is no “your exact configuration” if Many Worlds is true because discrete objects like “you” don’t even exist.

At a large scale you depend on your parents having existed. So you can only perceive the parts of the total state where your parents existed, because any parts of the state where they didn’t exist does not contain a you to perceive.

This analogy, again, doesn’t work. My parents existing could be said to be R and my parents not existing can be said to be T, and if I have a limited perspective where I only see R (due to the anthropic principle) then it naturally follows I would see R and not T, so that explains why I see one and not both.

But this is not applicable to Many Worlds at all because Many Worlds does not claim two events happen and that a limited perspective makes us just see one of them. Many Worlds claims no events happen. You cannot take the subset of the null set and get a non-empty set from it. The rest of your comment, again, stems from this misconception, which I did try to clearly address in my previous comment yet it still seems to not be understood.

What is meant by “determinism” here? If you mean nomological determinism in the sense of the “free will vs determinism” debate, quantum mechanics is still a nomologically deterministic theory, so it does not strengthen the “free will” position at all. If you mean predetermination in the sense of the “randomness vs determinism” debate, if you interpret quantum mechanics to be fundamentally random, then of course it is incompatible with predetermination. But this is still ultimately an interpretation as it is empirically impossible to distinguish between true randomness and a significantly chaotic system.

One might justify the former position on philosophical grounds, such as, Occam’s razor: it’s simpler to believe there is no cause than to posit a cause we cannot (currently) demonstrate, but that is still ultimately a philosophical argument. Take Plato’s cave, for example. If all they could see is the shadows, they might build theories about the shadows themselves, and then someone might posit that we should believe that the shadows are all that exists and nothing causes them because of Occam’s razor. They would clearly arrive at the incorrect conclusion using Occam’s razor, so Occam’s razor itself is debatable whether or not it is a reliable rule of thumb in cases like this.

Economics isn’t supposed to make sense, it’s just meant to justify the prevailing system for the time. It is like theology back when we used to live under religious monarchies. It treats itself as “academic,” has universities and degrees, very serious “scholarly” debate, entire textbooks written on it, all its adherents will insist that it is a genuine scholarly enterprise and anyone who disagrees just “doesn’t understand it,” but it is ultimately not a genuine scientific program but merely exists to justify the prevailing order at the time.

In many worlds, it’s not so much that the classical world is an illusion

Again, I think you have a false picture in your mind that Many Worlds is like an objective collapse model but where the “collapse” leads to a split into different parallel branches of all possible outcomes, and the reason we see just one discrete outcome and not all of them is because of a “limited perspective.”

But that’s not what Many Worlds is, in Many Worlds there are no discrete outcomes at all. If a photon hits a beam splitter and there is a 50% chance it will be reflected and show up on detector R and a 50% change it will be transmitted and show up on detector T, Many Worlds says that it will never show up on detector R or T at all. The discrete events of R or T simply never occur in Many Worlds.

Just saying there is a “limited perspective” does not get you anywhere. “Limiting a perspective” does not somehow get you to an eigenstate. Note that I am not saying in Many Worlds there are no “singular events.” Of course, if there are only “plural events,” i.e. if R and T both happen, then limiting your perspective to just one of them will give you a singular event. The point is more complicated than that: in Many Worlds there are simply not discrete events at all, so it is not as if R and T both happen in Many Worlds. Neither R or T happen, so there is no “limited perspective” that could give you R or T if neither the events R or T ever happen at all.

I don’t think there’s much difference between many worlds and random selection in the end, at least from our perspective. Either way we experience only state contingent on our state, so any quantum superposition that contains us (ok, sure, contains our brain) we can experience only one concrete resolution to, since the others would require our brain to be in a different state.

Randomness is not the problem. If fundamental randomness was the only difficulty in QM, there would not be so much debate around this subject. The issue is not randomness but that there is no possible way to construct a consistent statistical transition from the quantum state to a random classical outcome from the Schrodinger equation alone, you have to introduce some additional assumptions, and these additional assumptions inherently violate classical premises (such as locality or statistical independence).

Many Worlds does not escape this because it still posits the existence of a transition. To claim that the transition occurs only in our heads and not in physical reality is still positing the existence of a transition, and so there still needs to be some physical process by which the quantum state of waves in infinite-dimensional Hilbert space can be mapped onto the “limited perspective” that we consciously perceive. This mapping, again, requires additional assumptions, and these additional assumptions inherently violate classical premises, as all proposed assumptions for deriving Born probabilities in Many Worlds are manifestly nonlocal.

I’m not sure what it even means to say that you have a “limited perspective” in Many Worlds because you exist smeared out across all the possible branches and there is simply no discrete “you” localizable to a single branch.

Many worlds adds “but there is a disconnected copy of us experiencing the other valid states after we entered the superposition”, random chance says “and the other states disappeared when the superposition collapsed”, but without reaching past that horizon of our own state they’re measurably identical theories, so either both equally valid or both equally pointless speculation, depending on how strict you want to be.

Again, no, that is not Many Worlds. Many Worlds denies that we ever experience any outcome at all. Many Worlds is not a variant of an objective collapse model where all possible outcomes the quantum state could collapse to occur in a different branch of the multiverse. It denies that discrete events even occur at all. It does not argue that in the example of the photon that in one branch of the multiverse you perceive an event on detector R and another on detector T, but ultimately it is denying that the events associated with R and T even occur at all.

The problem that Einstein, Schrodinger, and Bell tried to point out is not “speculation” but that the orthodox interpretation and Many Worlds both make a claim about physical reality which they do not explain. They both take the form (1) there is an infinite-dimensional wave in Hilbert space, (2) you look at it, (3) a miracle happens, and (4) you perceive a discrete event localizable to 3D space. Both posit some sort of transition (which either occurs in external reality or in your head) which inherently implies a mapping from the invisible quantum state to observable reality yet neither actually provide this mapping.

Einstein first pointed this out with an example of radioactive decay, pointing out that the Schrodinger equation, no matter how much you evolve it, never gives you anything even with the appearance of a discrete event where an atom decays and emits a particle. It only gives you a quantum state for the emitted particle that its position spreads out over time.

Just saying abstract things like it’s a “limited perspective” does not address the issue because this is not a philosophical problem; you need a mathematical mapping or what the “interpretation” is claiming simply is not well-defined, i.e. it is not actually physically coherent. For many Copenhagenists they might argue being coherent doesn’t even matter because they only care about practical utility, and if a physical theory makes no physical sense, as long as this does not conflict with practical predictions it doesn’t matter, but Einstein wanted a physical theory that was coherent.

The gigantic, universal ψ wave that contains all the possible worlds is like Hegel’s dark night in which all cows are black: it does not account, per se, for the phenomenological reality that we actually observe. In order to describe the phenomena that we observe, other mathematical elements are needed besides ψ: the individual variables, like X and P, that we use to describe the world. The Many Worlds interpretation does not explain them clearly. It is not enough to know the ψ wave and Schrödinger’s equation in order to define and use quantum theory: we need to specify an algebra of observables, otherwise we cannot calculate anything and there is no relation with the phenomena of our experience. The role of this algebra of observables, which is extremely clear in other interpretations, is not at all clear in the Many Worlds interpretation.

— Carlo Rovelli, “Helgoland: Making Sense of the Quantum Revolution”

Yes, he was an Epsteinite, but his point was generally correct. Einstein, Schrödinger, and Bell all recognized that the orthodox interpretation of quantum mechanics makes no sense because it postulates a transition from quantum to classical with no explanation of how this actually occurs. They would not have liked Many Worlds either because it does not actually get rid of the transition, it just claims the classical world is an illusion so it shifts the transition to something happening in your brain, but then doesn’t explain how your brain could possibly create a world of discrete events happening in 3D space from a world where no discrete events ever happen that is just one giant continuous wave evolving in an infinite-dimensional space as there is no clear way to map the latter onto the former. Bell was around when Many Worlds was becoming popularized and so he did comment on it and pointed out it is nonsense and Everett never demonstrated any way to carry out this mapping consistently.

Don’t forget his contribution to paedomechanics.

real dragons? 🤔

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.

Quantum mechanics does not have much to do with consciousness.