Well if there was public daycare to take the stress off of parents who couldn’t deal with it then it wouldn’t be as big of an issue.

yeah yeah I’m sure quantum mechanics makes us all immortal if that makes you feel better 🤣

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.

I think it is more accurate to say no one agrees on how to understand it rather than no one understands it. Clearly people understand it or else they couldn’t use it to make new inventions and discoveries. It’s just that math doesn’t inherently give you an ontology. That requires you to apply a level of philosophical interpretation to the math, and quantum mechanics is very underdetermined when it comes to the ontology. 10 different physicists who are all experts at the math can look at the same equations and interpret the ontology differently. But I wouldn’t say they don’t “understand” it because their viewpoints can all be logically consistent.

Quantum mechanics does not have much to do with consciousness.

Well, what is boring and non-boring I guess is in the eye of the beholder. What I moreso was referring to is what is difficult to wrap your head around.

The nondeterminism is kind of unavoidable as long as you don’t want to change the mathematics of the theory itself, but I also don’t really consider nondeterminism to be that unintuitive or difficult to “understand.” I mean, throughout most of human history, it wasn’t that common for humans to actually believe in determinism in the Laplacian sense of being able to make absolute prediction to the future based on complete knowledge of the past, that was largely popularized with the rise of Newtonian mechanics, and even by the 19th century you had even a lot of materialist philosophers calling it into question on grounds of logical consistency. Personally, I think the strong desire to maintain Laplacian determinism is really a physicist thing. They work with Newtonian mechanics first and it becomes so intuitive some don’t want to let it go when it comes to quantum mechanics. But I doubt if you went and talked to the average person, most probably wouldn’t be that strongly adherent to Laplacian determinism.

The kinds of views I was talking about are more things like people who try to interpret the state vector as literally representing a physical wave spreading out in space that collapses like a house of cards when you perturb it, or try to envision a literal multiverse where everything is just a big “universal wave function.” A lot of these bizarre views are not only unintuitive but literally impossible to visualize, and they run into a lot of problems in logical consistency and there have been mountains papers and books published on the subject trying to work out all the conceptual issues. If you are a person just learning QM and the philosophical interpretation around it bothers you, if you listen to people who talk about these weird things, you will need to read through dozens of books and maybe even hundreds of papers just to get a general idea of what is going on, and even then most of these interpretations still have not resolved their mountain of conceptual issues.

To me this really bothered me when I got into quantum computing for the first time. I wanted to not just learn the math but have some sort of intuition of what is actually going on. I then went down a rabbit hole of reading tons and tons and tons of books and academic papers to try and find some way to make the math make sense on a philosophical level. Most of the mainstream views you see in the popular media just overcomplicate things for no reason because the person wants to make QM sound more mystical than it actually is. What I ultimately came to realize is that most of this confusion is just self-imposed in the sense that they are based on assumptions which are not actually demanded by the mathematics and entirely optional (such as interpreting a list of probability amplitudes a literal entity in a physical space) and thus most can be stripped away.

You can’t strip away every aspect of QM that makes it unique, because it clearly does differ from classical mechanics, but by dong this you do really hone down on what actually makes QM unique and what is genuinely an unavoidable consequence of the mathematics. And what you get down to is just interference effects, which arise from the fact that probability amplitudes are complex-valued, thus can cancel each other out, which can’t occur in classical probability theory. Nondeterminism and context-dependence then follow from this as a necessity for the theory to be logically consistent, but both of those are fairly easy to have an intuition for.

I am saying that assigning ontological reality to something that is by definition beyond observation (not what we observe and not even possible to observe) is metaphysical. If we explain the experiment using what we observe then there is no confusing or contradiction, or any ambiguity at all. Indeed, quantum mechanics becomes rather mechanical and boring, all the supposed mysticism disappears.

It is quite the opposite that the statistical behavior of the electron is decoupled from the individual electron. The individual electron just behaves randomly in a way that we can only predict statistically and not absolutely. There is no interference pattern at all for a single electron, at least not in the double-slit experiment (the Mach–Zehnder interferometer is arguably a bit more interesting). The interference pattern observed in the double-slit experiment is a weakly emergent behavior of an ensemble of electrons. You need thousands of them to actually see it.

What is it then? If you say it’s a wave, well, that wave is in Hilbert space which is infinitely dimensional, not in spacetime which is four dimensional, so what does it mean to say the wave is “going through” the slit if it doesn’t exist in spacetime? Personally, I think all the confusion around QM stems from trying to objectify a probability distribution, which is what people do when they claim it turns into a literal wave.

To be honest, I think it’s cheating. People are used to physics being continuous, but in quantum mechanics it is discrete. Schrodinger showed that if you take any operator and compute a derivative, you can “fill in the gaps” in between interactions, but this is just purely metaphysical. You never see these “in between” gaps. It’s just a nice little mathematical trick and nothing more. Even Schrodinger later abandoned this idea and admitted that trying to fill in the gaps between interactions just leads to confusion in his book Nature and the Greeks and Science and Humanism.

What’s even more problematic about this viewpoint is that Schrodinger’s wave equation is a result of a very particular mathematical formalism. It is not actually needed to make correct predictions. Heisenberg had developed what is known as matrix mechanics whereby you evolve the observables themselves rather than the state vector. Every time there is an interaction, you apply a discrete change to the observables. You always get the right statistical predictions and yet you don’t need the wave function at all.

The wave function is purely a result of a particular mathematical formalism and there is no reason to assign it ontological reality. Even then, if you have ever worked with quantum mechanics, it is quite apparent that the wave function is just a function for picking probability amplitudes from a state vector, and the state vector is merely a list of, well, probability amplitudes. Quantum mechanics is probabilistic so we assign things a list of probabilities. Treating a list of probabilities as if it has ontological existence doesn’t even make any sense, and it baffles me that it is so popular for people to do so.

This is why Hilbert space is infinitely dimensional. If I have a single qubit, there are two possible outcomes, 0 and 1. If I have two qubits, there are four possible outcomes, 00, 01, 10, and 11. If I have three qubits, there are eight possible outcomes, 000, 001, 010, 011, 100, 101, 110, and 111. If I assigned a probability amplitude to each event occurring, then the degrees of freedom would grow exponentially as I include more qubits into my system. The number of degrees of freedom are unbounded.

This is exactly how Hilbert space works. Interpreting this as a physical infinitely dimensional space where waves really propagate through it just makes absolutely no sense!