And it's a very bad interpretation IMO as it fails to provide a convincing interpretation of the probabilities QM produces, which are the main predictive content of the theory. If you believe every outcome happens then one thing happening with higher probability than another loses meaning. The decision theory argument put forward by some many worlds proponents fails to solve this as it just provides a calculation you can do that produces the Born probabilities. It doesn't provide a convincing interpretation of them.

Edit: Also the claim many worlds is somehow more parsimonious in its assumptions than Copenhagen is highly dubious. It assumes an infinite multiverse that is splitting into infinite variants in every infinitesimal instant, in possibly the grossest violation of conservation of momentum and energy (and therefore the corresponding symmetries) that could be imagined.

Also, MWI doesn't assume infinite worlds, it predicts them. It's a consequence, not a postulate.

"The most austere version of QM" is marketing bullshit and not widely accepted (or particularly meaningful). Many worlds does postulate its many worlds rather than predict them as it provides no way their existence can ever be tested for.

Equating Schrodinger's equation with many worlds is both intellectual dishonesty and begging the question. Any interpretation of conventional quantum mechanics involves Schrodinger's equation. Penrose's ideas (referenced by Sean Carroll in quotes others have posted on this thread) involve violations of Schrodinger's equation because they are actual new theories that differ from standard quantum mechanics. In other words, Penrose is doing actual physics rather than blowing smoke up people's arses. This fact doesn't convey some falsifiability on many-worlds, which is just one of several interpretations of QM that all make the same predictions.

Many Worlds just postulates that if you put a cat in a superposition of two quantum states (alive + dead), there is no sensation associated with that superposition. The cat is in a superposition of having the sensation of being alive, and having the sensation of being dead, but there is no observable quality of "being in a superposition of alive and dead". Both of the superimposed pure states of the cat feel decidedly one way or the other.

Hence, when a human is in a superposition, we would not know it. Both of our superimposed states are experiencing the feeling of being in a pure state of looking at an instrument measuring a photon with polarization ↑, or looking at an instrument measuring a photon with polarization ↓. Being in an (↑ + ↓) superposition doesn't feel like looking at an instrument display and seeing a blurry reading. We feel like we saw the instrument reading ↑, and we (as an element of the quantum system) cannot interact with the branch of the wavefunction that we are superimposed with in which we saw the instrument reading ↓.

This isn't a separate postulate, it's just straightforward quantum theory applied to quantum systems large enough to have opinions about their own state.

Your conclusion does not follow from your premise. MWI does postulate different stuff from Copenhagen, but the existence of many worlds is not one of them. Just because I postulate Peano arithmetic doesn’t mean I also postulate 2+2=4. The latter is but a consequence of the former.

My understanding of MWI, is that it just "believes" the equations. Specifically, if the equations say the complex amplitude is not null, then this stuff is real. And that’s about it. It’s not our fault the equations describe non-null amplitudes, that if real are many worlds.

This interpretation business could apply to macro-scale physics as well: when you send a probe so far out there that it crosses the limit of the observable universe, does it cease to exist? What if there’s people inside, will they die? One would be hard pressed to argue from their discontinued existence: why would we disbelieve the equations of relativity, that work so well for everything we can observe?

Conversely, what makes you think that the equations that work so well on everything we can observe, would somehow stop working (or stop applying) on the stuff we cannot observe? I know you didn’t explicitly disbelieve the equations, but since we can’t at the same time believe in their general applicability and disbelieve MWI, it’s hard to interpret your refusal to acknowledge MIW as the most probably hypothesis as anything but scepticism about the applicability of those equations.

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To throw you a bone I personally do have a reason to be wary: physics is currently inconsistent, and when we get a credible theory of everything, it may very well use different equations that do not describe many worlds, make long range probes vanish, or both.

Of course they cease to exist as do the people ... just like sailing ships and sailors did when they travelled past the visible horizon.

This isn't the conundrum you're looking for.

“Worlds” is inaccurate since there is still only one world (the ordinary, uncontroversial, tensor product Hilbert space of textbook qm). It the stuff in the world that enters into superposition, among which stuff are the atoms making up “observers”.

Similarly “completion” would be a better word than “interpretation”.

However, the absurdity of this consequence should mot lead us to doubt the existence of motion, but quite the opposite, it hints to us that the either the postulate is false, or otherwise we are missing some essential knowledge that allows to determine the correct consequences of that postulate.

No, you haven't. Standard "shut up and calculate" QM does not say time evolution always happens according to the Schrodinger equation. It only says that happens when a measurement is not being made.

No "photons bouncing around between atoms" would constitute a measurement. A measurement involves some kind of macroscopic, irreversible change.

> every detector is itself just built out of atoms

Yes.

> which evolve in accordance with Schrodinger's equation.

That's not what standard "shut up and calculate" QM says. The MWI is what you get if you insist that this is literally true, all the time, no exceptions. For many physicists, however, the fact that the MWI leads to claims that appear to grossly contradict observation is a reason to doubt that the Schrodinger equation really has to be literally true, all the time.

Is there an objective definition of "macroscopic" within the framework of QM? How does a system decide whether it's macroscopic or not? As far as I can tell I'm just swimming in a sea of bosons and fermions doing their thing, there's no "macroscopic" label that distinguishes the atoms of iron in my vacuum chamber wall from the atoms of rubidium in my vacuum chamber's magnetic trap. They're both following the exact same laws of physics at any given moment. (Laws which, AFAIK, satisfy time-reversal symmetry.) Of course, some systems have many degrees of freedom, which makes the calculation very difficult and exhausting, but that's our problem, not the system's.

When does your calculation get to drop branches of the wavefunction, and when does it have to keep track of them? Deciding this policy seems like an important step of shut-up-and-calculate.

And most importantly: which MWI claim contradicts observation?

Yes. All of those questions are open areas of research in the foundations of QM. There are no generally accepted answers to them, although of course each individual QM interpretation has its own set of preferred answers, and physicists who prefer a particular interpretation will often talk as though their preferred answers are generally accepted--but they're not.

> which MWI claim contradicts observation?

The fact that we observe measurements to have single results. The MWI claims that every measurement has all possible results, but that this fact is in principle unobservable. That makes the MWI unfalsifiable.

In a different thread you wrote that "as you say, the MWI claims that in each branch of the wave function, the human experiences a single result of a measurement." But here you're saying that MWI makes a claim that contradicts that we observe measurements to have single results. MWI, just like Copenhagen, claims that we will observe measurements to have single results (but that we won't be able to predict in advance which result we will experience observing). MWI's predictions are exactly in alignment with what we observe, and makes no claim that in any way differs from the reality that we experience. If it did, then it would be falsifiable.

I agree. Others (not you) elsewhere in this discussion have said otherwise.

> here you're saying that MWI makes a claim that contradicts that we observe measurements to have single results.

Yes, overall I think the MWI contradicts itself in this regard. It has to say that measurements have all possible results, because that's what Schrodinger's equation says, but it also has to say that we observe measurements to have single results, because it's clear that that's what we actually observe. I don't think its claimed resolution of that contradiction is valid. But you are correct that that is not an issue that can be resolved by experimental tests.

So you're an MWI proponent, then? If it's "obviously true" to you that everything, including measuring devices and ourselves, always evolves in time exactly according to the Schrodinger equation, no exceptions, then the MWI is your only option as an interpretation.

The fact that many people do not think the MWI is a viable interpretation is why interpretations like Copenhagen, which do not rest on the belief that the Schrodinger equation has to be literally true, everywhere, all the time, continue to exist.

But once you start trying to really tease apart the potentially metaphysical question of "what's actually happening?" (a question many people believe is just not within the realm of falsifiability/science), traditional collapse theories have a bunch of trouble setting up clear lines of what macroscopic is, how collapse actually occurs, etc. that do not seem to have easy resolutions, especially as we can create bigger and bigger quantum systems.

All of this is true. But none of it makes it "obvious" that the Schrodinger equation must always be exactly correct and that one should be "dumbfounded" (your word) that any interpretation (like Copenhagen) that says otherwise has any proponents at all. Yes, "collapse" is an obvious open question that hasn't been resolved. But the belief that some way of resolving it will eventually be found that doesn't require accepting the MWI is a perfectly reasonable belief to hold, and an interpretation like Copenhagen is a perfectly reasonable interpretation for people to use in the interim while we search for such a resolution.

Do you have experimental evidence for that claim? The "cat in a box experiment in a box" thought experiment suggests that it does.

Solutions to the Schrodinger equation all take the form `Psi(t) = U(t)Psi(0)` for some unitary operator `U`. "Collapse" applies a projection operator, which is nonunitary.

What further interpretation is needed? To the extent that we have a number we can measure, we have a calculation that can predict it. More interpretation would be nice, but fundamentally every interpretation of QM has this problem, it's not a unique problem with MWI.

> It assumes an infinite multiverse that is splitting into infinite variants in every infinitesimal instant, in possibly the grossest violation of conservation of momentum and energy (and therefore the corresponding symmetries) that could be imagined.

It assumes continuous unitary evolution that preserves momentum and energy in exactly the way you'd expect, exactly the way we already assume they work. Accepting that you can reasonably make calculations about infinitesimal changes is literally the foundation of physics, it's how Newton was able to make a theory of gravity (and people were just as unhappy about it then).