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.