There's an important distinction here which is often misunderstood. The loss of interference is due to the photon (really, light wave) interacting with the detector. That results in an entanglement, and it's easy to show (mathematically) that interference is lost.
That's not the difficult part. Any physical interaction that causes a correlation between the particle's state and some other state (effectively making that thing a detector) will cause this. This includes air molecules, which is why we have to isolate the systems very well.
After interacting with the detector, the particle+detector system is still in a superposition of two states (corresponding to the two slits). When the particle hits the screen, the screen gets in on the entanglement. At all times there are (effectively) two paths, including when your body and brain interact with the experimental setup.
So why do you see only one result?
One answer (the Many-Worlds Interpretation) is that you don't see one result. One "copy" of you sees this one and "another" sees the other. Another possibility is that there really is some particular event that "collapses" the wave function, but AFAICT people mostly use this interpretation for practical reasons, bypassing the philosophical conundrum.
Or, if you insist that you you sees only one result, but you still like MWI, then the question becomes: why did I end up on this branch rather than another one? But any explanation would have to come from outside the system (the multiverse) more or less, so I wouldn't hold my breath waiting for an answer.
but you still like MWI, then the question becomes: why did I end up on this branch rather than another one?
If you like MWI, then the answer is you end up on every branch. It's like another invocation of the hateful anthropic principle - in every branch you are asking about the result you saw in that branch. You are asking why you ended up in this branch because this is the branch in which you exist to ask that specific question.
The real question of MWI for me is, where the heck are all these squillions of universe copies every second and where does the energy come from and go to making squillions of universes worth of matter and creating universe sized places to put them and how does you interacting with a photon on Earth cause a copy of the Andromeda galaxy anyway?
The other real question with MWI is why you still experience one universe and not a superposition of universes?
How exactly, in moment-to-moment physical detail, do these splits happen?
And as for branching - how are observers assigned among universes?
The MWIers will shrug and tell you observer experiences are distributed at random.
Well - fine. So why can’t the collapse experience be inherently random too, without demanding squillions of Occam-busting unobservable universes?
It seems like the least plausible interpretation. Not only does it multiply entities for no good reason, and assume they can never be observed, but it doesn’t even solve the original problem.
“Yes, but the math...” isn’t a defence because there are plenty of situations in physics where technically correct solutions are discarded as unphysical. Why make an exception here?
> The other real question with MWI is why you still experience one universe and not a superposition of universes?
You experience a superposition of universes. Why do you think you aren't? What would you expect to be different?
> How exactly, in moment-to-moment physical detail, do these splits happen?
The wavefunction is what's physically real. When you can express the wavefunction as a linear combination of two other wavefunctions, then it makes sense to model this as a split, and think of it as the universe dividing in some process like cell division (a wavefunction that's almost a linear combination of two other wavefunctions, but has a small interaction term, is like a universe that has almost split into two, but the two parts are still slightly attached). Sometimes a split happens at a discrete point corresponding to a discrete physical event - e.g. if a proton and an antiproton in known spin states that sum to 0 collide to generate two gamma rays, then that's a distinct, discontinuous event (and you'll see that in the wavefunction) and after it the wavefunction is expressible a linear combination of two wavefunctions. So we interpret this as "when the particles collided, the universe branched", and like most models that's a simplification of reality, but it's a good one that generally gives correct intuitions. But if you ever want the exact physical detail, just look at the wavefunction.
> Well - fine. So why can’t the collapse experience be inherently random too
It can, but the collapse is a much bigger assumption than subjective probability. It violates unitarity, it's unclear when it happens, it introduces a notion of observer and a direction to time and all of that - and for what? It doesn't let you get rid of the superpositions (you still have to assume that the superpositions exist before the "collapse", so you still need all the machinery that you would need to implement many-worlds). Occam's razor says we can and should do without it.
> without demanding squillions of Occam-busting unobservable universes?
By that logic the assumption that the universe is billions of light years across should require a huge pile of evidence, much more than we have. Occam doesn't say we should be parsimonious about the size of the universe we assume, it says we should be parsimonious about how many physical rules we assume.
Your real question boggles my mind too. The way I visualize it is as another dimension where conservation of energy doesn’t apply. Which is just as hard to imagine as any extra dimension beyond the 3 or 4 we experience. But that doesn’t mean we shouldn’t try.
> where the heck are all these squillions of universe copies every second and where does the energy come from and go to making squillions of universes worth of matter and creating universe sized places to put them
It's the same "mass" of universe, just split across squillions of pieces. Like a tree trunk dividing into two branches, or a river confluence in reverse. (In fact it makes no difference if there were always two universe branches, it's just that they happened to contain exactly the same physical events up until the point where they didn't). And actually it's continuous rather than discrete: most of the time rather than dividing into distinct branches it's more like it's spreading out across a landscape. The branches are an approximation to let us compute more easily rather than something fundamental (e.g. we take this big continuous landscape of "universes" and draw a line between the region where a given photon was up and the region where that photon was down, and treat this as two distinct universes, because what we're interested in is whether the photon was up or down, and for our purposes any two points in the universe-landscape in which the photon was up are equivalent to each other).
> how does you interacting with a photon on Earth cause a copy of the Andromeda galaxy anyway?
It doesn't; when you interact with the photon which is, from your perspective, in two forks, you entangle your own fate with it, and from the perspective of an observer in Andromeda, both you and the photon are now in two forks (the one where the photon was up and you measured it as up, and the one where the photon was down and you measured it as down). As and when the observer in Andromeda interacts with you, then they can also be seen as divided into two forks. But if the observer in Andromeda is behaving exactly the same way in either fork, then it makes more sense to see them as being in a single unified universe. (Again, the very concept of "forks" is a simplification over continuous reality; the point isn't that there's any concrete sense in which the observer in Andromeda is in one rather than two realities whereas you are in two rather than one, it's just that when we want to talk about you we can draw a line in the universe landscape (between universes in which you measured up and universes in which you measured down) and treat the universe landscape as a fork between two universes, while when we want to talk about the observer in Andromeda there is no value in using that particular model (though there's nothing wrong with doing so, and if you compute predictions about the observer in Andromeda while treating them as being in such a superposition you will obtain correct results)).
Depends on what precisely you mean by that. It can allow you to exhibit correlations that are hard to explain within a classical framework, yes. But that requires very precise control over the system, so has no practical import most of the time. Things are being entangled pretty much nonstop around you all the time.
I think that basically the observer is considered "entangled" with a phenomenon after they have interacted with it. That's all.
The observer can be anything. You are you, so by the time information comes to you, you've become entangled.
I find this interpretation to be mostly wordplay. After all how can something be "real" if you can never observe it or detect it? It's sort of saying the theoretical construct "those other worlds" are real, but in what sense?
Do we ever observe or detect the larger universe outside our light cone? Yet astronomers say it exists.
What about other minds? Do you ever observer/detect someone else's experiences, thoughts, feelings? Or just their behavior from which you infer a mind?
That's not the difficult part. Any physical interaction that causes a correlation between the particle's state and some other state (effectively making that thing a detector) will cause this. This includes air molecules, which is why we have to isolate the systems very well.
After interacting with the detector, the particle+detector system is still in a superposition of two states (corresponding to the two slits). When the particle hits the screen, the screen gets in on the entanglement. At all times there are (effectively) two paths, including when your body and brain interact with the experimental setup.
So why do you see only one result?
One answer (the Many-Worlds Interpretation) is that you don't see one result. One "copy" of you sees this one and "another" sees the other. Another possibility is that there really is some particular event that "collapses" the wave function, but AFAICT people mostly use this interpretation for practical reasons, bypassing the philosophical conundrum.
Or, if you insist that you you sees only one result, but you still like MWI, then the question becomes: why did I end up on this branch rather than another one? But any explanation would have to come from outside the system (the multiverse) more or less, so I wouldn't hold my breath waiting for an answer.