Edit: If you try to actually define "wave function collapse" and "observer" you are back into the same mumbo-jumbo as "wave-particle duality". They are just placeholders that obscure the mystery.
That out of the way -
The "thing" is neither a particle or a wave, it's a propagating probability amplitude.
With one slit, you get a simple probability distribution. With two slits, you get a complex probability distribution, where the amplitudes are summed (and since they can be negative, sometimes sum to zero)
The detector adds constraints to the probability amplitudes, zeroing part of the complex probability of the double slit, thus reducing it to a non-complex distribution.
The expectation was that the electrostatic forces would create "bubbles" of empty around the ions, which (the bubbles, not the ions) would then be detected when they reached the bottom.
They found those bubbles. They also found many differently sized bubbles, all smaller, in a size distribution correlating to the probability that the ion would pierce the surface tension of the liquid.
The interpretation was that the smaller bubbles were the "pieces" of the waveform that traversed the surface tension. This was taken as evidence the waveform is an actual thing that actually exists.
I really wish I could find it...
https://en.m.wikipedia.org/wiki/Aharonov%E2%80%93Bohm_effect
But you can't look at a single photon/electron/whatever and get this contextual information out of what we know about the physical structure of the object itself. There is no physical marker we can read "inside" a photon that says it's entangled, or that it's passed through two slits by "interfering with itself".
So the information - and probably the process itself - is contextual. We don't know where this information lives. But it has to exist somewhere, because we can see the effects of its operation very easily.
So it's physical in the sense that we can see physical effects. Photons etc know what they're doing, and we can make excellent predictions about that behaviour. But the exact nature of the entities and relationships that makes this process work, and the way it maps to observable events and properties, remains a mystery.
The obvious implication is that what we see isn't really fundamental. Neither field excitations nor the fields of QFT are the ground truth, and there's another layer of reality that generates them - and probably spacetime too. But that layer has some very unusual properties from our POV, and we're going to need some new metaphors to understand it.
If you were modelling the universe and everything in it, say via a simulation, fancy probability distributions would be a good way of calculating behaviour. What if the "stuff of reality" we're looking for (and trying to map the math to), doesn't actually exist? What if the math is all there is? What if behaviour is all that matters?
In primitive cultures, spirits were hypothesised as the animating force behind behaviour of animals, stars, weird people etc. For a long time, it proved a reasonable model until it was supplanted by our more scientific notions of the observable universe. But what if quarks, leptons, photons, are just "spirits" i.e. a good enough approximation given what we know? What if it's "spirits" all the way down? i.e. the only fundamental is the math and everything else is a visualisation (of sorts)?
BTW, that kind of speculative question (and an awesome high school physics teacher), are what made me study physics at university. Shame (in one way) I was more interested in rocking out and drinking beer.
EDITED TO ADD: don't take anything I post too seriously. If you find yourself rolling your eyes, just downvote me and move on.
Behavior, observation, accurate prediction and empirical evidence is all that matters in science, yes. What difference does it make if you call it "particle" or "just math"? What practical difference does it make?
If none then that is not a question of science. It might be an interesting question of philosophy or spirituality, but not for science. Science deals with falsifiable hypothesis and accurate predictions and making functional machines.
The more you try to nail down MWI on specifics - like exactly when and how universes appear, and how many there are, and how this process is supposed to map back to a probability distribution in one of the universes - the less and less plausible it becomes.
I think it's a much simpler interpretation than the Copenhagen interpretation.
Here's an awesome discussion/debate between Carroll and a major MWI skeptic, David Albert: https://youtu.be/AglOFx6eySE
In this discussion, they cover what they consider "silly" and "important" criticisms of MWI. Respectfully, some of what you said in your comment is placed by them in the "silly" box.
To me it seems there can be two purposes for creating a good interpretation: 1) make it more in line with most people's intuitive thinking about the world, to make them understand the theories better and faster and 2) make it easier to produce new scientific results, or predict other things.
So far I don't see how the MWI manages either of those better than the other models.
Many worlds isn't the universe forking. It is more straightforward to think of the other worlds as probability spaces. The probability space simply divides as we call each division a "world".
On the contrary. When you look at the specifics it's all very obvious. Look at the wavefunction, not the TV-show verbal explanation, and the answer to your first two questions are very obvious, as is the ill-formedness of your final question.
It's a pretty simple concept. If you create a random hypothetical universe that follows all the laws of physics what reason is there for it to not exist? What constraints beyond the laws of physics ensure that our universe is the only one that can possibly exist?
Whether this is simpler than wave collapse or hidden variable interpretations is an almost purely subjective opinion.
From the perspective of the photon the time stands still, it arrives instantly at the destination.
You can see this in that the quantum eraser experiment only works with photons and not particles with mass like electrons.
It works with even larger particles as well such as molecular ions or buckyballs.
If you measure before/at the slit you affect the experiment. You affect how it travels through the slits, removing the interference pattern. So yes it’s not split but that has no bearing on when the detector is not at the slit. They’re different situations and have different outcomes.
So in reverse then, when absorbed the wave could be cancelled out and it is forced into particle behaviour?
With a TIRF microscope you use the 'Evanescent Wave' to image fluorescent molecules in macro objects like neurons or diamonds.
Like, you can see, even more clearly, the yeast that makes your beer.
https://en.wikipedia.org/wiki/Total_internal_reflection_fluo...
https://lightmicroscopy.ucdenver.edu/img/seminars/June17.png
So my thought was maybe the particle in the two-slit experiment was entangled with itself? Like I said, idle thoughts of a layman so not sure how right, wrong or not-even-wrong this is but hey, always fun to think about QM :)
Edit: I think I understand what you're saying. The field isn't separate from the particle - the particle is the quantized field itself.
This lecture is long but goes into the everything is fields idea https://youtu.be/gEKSpZPByD0
As strange as it sounds, it does make some sense. Think of elementary particles; we never observe them as such, we only observe them when they're interacting with something.
Yes. A particle is nothing but a very localized wave. Aka. "everything is fields."
(At least that's may layman's understanding having read quite a bit about this in recent years.)
(In Ukrainian) https://www.facebook.com/groups/194031297610629/permalink/10...
This is to me the most baffling aspect of QM.
"that we can see". That's exactly the difference. The observation (measurement) affects the state of the system. That's like quantum mechanics 101.