My background is as a PhD physicist whos main association with quantum mechanics wave particle duality was in 1) solid state physics: electrons and 'cooper pairs' of electrons in superconductors and crossing barriers by tunneling 2) figuring out the noise properties of amplifiers treating high frequency radio waves as both waves and particles.
The diffracted electron does not go through one slit or the other. To see if it goes through one slit or another you have to, for example, turn a light on to look at the electron. You see the electron by bouncing photons off the electron. When you turn a light on of sufficiently short wavelength to see which slit the electron goes through two things happen: 1) you see the electron go through one slit or the other 2) the diffraction pattern formed by electrons disappears! Bouncing photons off the electrons in order to see them with sufficient resolution to determine which slit they go through also screws up the correlation required to get two-slit diffraction patterns.
So now you turn the light back off, you can't see which slit the electrons are going through, and you see the banded diffraction pattern past the slits building up again.
This is the uncertainty principle. In order to localize the electron sufficiently to restrict it to one slit or the other, you have to randomize its momentum to the point that two-slit diffraction effects are blurred out. In order to "localize" the electrons momenta to the point that you see diffraction patterns, you have to randomize its location to the point that you cannot actually, even in principle, say it went through one slit or the other.
I may be giving you one QM "interpretation" here. But I have not seen any "Many Worlds" interpretations that overcome the uncertainty principle and can also be used to calculate the results of experiments. I admit these descriptions may not exist, but the way I describe the electrons and slits absolutely can be used to make quantitative predictions that fit experiments.
I don't think any interpretation provides a novel set of tools for measurement or prediction.
I've read through the Quantum Physics sequence and feel that I managed to understand most of it. But now it seems to me that the Double Slit and Schrodinger's cat experiments are not described quite correctly. So I'd like to try to re-state them and see if anybody can correct any misunderstandings I likely have.
With the Double Slit experiment we usually hear it said the particle travels through both slits and then we see interference bands. The more precise explanation is that there is an complex valued amplitude flow corresponding to the particle moving through the left slit and another for the right slit. But if we could manage to magically "freeze time" then we would find ourselves in one position in configuration space where the particle is unambiguously in one position (let's say the left slit). Now any observer will have no way of knowing this at the time, and if they did detect the particle's position in any way it would change the configuration and there would be no interference banding.
But the particle really is going through the left slit right now (as far as we are concerned), simply because that is what it means to be at some point in configuration space. The particle is going through the right slit for other versions of ourselves nearby in configuration space.
The amplitude flow then continues to the point in configuration space where it arrives at the back screen, and it is joined by the amplitude flow via the right slit to the same region of configuration space, causing an interference pattern. So this present moment in time now has more than one past, now we can genuinely say that it did go through both. Both pasts are equally valid. The branching tree of amplitude flow has turned into a graph.
So far so good I hope (or perhaps I'm about to find out I'm completely wrong). Now for the cat.
I read recently that experimenters have managed to keep two clouds of caesium atoms in a coherent state for a hour. So what would this look like if we could scale it up to a cat?
The problem with this experiment is that a cat is a very complex system and the two particular types of states we are interested in (i.e. dead or alive) are very far apart in configuration space. It may help to imagine that we could rearrange configuration space a little to put all the points labelled "alive" on the left and all the dead points on the right of some line. If we want to make the gross simplification that we can treat the cat as a very simple system then this means that "alive" points are very close to the "dead" points in configuration space. In particular it means that there are significant amplitude flows between the two sets of points, that is significant flows across the line in both directions. Of course such flows happen all the time, but the key point is here the direction of the complex flow vectors would be aligned so as to cause a significant change in the magnitude of the final values in configuration space instead of tending to cancel out.
This means that as time proceeds the cat can move from alive to dead to alive to dead again, in the sense that in any point of configuration space that we find ourselves will contain an amplitude contribution both from alive states and from dead states. In other words two different pasts are contributing to the present.
So sometime after the experiment starts we magically stop the clock on the wall of the universe. Since we are at a particular point the cat is either alive or dead, let's say dead. So the cat is not alive and dead at the same time because we find ourselves at a single point in configuration space. There are also other points in the configuration space containing another instance of ourselves along with an alive cat. But since we have not entangled anything else in the universe with the cat/box system as time ticks along the cat would be buzzing around from dead to alive and back to dead again. When we open the box things entangle and we diverge far apart in configuration space, and now the cat remains completely dead or alive, at least for the point in configuration space we find ourselves in.
How to sum up? Cats and photons are never dead or alive or going left or right at the same moment from the point of view of one observer somewhere in configuration space, but the present has an amplitude contribution from multiple pasts.
If you're still reading this then thanks for hanging in there. I know there's some more detail about observations only being from a set of eigenvalues and so forth, but can I get some comments about whether I'm on the right track or way off base?