According to our current model, we would probably observe un-collapsed quantum field waves, which is a concept inaccessible from within the universe, and could very well just be an artifact of the model instead of ground truth.
It so strange to me that this is the popular way people think about quantum mechanics. Without reformulating quantum mechanics in any way or changing any of its postulates, the theory already allows you to recover the intermediate values of all the observables in any system through retrospection, and it evolves locally and deterministically.
The “spreading out as a wave” isn’t a physical thing, but an epistemic one. The uncertainty principle makes it such that you can’t accurately predict the outcome of certain interactions, and the probability distribution depends upon the phase, which is the relative orientation between your measurement basis and the property you’re trying to measure. The wave-like statistical behavior arises from the phase, and the wave function is just a statistical tool to keep track of the phase.
The “collapse” is not a physical process but a measurement update. Measurements aren’t fundamental to quantum mechanics. It is just that when you interact with something, you couple it to the environment, and this coupling leads to the effects of the phase spreading out to many particles in the environment. The spreading out of the influence of the phase dilutes its effects and renders it negligible to the statistics, and so the particle then briefly behaves more classically. That is why measurement causes the interference pattern to disappear in the double-slit experiment, not because of some physical “collapsing waves.”
People just ignore the fact that you can use weak values to reconstruct the values of the observables through any quantum experiment retrospectively, which is already a feature baked into the theory and not something you need to add, and then instead choose to believe that things are somehow spreading out as waves when you’re not looking at them, which leads to a whole host of paradoxes: the Einstein-Podolsky-Rosen paradox, the Wigner’s friend paradox, the Frauchiger-Renner paradox, etc.
Literally every paradox disappears if we stop pretending that systems are literally waves and that the wave-like behavior is just the result of the relationship between the phase and the statistical distribution of the system, and that the waves are ultimately a weakly emergent phenomena. We only see particle waves made up of particles. No one has ever seen a wave made up of nothing. Waves of light are made up of photons of light, and the wave-like behavior of the light is a weakly emergent property of the wave-like statistical distributions you get due to the relationship between the statistical uncertainty and the phase. It in no way implies everything is literally made up waves that are themselves made of nothing.
Many-worlds is nonsensical mumbo jumbo. It doesn’t even make sense without adding an additional unprovable postulate called the universal wave function. Every paper just has to assume it without deriving it from anywhere. If you take MWI and subtract away this arbitrary postulate then you get RQM. MWI - big psi = RQM. So RQM is inherently simpler.
Although the simplest explanation isn’t even RQM, but to drop the postulate that the world is time-asymmetric. If A causes B and B causes C, one of the assumptions of Bell’s theorem is that it would be invalid to say C causes B which then causes A, even though we can compute the time-reverse in quantum mechanics and there is nothing in the theory that tells us the time-reverse is not equally valid.
Indeed, that’s what unitary evolution means. Unitarity just means time-reversibility. You test if an operator is unitary by multiplying it by its own time-reverse, and if it gives you the identity matrix, meaning it completely cancels itself out, then it’s unitary.
If you just accept time-symmetry then it is just as valid to say A causes B as it is to say C causes B, as B is connected to both through a local causal chain of events. You can then imagine that if you compute A’s impact on B it has ambiguities, and if you compute C’s impact on B it also has ambiguities, but if you combine both together the ambiguities disappear and you get an absolutely deterministic value for B.
Indeed, it turns out quantum mechanics works precisely like this. If you compute the unitary evolution of a system from a known initial condition to an intermediate point, and the time-reverse of a known final condition to that intermediate point, you can then compute the values of all the observables at that intermediate point. If you repeat this process for all observables in the experiment, you will find that they evolve entirely locally and continuously. Entangled particles form their correlations when they locally interact, not when you later measure them.
But for some reason people would rather believe in an infinite multiverse than just accept that quantum mechanics is not a time-asymmetric theory.