I've been interested in physics since I was a kid, and read many books on the topic. The thought experiments of Einstein that led to his theories of relativity were some of the earliest topics I encountered. If you have not read of that, do so . . . I will wait.
So we come to the EPR paradox. The new field of quantum mechanics in the 1920s presented this conundrum - that particles could have entangled properties but that those properties would not become determined until a measurement event, at least according to Bohr. But upon one measurement, both particles states would be determined even if they were separated, and this determination would be instantaneous - faster than light.
The EPR paradox received further attention in the 1950s and led to the Bell's Inequalities - describing the paradox in some detail. Bell proposed solutions to the paradox which are each a bitter pill in their own way. Some have received greater press, but there is nothing yet known to choose among them. Two that are most conspicuous are 1) a multiverse - all the outcomes exist in separate parallel universes, and 2) hard determinism - the paradox arises from quantum mechanics being predictive, but spacetime is complete and only one outcome actually exists - always has and always will.
The more I have thought on these options, the less possibility I can grasp for matters spiritual. The multiverse scenario seems ridiculously uneconomical to my admittedly-Calvinist upbringing, but if all outcomes exist, what judgement can there be for how a person lives (i.e. we live in ALL the ways we can). The hard determinism scenario is crystalline. We do not actually have any free will whatsoever - not even the free will to take advantage of being completely inculpable for our actions.
I think there may be a more mystical way of thinking of hard determinism though - a koan, if you will. We are agents of causality within a complete four-dimensional spacetime. We bring the crystalline structure of the universe into existence by virtue of our own existence in some way.
I haven't given this a lot of thought, but here are some things that spring to mind -- I'm open to a more in depth conversation around this.
I think it's a stretch to apply anything which happens on a quantum level to larger particles or collections of particles. It's difficult enough to keep multiple qubits in sync under tightly controlled conditions in a quantum computer -- extrapolating quantum behavior to a human being running loose in the universe seems to missing a step or two in the middle.
Sure, photons go through both slits in the double-slit experiment, but when I try to walk through both the enter and exit doors of the supermarket, I usually bump into the wall between them, whether or not someone else is looking at me.
At the risk of being simplistic (and I'm aware this argument is not rigorous at all), does a flock of birds act like a single bird? A school of fish? A group of people? One electron is just there -- billions of electrons can light up a room.
Again, I haven't delved deep into this, but I think what we casually perceive as reality -- the classic Newtonian universe with solid things moving around and falling onto people's heads -- is the average of all these little quantum things going on down at Planck scale. All these entanglements and tunnels and spontaneous particle creations average out to give us something that looks like an electron or a proton, and put enough of those together and you've got a gaseous cloud that behaves differently than the individual quantum elements.
Anyway, just my US$0.02. Your mileage may vary, void where prohibited by law, terms and conditions apply, close cover before striking, and I may be completely wrong.
the things which happen on quantum level can affect the larger outcomes - a photon is emitted or it isn't and we can choose actions based on that outcome. In a lot of situations, large numbers of quantum events contribute to the outcome - and I believe they refer to that as "quantum degeneracy", basically it does not matter which quantum event(s) contribute; the outcome will be the same. So a rock behaves in ways that belie its quantum composition. But we do not know where the dividing line is; where quantum degeneracy ceases. It might happen in the nerve system that single quantum events cause bifurcations in outcome chances.