Warning: This “free will” post is for those interested in quantum mechanics, and who have a general understanding of the field and terms used within it.
If there was one theorem that has driven physicists to accept an indeterministic model of quantum mechanics the most, Bell’s theorem would be on the top pedestal. With the acceptance of such a theorem, certain quantum events simply cannot have a local “hidden variable”. This means that if one is to suggest a cause that we cannot “observe” for the event (a “hidden” variable), the cause has to be “non-local”. This idea of a non-local cause means that there is instantaneous action at a distance, something Einstein labeled as “spooky action at a distance”. And though quantum entanglement has been “demonstrated” (but with the loophole I’ll be discussing below), many physicists prefer different quantum interpretations that do not rely on non-local hidden variables, and tend to lean toward indeterministic models which says that there actually is nothing that determines the event. A less common leaning is toward a deterministic model that postulates an almost infinity of invisible worlds. The least common, though still accepted by many, are non-local hidden variable “deterministic” models such as pilot-wave theory (Bohmian Mechanics) – a model I have great appreciation for.