I started writing a number of responses to many posts on this thread and these became far too numerous and far too lengthy. I still intend to post these responses, but I am including a small number of references I have scanned or linked to that are worth reading in general but which relate particularly to issues in this thread and still more particularly contain many points I would like to be able to refer to and expand on/explain rather and let readers choose whether or not they feel like they require a deeper basis or wider context to understand the points I will be touching on. The chapter on the failure of classical determinism is from a must-read on the subject of determinism in general by Earman, and I am including one volume simply because it is freely available (and relevant) and references much I may refer to (both topics and texts). I decided to keep to only a few choices and thus had to sadly exclude some papers/chapters I would really have liked to, but oh well. So see the link below for the free ebook and see attached:
Physical (A)Causality Determinism, Randomness and Uncaused Events
I didn't read all of them, but the one on determinism in classical physics seems to be mostly a straw man argument. The definition of determinism in that paper isn't one that would have used, nor is it the most common one in physics. The singularities described are ALL deterministic effects by the typical definition of determinism. Now, the examples *do* show some of the problems inherent in the specific classical laws described. In particular, situations where particles go off to infinity in a finite time are inherently problematic. But they are still quite deterministic. And, they cannot be 'run backwards' and get valid solutions, as claimed.
The lack of determinism in quantum thoery is of a much different character than these examples. For one, QM is not a realist theory, nor is it causal. Both are relevant to determinism.
First, the lack of causality is pretty directly related to non-determinism: that several different results are possible from a given initial condition is *precisely* the lack of determinism. So, even though the evolution of the wave function is deterministic, the actual physical results are only probabilistic once the wave function is known. And the wave function is taken to be *all* that is knowable about a physical situation.
Related to this is the lack of realism. In QM, things do NOT have definite properties at all times. In fact, any non-stationary state is going to be a superposition of more than one state with definite energy. That means, already, that the energy of such a state is not definite. But the same can be said for almost all observables: most real states are superpositions of eigenstates and so do not have definite values for the observable.