While I very much enjoyed Steven Weinberg’s article “Einstein’s Mistakes,” I am puzzled by the author’s statement about quantum mechanics: “The difficulty is not that quantum mechanics is probabilistic—that is something we apparently have to live with. The real difficulty is that it is also deterministic, or more precisely, that it combines a probabilistic interpretation with deterministic dynamics.”
Quantum mechanics is an acausal deterministic theory in the sense that a physical system’s state (mathematically described by a state vector) at a given initial time determines its state at a specified later time, but its state is not in one-to-one correspondence with sharp values of all its dynamical variables; that correspondence is probabilistic. Therefore events, identified by sharp values of those variables at one spacetime point, are not causally connected with other events. That is something we have to live with.
Why does the combination of these two attributes—acausality and determinism—constitute a special difficulty? Weinberg asks, “So where do the probabilistic rules of the Copenhagen interpretation come from?” Why do they have to come from anywhere other than from human brains? Nature exists out there, independent of human thought, but its mathematical description surely is a human construction rather than an immutable law given to us on a stone tablet.
