I continue to struggle with basic concepts. On the one hand, I have been watching some of Leonard Susskind's lectures on QM, in which he essentially equates measurement with entanglement between an isolated system and another system. This is confusing to me. Measurement in the QM sense seems to involve the acquisition of a particular value for an observable at a particular time. Entanglement, on the other hand, does not seem to occur at a particular time in the experiment, but is merely introduced as part of the framework for calculating the time evolution of the system, given the experimental apparatus. Perhaps there is some ultimate equivalence but it is beyond me at the moment
The issue of time seems to be at the heart of varying interpretations of QM. Whether a system is in a particular state at a particular time, whether it is in multiple states, or whether it exists merely as potentialities until it interacts with some other system. What exactly causes it to exhibit certain behavior, and when exactly these causal relationships come into play in the course of an experiment. These are very unresolved questions in my mind, and perhaps they can't be resolved under current paradigms.