How is orthodoxy possible?

How can an interpretation maintain both the eigenstate–eigenvalue link and indeterminism? Given the former, the properties possessed by a system are completely fixed by its quantum-mechanical state, but the quantum-mechanical state evolves deterministically, as I noted at the end of chapter 1. By themselves, then, the eigenstate–eigenvalue link and the quantum-mechanical equation of motion lead to determinism. Orthodoxy must change one of these things if it wants to maintain indeterminism.

Of course, it cannot change the eigenstate–eigenvalue link, lest it no longer be orthdoxy. Hence it changes the equation of motion. In this chapter, I will discuss two ways to change the quantum-mechanical equation of motion: by ‘interupting’ it from time to time with some other (indeterminsitic) equation, or by making a wholesale replacement. The first strategy I discuss in the next section, and the second in the subsequent section.

The projection postulate

Collapse as an analogue of Lüder's rule

Thus far, we have been working in the ‘Schrödinger picture’, according to which states evolve in time (according to the Scrödinger equation) and any given observable is at all times represented by the same operator. The Heisenberg picture reverses things: the states are constant in time and the operators representing observables change.