Schrödinger’s approach to the equations of quantum theory is not unique. Heisenberg worked out an alternative formulation. We already know how to calculate expectation values in principle, even if we don’t have the mathematical skill to do it in practice. If a system is changing and developing over time, then it makes sense to suppose that the expectation values of its physical variables will also change over time. en, the expression ∇t〈O〉 will tell us the rate at which this change is happening (look back at the de nition of ∇t in Chapter 12).