Operators and the Second Postulate
M.S. Rogalski, Stuart B. Palmer · 2020
Physical observables which are measured in a particular state of a dynamical system can be conveniently represented in classical mechanics by mathematical variables, on the grounds that all have definite values in any classical state. In contrast, the statistical interpretation of wave functions describing the quantum states implies that there are uncertainties in the measurement of any two observables which can be represented by canonically conjugate variables, such as position q and momentum p q . This requires any measured dynamical observable A in a quantum state to be associated with an expectation value 〈 A 〉.