WHAT IS THE TIME DERIVATIVE OF A QUANTUM OBSERVABLE
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A reasonable-sounding prescription for measuring dA dt is to measure A itself at times t and t + t and to calculate [A(t + t) - A(t)] t {reversed tilde equals} dA dt, with t sufficiently small. However, this interpretation is shown to be untenable if dA dt is defined by the Heisenberg equation of motion for A. Thus a phenomenon noted by Ruijsenaars for the position and velocity observables of a relativistic particle is actually a general feature of quantum theory. 1985.