The issue of measurement has been part of the discussion of quantum mechanics. In addition, this has been complicated by the argument over reality and when and where it arises in the quantum system. One of the arguments of A. Einstein, with B. Podolsky and N. Rosen was for the existence of a reality in the system before the measurement. Modern approaches to time-resolved studies require the fast lasers simply because they provide the finest time scale over which measurements can be made at the present time. There are problems with the ideas, in that such measurements might be thought to be incompatible with the standard Copenhagen–Gottingen viewpoint. While the earlier measurements were simple spin measurements, more modern approaches have concentrated on a continuous measurement in which one integrates the weak signal over time. One problematic aspect of continuous measurements is the time evolution of the system, where what is being measured may be a function of time.