Werner Heisenberg arrived at quantum mechanics by a quite different route. For him, the essence of the theory was in the fact that the process of measurement of one dynamical variable of a system (the position of a particle, for instance) caused a disturbance of the system that rendered the determination of a complementary variable (in this case momentum) uncertain. Heisenberg thus started with the concept of a point particle and considered the resulting “uncertainty” to be a consequence of the measurement process. In this way, he rejected the classical goal of describing the behavior of physical systems and made the theory into one in which the central goal was to determine the results of experiments. It was through this approach that the human observer came to be a central actor in the theory. Whereas Schrödinger’s waves suggested a physical reality, Heisenberg’s theory was concerned not with the physical world itself but with how we came to know it.
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