Appendix - How quantum mechanics is used

Summary

Our large-scale classical view of Nature assumes we can know the current state of things that can be used as an initial condition together with the laws of motion to predict future states (Chapter 2). Newton, Gauss, and others invented procedures for finding the orbits of planets given a few observations of their actual positions in the sky. Since Schrödinger’s equation is a law of motion for the wave function, we might attempt similar applications in quantum theory. But how can we know the current wave function of a system? All we can know empirically of Nature, in the quantum view, is whether a detector clicks. This limitation leads to major differences in strategy for using quantum vs. classical mechanics. Keep in mind that the future of one individual atom out of the trillions of trillions in human-sized matter is rarely significant even when the concept of “individual atom” makes sense. By contrast, predicting the future path of a particular near-Earth asteroid may be urgently important. Quantum mechanics is useful despite its ambiguities because we require very different information in the macroscopic and microscopic regimes.