No doubt about it, quantum mechanics is hard. Relativistic mechanics is unfamiliar and counterintuitive, but it is no harder than classical mechanics: In classical mechanics, the state of a single particle moving in a single dimension is specified through two real numbers—a position and a momentum. In special relativity, that particle’s state is again specified by two real numbers—a position and a momentum. Position and momentum behave in a less familiar way, but the state is still specified through two real numbers. But in quantum mechanics, the state of a single particle moving in a single dimension is specified through a complex-valued function—an infinite number of numbers.
This more-intricate specification reflects the fact that quantum mechanics is richer than classical or relativistic mechanics. Quantal systems can interfere and entangle—options that are simply not available to classical systems. But keeping track of this richness is, as mentioned, hard. It gives rise...
