Quantum theory says that in Young’s experiment every photon goes through both slits at once in a superposition. While solving a normal equation gives one solution that satisfies its conditions, solving the quantum wave equation gives a set of solutions, each a physical event state with an associated probability. These orthogonal solutions evolve over time but at each moment only one of them can actually occur. The mathematics has the unusual feature that if any two states are solutions so is their linear combination. While single states match familiar physical events, these combination states never physically occur, yet they underlie the mysterious efficacy of quantum theory. It is in just such a combination that one photon goes through both Young’s slits at once.
Not only photons can superpose, e.g. ammonia molecules have a pyramid shape (Figure 3.20) with a nitrogen atom apex (1) and a base of hydrogen atoms (2, 3, 4) that can manifest in either right or left-handed forms. To turn a right-handed molecule into a left-handed one, a nitrogen atom must pass through the pyramid base which is physically impossible (Feynman et al., 1977) III, p9-1 but in quantum theory, if each state is valid then so are both at once. So an ammonia molecule can be left-handed one moment and right handed the next, even though it can’t physically change between these states. To call superposition ignorance of a hidden physical state is to misunderstand it, as superposed quantum currents can flow both ways round a superconducting ring at once even though physical currents would cancel (Cho, 2000).
In quantum realism, a superposition is when the processing of a quantum entity simultaneously distributes to two or more outcomes. So when a photon wave spreads through two slits in Young’s experiment, it literally half-exists in both. When the photon is later observed in a physical event, that is the photon restarting from a specific instance. Superposition is physically impossible but that quantum processing tries every option is just is business as usual in the quantum world.