According to quantum theory, every photon goes through both Young’s 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 of which is a physical event with a known probability. These orthogonal solutions evolve over time as the wave spreads but at each moment only one can occur as a physical event. The mathematics has the strange feature that given any two solutions, their linear combination is also a solution, but while single solutions match familiar physical events these combination solutions never physically occur (Note 1). It is in just such a combination that one photon goes through both Young’s slits at once. That quantum solutions can superpose underlies the mysterious efficacy of quantum theory.
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). This structure can manifest in either right or left-handed forms but to turn a right-handed molecule into a left-handed one, a nitrogen atom must pass through the pyramid base, which isn’t physically possible (Feynman et al., 1977) III, p9-1. Yet in quantum theory, if two solutions are valid then so are both at once. This explains how 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, superposition is quantum processing simultaneously spreading to two or more outcomes regardless of their physical compatibility, so when a photon wave spreads through two slits in Young’s experiment, it literally half-exists in both. If the photon is later observed in a physical event, that photon restart is based on a specific instance. Superposition is physically impossible but is just business as usual in the quantum world.
Note 1. If Y1 and Y2 are state solutions of Schrödinger’s equation then (Y1 + Y2) is also a valid solution.