Quantum theory lets one photon go through both Young’s slits at once in a superposition. Solving a normal equation gives one solution that satisfies its conditions, but solving a quantum equation gives a set of solutions, each a physical event with a known probability. These orthogonal solutions evolve over time, as the wave spreads, but at each moment there can be only one physical event. Quantum mathematics has the strange feature that for any two solutions, their linear combination is also a solution but while single solutions match familiar physical events, these superposed solutions never physically occur (Note 1). For example, in Young’s experiment, one photon goes through both slits at the same time in a superposed state, but we never observe the photon in both slits at once. That quantum solutions can superpose underlies the mysterious efficacy of quantum theory.
molecule states
Superposition applies not only to photons but also to molecules. For example, 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 molecule can occur 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 is physically impossible (Feynman et al., 1977) III, p9-1. Yet in quantum theory, if these 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. In the quantum world, ammonia molecules exist in a superposition of left and right-handed states, so we can see either at any moment, just as a photon go through either of two slits.
To think that superposition is just 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). As the interference pattern of Young’s experiment shows, the superposed photon really does go through both slits at once. Superposition is physically impossible but is just business as usual in the quantum world.
In this model, superposition is quantum processing simultaneously exploring possible options regardless of their physical compatibility, so when a photon spreads through two slits in Young’s experiment, it literally half-exists in both. Why then don’t quantum combinations occur physically?
This isn’t possible because a physical event is a processing restart. When our computers restart, only one event can cause it. Any event after that doesn’t affect the restart, and any event before would replace it. Restarting a process stops anything else it is doing, and the same is true for a quantum process. It follows that while a photon, or an ammonia molecule, can be in two quantum states at once, it can only restart from one of them. Superposition then never occurs physically because a physical event is a processing restart. Even so, we struggle to imagine how, as Schrödinger’s example of a cat that is both alive and dead shows in the next module.
Note 1. If Y1 and Y2 are state solutions of Schrödinger’s equation then (Y1 + Y2) is also a valid solution.