A tuning fork rings a single note until the trembling of the floor touches it. A photon in empty space is like such a tuning fork: its polarization can be strictly vertical or horizontal, and that is a pure quantum state. But the space around it is not empty. It is threaded by gravitational waves that stretch and squeeze everything they meet along the way. Decoherence is exactly that moment when the trembling of the floor starts throwing the tuning fork's pitch off.

Previously, this process was calculated for an object with no direction. A photon is trickier: it has polarization, and you have to make sure the calculation does not depend on how we look at the system. Scientists got around this difficulty by choosing the gravitational field itself as the clock. The idea is simple: time is set not by an external observer but by the course of events themselves.

What came out of it is an equation that describes how a photon gradually loses its quantum purity. It contains none of the simplifications usually made to keep a calculation manageable. This means the equation also applies when the gravitational noise changes rapidly. The practical payoff will not arrive tomorrow, but it is there. If we manage to measure how the gravitational background erases the quantum purity of a single photon, that will be a test of ideas in quantum gravity. For now, the model is a reminder: even in the emptiest room, the floor is always trembling a little.