A team of Norwegian physicists has explored a deceptively simple thought experiment: what happens to a single photon if it is only partway through reflecting off a perfect mirror when that mirror is suddenly pulled away? Because a photon is not a point-like particle but an extended wave-like object, the answer turns out to be considerably more complicated than the intuitive guess, and it sheds light on the subtle, nonlinear physics that governs how light behaves when its surroundings change abruptly.

The article explains that photons do not normally divide or combine, since everyday interactions between light and matter are linear, but nonlinear effects can occur under special conditions, typically requiring a highly responsive medium or intense laser light. Removing a mirror mid-reflection seems unrelated to this kind of nonlinearity, yet the researchers show it is in fact a nonlinear event. Drawing an analogy with a partially reflective mirror, where a single photon enters a superposition of being reflected and transmitted until measured, the physicists demonstrate that the naive expectation — that a photon caught mid-reflection would simply collapse into one full path or the other — does not hold, with the explanation resting on the relationship between time and frequency in describing light signals.