Vanish the Mirror, Create a Multi-Photon Edge
Physicists model removing a mirror while a photon's wave packet is reflecting and find that a sharp edge cannot form in a single photon; instead, a time-dependent boundary creates a multi-photon edge via Bogoliubov transformations, pulling photons from the vacuum. Instantaneous removal would diverge the photon number, while gradual changes yield only a small finite amount.