Spontaneous emission occurs when an excited atom gives up its energy by releasing a photon. However, if the excited atom is placed near a mirror, the spontaneous emission can be inhibited, because the photon’s wavelength does not allow the electric field to be zero at the reflecting boundary.

This raises the question, "if the atom does not emit the photon, how does it know there is a mirror at all?" Suppose the mirror were replaced instantaneously by a photodetector. The question is, how long would the atom have to wait before it could emit the photon, and how soon could this photon be detected? I will show the results of an experiment that attempts to answer this question.