Do photons know?
Wheeler proposed his 1978 thought experiment to determine, once and for all, whether photons in the famous "double slit" experiment can in some way infer the presence of an observer before they are emitted.
The image above shows the single photon source (pumped by a green laser) that Jean-François Roch and colleagues from the École Normale Supérieure de Cachan used in their faithful version of Wheeler's experiment. (Credit: Vincent Jacques.)