Carroll Alley and colleagues at the University of Maryland perform the practical delayed-choice experiment devised by John Archibald Wheeler; it is the Young slit experiment modified with a sort of Venetian blind instead of a screen, which can be opened or closed after the photons have passed through the slit; in this way Alley demonstrates what Wheeler had theorized: the photons that hit the Venetian blind (and are therefore not observed individually) form an interference pattern, while those that pass through it (and are thus observed by sensors capable of seeing them) do not.
