From David Dunstan, Queen Mary, University of London
Shariar Afshar has certainly built an ingenious set-up (24 July, p 30). However, if his double-slit experiment performed as claimed, it would refute classical optics rather than standard quantum mechanics. His grid of fine wires constitutes a diffraction grating, which splits an incident beam into multiple beams at angular separations equal to the angular separation of his pinholes. The image formed of each pinhole is not a single spot, but a row of spots. When both pinholes are open, so that the interference pattern exists, the spots imaged from each pinhole are superposed. A photon detected at one of these spots may therefore have come through either pinhole or, as quantum mechanics demands, through both pinholes.
There is no “which-way” information, and Bohr’s ideas do not need to be thrown on the scrap-heap just yet. Afshar has not succeeded where Einstein failed.
London, UK
