Cosmos Community: Blog

This concerns physics, not astronomy, but since it challenges a foundational principle of quantum mechanics, I thought it well worth a blog. Several years ago a physicist with, so far as I know, no prior published research and working on his own, Shahriar Afshar, apparently found a way to simultaneously measure both the wave and particle properties of light using an almost trivially simple apparatus to carry out a novel version of the classical double-slit experiment. As I recall this experiment -- which it was said could be done for $20 in a high-school lab -- was "published" in a popular article in New Scientist in 2004. That's a great magazine, which I myself subscribe to, but it is not a research journal. A storm of criticism, some of it very vitriolic, ensued. Now at last a proper scientific paper has been published in Foundations of Physics, a journal whose prestige has risen substantially with the new editor-in-chief, Nobel laureate and general relativity guru Gerard 't Hooft. So has Afshar succeeded in demolishing a foundation of quantum physics? Comments are welcome.

Following is a summary extracted from a new article in New Scientist (17 February 2007) by Marcus Chown.

His set-up is based on the classic "double-slit" experiment that first showed the wave-like properties of light. In that experiment, light is shone onto a screen with two pinholes in it. The light that passes through the pinholes fans out from them like ripples from a stone thrown into a pond and produces an interference pattern of light and dark fringes where the spreading waves either reinforce or cancel each other.

Afshar's apparatus is similar, but with a lens on the far side of the pinhole screen (see Diagram). The lens refocuses the spreading beams onto two mirrors, which reflect them onto two photon detectors, allowing Afshar to work out the path taken by the photons. According to Bohr's complementarity principle, that means there should be no evidence of an interference pattern, as Afshar is observing the light as particles, not waves.

However, Afshar observes the interference indirectly, by placing wires in front of the lens at the positions where he would expect the dark, lightless fringes of an interference pattern to be. If the photons do not interfere, Afshar argues, there will be no dark fringes and the wires will block some of the photons hitting the lens, reducing the photon count at the detectors. No such dip in the signal is seen - implying that the light does form an interference pattern, violating the complementarity principle.