Contradiction of wave-particle duality?

I can't read the whole article, so it's hard to be sure with these things, but I for one would be sceptical. Such an experiment would fly in the face of hundreds of other experiments which make suggestions to the contrary. Also, if it was that big a breakthrough, why take so long to publish?

17th Feb 2007:

IT ROCKED quantum theory when it was first proposed in 2004, unleashing a dam-burst of vitriol in the physics community. Now the controversial experiment that questions our understanding of the wave-and-particle nature of light has finally been published, forcing some of its initial opponents to take it seriously.

The experiment, carried out by physicist Shahriar Afshar - then at the private, Boston-based Institute for Radiation-Induced Mass Studies - challenges a principle proposed by Niels Bohr 80 years ago. It relates to subatomic entities such as electrons and photons that seem to behave as if they were particles in some experiments and as waves in others.

According to Bohr, these wave-like and particle-like properties are complementary - no experiment can reveal both at once. Until Afshar's results, no experiment had seemed to contradict this principle.

When Afshar went public with his first findings in New Scientist (24 July 2004, p 30), he raised a storm of criticism. "I even had my religion and ethnicity attacked," says Afshar, an Iranian-American, now at Rowan University in Glassboro, New Jersey.

“I had my religion and ethnicity attacked. I felt like giving up because of the emotional intensity of the opposition”"This was especially unfortunate," says Antony Valentini of the Perimeter Institute for Theoretical Physics near Toronto, Canada. "One can understand that different people interpret the experiment differently, but that he should have had such trouble is peculiar."

Afshar believes the simplicity of his experiment - which could be done for $20 in a high-school lab - made people feel it could not be right. "Nevertheless, it's hard to understand how anyone could not see that this was an intriguing experiment," says Valentini.

"There were a few times when I felt like giving up because of the emotional intensity of the opposition," says Afshar. However, he persevered and has now refined his experiment so that he can measure the path of photons - the particles of light - while also observing their wave-like interference pattern.

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.

The revised experiment is being published in the journal Foundations of Physics (vol 37, p 295), which is currently edited by Nobel laureate physicist Gerard't Hooft, who also believes that there are fundamental problems with quantum theory (New Scientist, 6 May 2006, p 8). Afshar himself now hopes for a more cool-headed discussion of his work. "I do think he was treated unfairly," says Lucien Hardy, also at the Perimeter Institute, who was initially sceptical about the experiment, but has now been won round.

Neil Gunther, a physicist at Performance Dynamics in Castro Valley, near San Francisco, interprets Afshar's findings in a way that does not violate Bohr, and is now designing an experiment to test that theory. "Afshar may have inadvertently put his finger on some new physics that could have important implications for quantum imaging," he says.

From issue 2591 of New Scientist magazine, 17 February 2007, page 13