What if one of the most deep-seated ideas in science was wrong? What if an edict propounded by one of the towering figures of 20th- century physics and unchallenged for nearly 80 years turned out to be incorrect? And what if this was no mere conjecture by some crackpot, but the unequivocal ruling of a reproducible experiment?
Enter Shahriar S Afshar, a 33-year-old Iranian-American physicist. Afshar has tested the foundations of quantum theory, our most successful description of the reality that underpins the everyday world. According to quantum theory, the basic building blocks of our world such as atoms can behave both as particles - like tiny bullets - and waves - like ripples on a pond. This bizarre behaviour turns out to be at the root of a multitude of quantum phenomena which have made the modern world possible, given us everything from computers and lasers to nuclear reactors.
But, though atoms and their like can behave both as particles and waves, it has long been believed that nature never permits us to observe both aspects at the same time. "I have shown this is wrong," says Afshar. "And I've done it with an experiment so simple that a version could have been done any time in the past 200 years." Afshar has carried out a version of the double-slit experiment, widely believed to embody the central mystery of quantum theory. In the experiment, quantum particles - commonly particles of light, or photons - are shone on an opaque screen with two closely spaced vertical slits in it. The photons go through the slits and, on a second screen beyond the first, create a pattern of alternating light and dark bands much like a supermarket bar code. The bright bands are where a lot of photons end up and the dark bands where there are none at all.
What is so peculiar about this pattern is it is exactly what would be expected if waves were spreading out from each slit and overlapping with each other. Where the crests of the two waves coincided and reinforced each other, there would be a big wave, and where crests of one wave coincided with troughs of the other and they cancelled each other out, there would be no wave. Such an interference pattern for light was in fact observed in 1801 by Thomas Young. It was the definitive proof that light was a wave which spread through space like a ripple. But how can photons - which are tiny bullets not waves - produce an interference pattern? Things get even more mysterious when the source of photons impinging on the double slit is made so weak they arrive one at a time. If the photons are counted by detectors as they arrive at the second screen, they still reveal the gradual build-up of a barcode-like pattern. The puzzling thing is that the prerequisite of such an interference pattern is that two things mingle, or "interfere". Yet the particles emerge from the slits one at a time. Somehow each appears to go through both slits simultaneously and mingle with itself.
In the double-slit experiment, particles - whose defining characteristic is they are localised at a particular point in space - behave like waves, whose characteristic is they are smeared throughout a relatively large region of space. This is the central weirdness of the quantum world. The basic building blocks of reality behave both as particles and waves. Or, rather, they behave like something else for which we have no word and for which there is no analogue in the everyday world - an entity with both a particle- like face and a wave-like face.
Albert Einstein absolutely hated this idea and tried to scupper quantum theory. All he needed to do was prove that the ultimate building blocks of reality were not the airy-fairy wave-particles of quantum theory. And that meant finding a means of pinning them down to a localised point in space, thus showing them beyond any doubt to be particles and not waves. The scheme Einstein came up with involved bouncing high-energy light off a moveable double-slit. …