19:00 22 July 2010 by David Shiga
It is one of the all-time greatest physics experiments: such a classic that it's taken a century to go one better.
In the double-slit experiment of 1908, a photon fired at a pair of slits passed through both simultaneously and interfered with itself. This surprising effect provided one of the first clues to the weird world of quantum mechanics.
Now precise measurements have been made on a version with three slits - and they again confirm the predictions of quantum mechanics.
Why are we still testing such predictions? It is not just tilting at windmills: physicists have long struggled to unite quantum mechanics with general relativity, which describes gravity, and some believe quantum mechanics will need tweaking to make this work.
Those tweaks, some physicists have argued, might include altering a quantum dictum called Born's rule. It predicts that interference patterns from three or more slits is equivalent to combining the effects of several double-slit experiments.
But although it is easy to add a third slit to the double-slit experiment, it has been more challenging to do it in a way that allows the precise measurements needed to check the validity of quantum mechanics.
"The experiment is much harder than it might seem," says James Franson of the University of Maryland, Baltimore County, who was not involved in the study. "The slightest misalignment of the slits might produce errors."
Urbasi Sinha of the University of Waterloo in Ontario, Canada, and colleagues managed to do it by carving into a metal coating on a glass plate to make three equally spaced transparent slits, 30 micrometres wide and 100 micrometres apart. Then they used a laser to fire photons one at a time at the plate and recorded the pattern of light on the other side.
The result was a wavy interference pattern that matched the predictions of Born's rule to within the experiment's error margin of 1 per cent.
Franson says it is important to check for deviations from what theory predicts, even though the results were unsurprising in this case. "People tend to take things for granted, but physics is an experimental science and we should test these things," he says.
Journal reference: Science, DOI: 10.1126/science.1190545
Original Article
