From the Aug. 18, 2007 issue of New Scientist magazine

Light seems to defy its own speed limit

IT'S a speed record that is supposed to be impossible to break. Yet two physicists are now claiming they have propelled photons faster than the speed of light. This would be in direct violation of a key tenet of Einstein's special theory of relativity that states that nothing, under any circumstance, can exceed the speed of light.

Günter Nimtz and Alfons Stahlhofen of the University of Koblenz, Germany, have been exploring a phenomenon in quantum optics called photon tunnelling, which occurs when a particle slips across an apparently uncrossable barrier. The pair say they have now tunnelled photons "instantaneously" across a barrier of various sizes, from a few millimetres up to a metre. Their conclusion is that the photons traverse the barrier much faster than the speed of light.

To see how far they could make photons tunnel, Nimtz and Stahlhofen sandwiched two glass prisms together to make a cube 40 centimetres on its sides. Since photons tunnel most readily over distances comparable with their wavelength, the physicists used microwaves with a wavelength of 33 cm -- long enough for large tunnelling distances yet still short enough that the photons' paths can be bent by the prism.

As expected, the microwaves shone straight through the cube, and when the prisms were separated, the first prism reflected the microwaves (see Diagram). However, in accordance with theory, a few microwave photons also tunnelled across the gap separating the two prisms, continuing as if the prisms were still sandwiched together.

Nimtz and Stahlhofen found that the reflected microwaves and the few microwaves that tunnelled through to the second prism both arrived at their respective photodetectors at the same time. This suggests an ultra-fast transit between the two prisms -- so much faster than the speed of light that the experimenters couldn't measure it. Moreover, the pair found that the tunnelling time, if any, did not change as they pulled the prisms further apart. Because tunnelling efficiency also drops off with distance, however, Nimtz says that they could not observe the effect across distances greater than 1 metre (

"For the time being," he says, "this is the only violation [of special relativity] that I know of."

How can this be explained? The Heisenberg uncertainty principle dictates that a particle's energy and the time it spends in any one place cannot both be known with absolute precision. This means particles can sometimes sneak over a barrier if the time they spend traversing that barrier is short enough. Bizarre as it may seem, quantum tunnelling is not only a commonplace phenomenon in the quantum world, it also lies at the core of many processes we take for granted.

"In my opinion, tunnelling is the most important physical process, because we have it in radioactivity and we have it in nuclear fusion," Nimtz says. "The temperature of the sun is not high enough to organise regular fusion of protons into helium [without tunnelling]. Some people are saying that the big bang happened because of tunnelling. Recently, many people have argued that processes in biology and in our brain are based on tunnelling."

Aephraim Steinberg, a quantum optics expert at the University of Toronto, Canada, doesn't dispute Nimtz and Stahlhofen's results. However, Einstein can rest easy, he says. The photons don't violate relativity: it's just a question of interpretation.

Steinberg explains Nimtz and Stahlhofen's observations by way of analogy with a 20-car bullet train departing Chicago for New York. The stopwatch starts when the centre of the train leaves the station, but the train leaves cars behind at each stop. So when the train arrives in New York, now comprising only two cars, its centre has moved ahead, although the train itself hasn't exceeded its reported speed.

"If you're standing at the two stations, looking at your watch, it seems to you these people have broken the speed limit," Steinberg says. "They've got there faster than they should have, but it just happens that the only ones you see arrive are in the front car. So they had that head start, but they were never travelling especially fast."

DIAGRAM: FASTER THAN THE SPEED OF LIGHT? Photons "tunnel" across a gap between two prisms yet arrive at same time as reflected photons that travelled a shorter distance


By Mark Anderson


photo by NASA Goddard Space Flight Center