IT鈥橲 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鈥檚 special theory of relativity that states that nothing, under any circumstance, can exceed the speed of light.
G眉nter 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 鈥渋nstantaneously鈥� 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 millimetres 鈥� long enough for large tunnelling distances yet still short enough that the photons鈥� paths can be bent by the prism.
Advertisement
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 鈥� so much faster than the speed of light that the experimenters couldn鈥檛 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 ().
鈥淔or the time being,鈥� he says, 鈥渢his is the only violation [of special relativity] that I know of.鈥�
How can this be explained? The Heisenberg uncertainty principle dictates that a particle鈥檚 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.
鈥淚n my opinion, tunnelling is the most important physical process, because we have it in radioactivity and we have it in nuclear fusion,鈥� Nimtz says. 鈥淭he 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.鈥�
Aephraim Steinberg, a quantum optics expert at the University of Toronto, Canada, doesn鈥檛 dispute Nimtz and Stahlhofen鈥檚 results. However, Einstein can rest easy, he says. The photons don鈥檛 violate relativity: it鈥檚 just a question of interpretation.
Steinberg explains Nimtz and Stahlhofen鈥檚 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鈥檛 exceeded its reported speed.
鈥淚f you鈥檙e standing at the two stations, looking at your watch, it seems to you these people have broken the speed limit,鈥� Steinberg says. 鈥淭hey鈥檝e 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.鈥�
