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ANYONE struggling with a New Year鈥檚 fitness regime knows that you move more slowly when you鈥檙e out of shape. Now it seems the same is true for light, which until now was thought to travel at a constant speed.
It鈥檚 well known that light travels at different speeds as it passes through different materials; it travels more slowly in water than air, for example. But the speed of light in a vacuum, at a little under 300 million metres per second, is an unwavering constant that underpins much of modern physics, including Einstein鈥檚 theory of special relativity. It鈥檚 so important that physicists give it a special letter: c.
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Now, Miles Padgett at the University of Glasgow, UK, and his colleagues have shown this isn鈥檛 quite right. Light travelling in a plane wave 鈥� the traditional up-and-down squiggle you learn about in school 鈥� always travels at c, but light with a more complex wave structure travels slightly slower, by about one-thousandth of a per cent.
The team revealed this oddity by studying two kinds of shaped light: a Bessel beam, which looks like concentric rings of light, and a Gaussian beam, which spreads out as it travels. They used an ultraviolet laser to produce pairs of photons and passed one photon through a filter to shape it into either a Bessel or Gaussian beam. Both travelled 1 metre before hitting a detector, so they should have arrived at the same time, but the shaped photon was a bit delayed (Science, ).
Why does this happen? One way of thinking about it is that some of the light in a structured beam is moving in the 鈥渨rong鈥� direction 鈥� sideways rather than forwards. This isn鈥檛 a strictly accurate picture of the energy distribution within the beam, warns Padgett, but it is a way to imagine what might be going on.
鈥淧ersonally I think that鈥檚 a useful concept, though the scientific rigour police might not welcome it,鈥� he says.
Don鈥檛 rip up your physics textbook just yet, though 鈥� the implications are likely to be minor, for now at least, only affecting things like short-range experiments that rely on very precise time-of-flight measurements. 鈥淲e鈥檙e not challenging Einstein,鈥� says Padgett.
Hints of this effect have been seen in other experiments, but no one had quite pinned it down before, says Ulf Leonhardt at the Weizmann Institute of Science in Rehovot, Israel.
鈥淸This] is really the first clean and clear experiment where the speed of photons in structured light beams is directly measured,鈥� Leonhardt says. Now that physicists understand it, they might be able to exploit it. 鈥淚 do not foresee immediate applications in the short run, but important fundamental physics always has implications and applications in the long run.鈥�
This article appeared in print under the headline 鈥淪hapely light likes to take it slow鈥�