Anyone struggling with a New Year’s fitness regime knows that you move slower when you’re out of shape. Now it seems the same is true even for light, which up until now physicists had thought travelled at a constant speed.

It’s well known that light travels more slowly when it passes through different materials. This phenomenon, known as refraction, creates optical illusions like a seemingly broken drinking straw sticking out of a glass of water. But the speed of light in a vacuum, a little under 300,000,000 metres per second, is an unwavering constant that underpins much of modern physics, including Albert Einstein’s theory of relativity. It’s so important that physicists give it a single letter: c.

Now, Miles Padgett at the University of Glasgow, UK, and his colleagues have shown this isn’t quite right. Light travelling in a plane wave – the traditional up and down squiggle you learn about in school – always travels at c, but light with a more complex wave structure travels slightly slower, by about a thousandth of a per cent.

Light on time?

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 photons travelled one metre before hitting a detector, so they should have arrived at the same time, but the shaped photon was slightly delayed.


Why does this happen? One way of thinking about it is that some of the light in a structured beam is moving in the “wrong” direction – sideways rather than forwards. This isn’t 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. “Personally I think that’s a useful concept, though the scientific rigour police might not welcome it.”

Don’t rip up your physics textbook just yet though – the implications are likely to be minor, only affecting certain short-range experiments that rely on very precise time-of-flight measurements, for example. “We’re not challenging Einstein,” 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. “[This] is really the first clean and clear experiment where the speed of photons in structured light beams is directly measured,” he says. Now that physicists understand it, they might be able to exploit it. “I do not foresee immediate applications in the short run, but important fundamental physics always has implications and applications in the long run.”

Correction: The original version of this article reported the speed of light in a vacuum as 300,000 metres per second.

Journal reference: Science, DOI: 10.1126/science.aaa3035