Light can change speed, even in a vacuum, a new paper reports. The discovery could change the way we think about one of the constants of the universe.

The importance of the speed of light to physics can hardly be overstated. The number 2.997 x 108 m/s governs our lives, even if we seldom notice it. It forms the speed limit of the universe; the c in the famous equation e=mc2, and also defines the way we measure distance. High school physics teaches us that this speed is not quite universal – when travelling through water or glass, light slows down; an effect put to good use in lenses and prisms.

For this reason, c is correctly referred to as “the speed of light in a vacuum.” However, in a paper on arXiv, Miles Padgett from the University of Glasgow has shown that even this needs a rethink. He manipulated the wave structure of some photons and sent them on a path of the same length as unaltered packets of light. The manipulated photons arrived later, indicating they were travelling more slowly.

The manipulation occurred by twisting a plane wave (one where the wave front is a parallel plane at right angles to the direction of travel) into a conical wave front, which is analogous to focusing a wave from a spread-out source onto a single point.

Credit: Giovannini et al. A Bessel beam is created when a conical wavefront is created for a single photon with angle α.

The slowing occurs at a rate of about one part in a hundred thousand. So in the time it takes unmodified light to travel a meter, the adjusted light makes it 0.01 millimeters less. With some understatement, Padgett and his co-authors note, “Measuring the arrival time of single photons with femtosecond precision is challenging.” The team achieved this by producing strongly correlated photon pairs and having them meet at the destination so that tiny variations in their arrival times would be revealed as phase differences.

Credit: Giovannini et al. Plane wave photons were seperated from those converted to conical wavefronts and then brought together so their speeds could be compared.

It's not the first time that the speed of light in a vacuum has been shown to be flexible. The velocity of light can be reduced within a hollow waveguide, and discrepancies in the time of arrival of light and neutrinos have led to suggestions of unrecognized effects from gravity, albeit on a scale many times smaller than Padgett proposes.

Professor Robert Boyd of the University of Rochester, New York, told Science News, “I’m not surprised the effect exists. But it’s surprising that the effect is so large and robust.”

“Our work highlights that, even in free space, the invariance of the speed of light only applies to plane waves,” the authors conclude. It is unclear whether the work has any practical applications. There are no indications to suggest that since light speed can be lowered, it can also be raised or breached for interstellar travel.