Many hands make light slow down – permanently

The speed of light never changes, right?

Wrong.

Scientists from two universities in Scotland have managed to slow down light as it moved through free space.

It required a bit of hocus pocus, and the slowdown was rather limited – a few millionths of a metre across a distance of one metre. However, it was accomplished and the slowdown in speed became permanent.

The University of Glasgow and Heriot-Watt University joined forces to accomplish this feat, with the team led by Prof Miles Padgett at Glasgow. Their results provide new ways to think about the properties of light, Prof Padgett said.

Many will scoff, remembering those experiments at school where one could prove how light was slowed as it passed through materials such as glass and water. There is no comparison however the researchers say.

With glass or water the light photons slow down, but then resume light speed once they exit. In the case of this new finding, once slowed the photons stay slowed, they don't speed up again. Details were published early this morning in Science Express, the early online version of journal Science.

"We have achieved this slowing effect with some subtle, but widely-known optical principles, " said co-lead author Jacquiline Romero of Glasgow's optics group.

It involved sending the single light photon through a "mask", something that causes the photon or group of photons to adopt a spatial structure. Once it adopts this enforced structure it just moves less quickly, coming in about 20 wavelengths of light – about 11 millionths of a metre on average – behind an unencumbered photon.

Their approach worked whether the two racing photons were zipping through air or through a vacuum. It also works if you use a beam of light and the researchers belief this method will work for other kinds of waves.

“We expect that the effect will be applicable to any wave theory, so a similar slowing could well be created in sound waves, for example,” Prof Padgett said.