Moving at the speed of light

By: Bella Dalba

It is a basic principle of science that the speed of light slows as it travels through a buffer (water or glass, for example), as this distorts the wavelengths. Until January 23, however, it has generally been thought impossible for particles of light, known as photons, to be slowed as they travel through free space, unimpeded by interactions with any materials. In a new paper published in Science Express, researchers from the University of Glasgow and Heriot-Watt University describe how they have managed to slow photons in free space by applying a mask to an optical beam, giving photons a spatial structure that can reduce their speed.

“Most people think of light as one continuous beam, but there are millions of photons moving around at an incomprehensible speed. The fact that they can isolate these particles shows you just how far technology has come,” explains Aimee Gaylord, a biology teacher at Seckman High School.

The team compares a beam of light (which contains a multiplicity of photons) to a team of cyclists, in which each person takes a turn leading the group. Although the group travels along the road as a unit, the speed of individual cyclists can vary as they swap position, making it difficult to measure velocity.

The experiment was configured like a time trial race, with two photons released simultaneously across identical distances towards a defined finish line. The researchers found that one photon reached the finish line as predicted, but the structured photon, which had been reshaped by the mask, arrived later, indicating that it traveled slower while still in free space. Over a distance of one meter, the team measured a decelerate of 20 wavelengths, which is crucially different to the slowing effect of passing light through a medium, where the light is only slowed during the time it is passing through the material. Passing the light through the mask limits the maximum speed the photon can achieve.

Daniel Giovannini, one of the lead authors of the paper, said: "The delay we've introduced to the structured beam is small, measured at several micrometers over a propagation distance of one meter, but it is significant. This finding shows unambiguously that the propagation of light can be slowed below the commonly accepted figure of 299,792,458 meters per second, even when travelling in air or vacuum.”

The effect is applicable to any wave theory, and could well be used to manipulate sound waves. These results provide a new way to think about the properties of light and explore the potential of future applications. “Think about it,” says Kelly Steinbrueck, a Physical and Earth Science teacher at Seckman High School. “This experiment could be the catalyst for future space exploration. If we can begin to manipulate sound and light, soon we could control the particles of matter. This is the first step towards actualizing warp-speed.”