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The speed of light travelling through free space has been slowed down for the first time, breaking what was thought to be a constant physical measurement.

A team of physicists at the University of Glasgow sent photons through a mask to change their shape and then raced an altered photon against an unaltered one. Over a distance of one metre the team observed that the altered photon was slowed by up to 20 wavelengths, demonstrating for the first time that light can be slowed in free space. "The results give us a new way to think about the properties of light," said professor Miles Padgett from the University of Glasgow's optics group. The research was carried out in conjunction with Heriot-Watt University with the findings being published in the journal Science Express.


The speed of light in free space is 186,282 miles per second, which until now was thought to be a constant. While light slows down when passing through water or glass it returns to the speed of light when it comes out the other side. In this experiment the speed of a photon remains slightly slower.

As light behaves both like a wave and a particle it is possible to change the shape of an individual photon as if it were a wave and then race the two particles of light.

To explain how the experiment worked the researchers compared the behaviour of a beam of light to that of a team of cyclists. While the team of cyclists travels at a constant speed, the individual cyclists will all be travelling at their own speeds as they swap positions.


The same applies to a beam of light, which has a number of different velocities. In this analogy the group of cyclists is the beam of light travelling at the speed of light, but each cyclist is an individual photon with its own speed. "The delay we've introduced to the structured beam is small, measured at several micrometres over a propagation distance of one metre, but it is significant," said Daniel Giovannini, one of the lead authors of the paper.

Importantly the experiment is only applicable over short distances. The researchers explained that the effect was biggest when the lenses used to create the beam of light were large and the distance travelled was small. This means our fundamental understanding of how the universe works remains unaltered, but our understanding of light has changed.

An earlier version of this article incorrectly defined free space as air.