Scientists have long known that the speed of light can be slowed slightly as it travels through materials such as water or glass.
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.
But now in new research, scientists have managed to slow photons in free space for the first time - suggesting the speed of light is even more complex than we imagined.
The research was outlined in a paper published in Science Express today.
Researchers from the University of Glasgow and Heriot-Watt University described how they have managed to slow photons in free space.
They demonstrated that applying a 'mask' to an optical beam, which changed the shape of photons, can reduce their speed.
The team compare a beam of light, containing many photons, to a team of cyclists who share the work by taking it in turns to cycle at the front.
Although the group travels along the road as a unit, the speed of individual cyclists can vary as they swap position.
The group formation can make it difficult to define a single velocity for all cyclists, and the same applies to light.
A single pulse of light contains many photons, and scientists know that light pulses are characterised by a number of different velocities.
The speed of light is regarded as an absolute. It is 186,282 miles per second in free space.
Light propagates more slowly when passing through materials like water or glass but goes back to its higher velocity as soon as it returns to free space again.
Or at least it did until now.
Two and a half years ago, the experimenters set out to see if they could slow down light just a little - and keep it moving more slowly