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A team of Scottish scientists has made light travel slower than the speed of light.
They sent photons - individual particles of light - through a special mask. It changed the photons' shape - and slowed them to less than light speed.
The photons remained travelling at the lower speed even when they returned to free space.
The experiment is likely to alter how science looks at light.
The collaborators - from Glasgow and Heriot-Watt universities - are members of the Scottish Universities Physics Alliance. They have published their results in the journal Science Express.
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.
But hang on a minute. If a photon is a particle, how is it possible to impose a pattern on it?
It's because photons exist in the exotic and rather wonderful quantum realm, where the rules of the reassuringly solid world in which we live tend to lose their grip.
They exhibit what physicists call "wave-particle duality": they behave like both a wave and a particle. So you can send them round a racetrack two by two like particles, yet change the shape of one of them as if it was a wave.
There are some practical implications. Light is used to make extremely precise measurements such as how far the Moon is from Earth.
The good news is that we are not in for any nasty surprises on that scale. But researchers using large aperture lenses to accurately measure very short distances may be forced to take a second look for tardy photons.
Beyond that, Dr Giovannini says practical, everyday uses for the discovery are possible. Although he concedes the physics is more fundamental than applied right now.