Can slow down the speed of light

Strolling photons : Slow light

Light flies at the speed of light. This is a central law of Einstein's theory of relativity. Nothing can move faster than the speed of light in a vacuum. But can light particles also move more slowly in a vacuum than at the speed of light? Physicists at the University of Glasgow have sent individual photons, as light particles are called, onto the race track. As they report in the journal “Science”, photons with an embossed structure on the side actually arrived a tiny bit later than others.

Slowed down light

To slow down light, it is usually sent into a dense medium such as glass or water. There, light is a third to half slower than the speed of light of almost 300,000 kilometers per second. Photons are not just point-like particles, they also have wave properties. That is why you can give them different spatial structures with the help of special optics.

To measure the speed of such photons, the scientists compared them with unchanged light particles. To do this, they first sent a laser beam into a special crystal. There, the photons of the laser beam were transformed into two identical light particles each with half the energy.

A millionth of a meter ahead

The researchers let one of these photons travel over a distance of one meter. In between, they imprinted a lateral structure on the other using a special optic. As the measurements showed, the modified photons arrived with a tiny delay. "This delay is small, only a few millionths of a meter over a distance of one meter," says Daniel Giovannini, one of the Glasgow researchers. But this is the first time that it has been proven that individual light particles can be slowed down - even if only minimally. Until now, this effect was only known from light rays composed of many photons. The theorist Stephen Barnett has calculated that the effect only occurs at short distances and large optics. Due to their structure, the light particles run straight ahead, but at the same time have a lateral component that slows them down.

"The effect probably doesn't play a role at great distances, like in astronomy," says Miles Padgett, head of the Glasgow working group. But even if there are no signs of this basic research being used for such light rays, the effect should also occur with other waves, such as sound waves.

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