The speed of light is a limit, not a constant -- that's what researchers in Glasgow, Scotland, say. A group of them just proved that light can be slowed down, permanently.

Scientists already knew light could be slowed temporarily. Photons change speeds as they pass through glass or water, but when they exit the other side and return to a vacuum (like outer space) they speed back up.

In a new experiment at the University of Glasgow, however, scientists were able to permanently manipulate light's speed by passing photons through a device that alters their structure. The device, created in collaboration with researchers at Heriot-Watt University in Edinburgh, is a filter of sorts that the scientists refer to as a mask.

"That mask looks a little bit like a bull's-eye target," researcher Miles Padgett told BBC News. "And that mask patterns the light beam, and we show that it's the patterning of the light beam that slows it down.

"But once that pattern has been imposed -- even now the light is no longer in the mask, it's just propagating in free space -- the speed is still slow," Padgett added.

In other words, the beam of light is reorganized in a way that slows down each individual photon. When tested in a vacuum next to a regular light beam. Photons that had been filtered through mask were milliseconds behind in a sprint to the end of the vacuum racetrack.

Researchers, whose latest work was published this week in the journal Science Express, say the findings prove the speed of light is not an absolute, more like a ceiling.

"It's very impressive work," Robert Boyd, an optical physicist at the University of Rochester who wasn't involved with the study, told Science Magazine. "It's the sort of thing that's so obvious, you wonder why you didn't think of it first."