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 reporters. "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.
"The exciting discovery here is that this speed is the true speed of light only for plane waves, that is waves that are perfectly flat."
"In everyday situations however, we interact with light that is not a plane wave but has some kind of structure on it."
"The presence of this structure (think of the light beam emitted from a laser pointer) forces the light to actually move slower."
"There are lots of technicalities involved in the actual experiments used to measure this slow-down, but the result is widely applicable. A very appropriate discovery for the 2015 international year of light".
Professor Padgett added, “It might seem surprising that light can be made to travel more slowly like this, but the effect has a solid theoretical foundation and we’re confident that our observations are correct.
“The results give us a new way to think about the properties of light and we’re keen to continue exploring the potential of this discovery in future applications."
"We expect that the effect will be applicable to any wave theory, so a similar slowing could well be created in sound waves, for example.”
Spatially structured photons that travel in free space slower than the speed of light - Science Magazine January 22 2015 - Science DOI: 10.1126/science.aaa3035