Scientists Slow Down the Speed of Light Traveling in Free Space

Scientists may have just managed to slow down the speed of light traveling in free space. The new feat is one that, before now, was thought to be impossible.

Researchers have long known that the speed of light can be slowed slightly as it travels through materials such as water or glass. That said, slowing photons-also known as light particles-in free space has long thought to be impossible. Now, though, researchers have applied a mask to an optical beam to give photons a spatial structure that can actually reduce their speed.

Think of a beam of light, containing many photons, like a team of cyclists. This team shares the work load by taking 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, though, can make it difficult to define a single velocity for all cyclists; this same issue applies to light. A single pulse of light, as an example, contains many photons, and scientists know that light pulses are characterized by a number of different velocities.

The scientists created an experiment that was a bit like a time trial race. Two photons were released simultaneously across identical distances toward a defined finish line. The researchers found that one photon reached the finish line as predicted, but the structured photon that had been reshaped by the mask arrived later. This, in particular, meant that it was travelling more slowly in free space-by up to 20 wavelengths. That's many times greater than the measurement precision.

The findings reveal that after passing a light beam through a mask, photons move more slowly through space. This is very different to the slow effect of passing light through glass of water, which is when light is only slowed while it's actually within the material; it returns to regular speed after it enters free space once more.

"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," said Miles Padgett, one of the researchers, in a news release. "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."

The findings are published in the journal Science.

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