Normally, light zips through a vacuum at a blazing 186,282 miles per second. But as a new experiment by Scottish physicists has shown, this isn't always necessarily the case.


We know that light slows down when it travels through a dense medium, such as water or glass. But when there's no obstruction — like when it's travelling through a vacuum — there's no reason for it to go any slower than the cosmological speed limit of 299,792,458 m s–1. Or so we thought.

Researchers have now shown that the speed of an individual photon can decrease by small amount if it's sent through a patterned mask. Physics World explains:


Over two years, Miles Padgett and colleagues at the University of Glasgow, together with Daniele Faccio of Heriot-Watt University in Edinburgh, designed an experiment that can determine whether light with a certain "spatial structure" travels substantially slower than regular light in a vacuum. The researchers created a source that emitted pairs of photons simultaneously. One of the photons went straight to a highly precise photon counter, while the other went via two liquid-crystal masks, which imparted their profile onto the passing particle of light.

Across a propagation distance of 1 m, the team found that the spatially structured photon lagged behind its partner by between 10 and 20 wavelengths. That equated to a drop in speed of about 0.001%, says team member Jacquiline Romero.

"Using time-correlated photon pairs we show a reduction of the group velocity of photons in both a Bessel beam and photons in a focused Gaussian beam," note the researchers in their study. "Our work highlights that, even in free space, the invariance of the speed of light only applies to plane waves."

The physics behind this is quite complicated. To simplify things, the BBC analogizes it to a bicycle race:


The peloton - the main bunch of riders - may be moving at a constant speed. But within the bunch an individual rider may moving more slowly, dropping back for a rest or a drink.

Meanwhile other riders in the bunch are moving faster to get to the front.

The bunch is a beam of light, travelling at - yes - the speed of light. The riders are photons, travelling at their individual speeds.

Interestingly, the same thing should apply to sound waves.


Read more at Physics World and the BBC. You check out the scientific study here.

Image: Glasgow/Heriot-Watt Universities