Thanks to a quirk of quantum theory, subatomic particles can emit light as they travel through a seemingly empty vacuum.

The speed of light varies in water, ice and other media. In some cases, an electron or other charged subatomic particle passing through a medium travels more quickly than light moving through the same medium. Such a speedy particle creates a cone of compressed waves as it zips through its surroundings. These waves emit light, called Cherenkov radiation, that has a bluish tinge.

Alexander Macleod, Adam Noble and Dino Jaroszynski at the University of Strathclyde in Glasgow, UK, find that this phenomenon can also occur in a vacuum. According to quantum theory, a vacuum is actually filled with pairs of ephemeral particles that spontaneously come into being before cancelling each other out again. The team’s calculations show that these particles can momentarily reduce the speed of light around them. As a result, a particle travelling at near the speed of light might emit Cherenkov radiation.

Under certain conditions, Cherenkov radiation in a vacuum should be detectable. If so, observations of the radiation could verify some interactions between light and matter predicted by quantum theory.