Flashes from 'Photonic Booms' May Illuminate Cosmic Secrets

In theory, if you sweep a laser pointer across the moon fast enough, you can cast spots that move faster than light across the surface. Now, scientists have reported that this theoretical curiosity may be practically useful out in the cosmos.

When a superluminal sweep occurs, it typically starts with a flash that may reveal previously unknown three-dimensional information about the scattering object. While these superluminal sweeps may seem odd, though, the well-known phenomena do not violate Einstein's theory of special relativity.

These flashes are dubbed "photonic booms," since they're directly analogous to sonic booms. These photonic booms could potentially be detected on the moon, on passing asteroids, on fast moving shadows cast on reflecting dust clouds near variable stars and on objects illuminated by the rapidly rotating beam of a pulsar. If they were detected, they could tell scientists quite a bit more about all of these objects.

How do they work? The physics of a photonic boom is tied to a faster-than-light sweep speeds of the illuminating spots and cast shadows. More specifically, a flash is seen by an observer when the speed of the scattered spot toward the observers drops from above the speed of light to below the speed of light. This effect hinges on the interplay between the time it takes for a sweeping light beam to cross an object, and the time it takes for the light beam to traverse the depth of the object. Measuring photonic booms, therefore, could give us information about the depth of the scatterer.

In order to reveal the size and surface features of asteroids passing near Earth, for example, a laser beam should be swept across the rock's surface thousands of times per second. Each sweep needs to force a harmless but telling photonic boom. Then, the flashes should be recorded with high-speed cameras attached to large telescopes, potentially mapping out major features on the asteroids.

Photonic booms could also be seen further out in the universe. In Hubble's Variable Nebula in the constellation of Monoceros, shadows cast by clouds moving between the bright star "R Mon" and reflecting dust move so fast that they could create photonic booms visible for days or even weeks.

"Photonic booms happen around us quite frequently-but they are always too brief to notice," said Robert Nemiroff, one of the researchers, in a news release. "Out in the cosmos they last long enough to notice-but nobody has thought to look for them!"

The findings are published in the journal Publications of the Astronomical Society of Australia.

For more great science stories and general news, please visit our sister site, Headlines and Global News (HNGN).