If you sweep a laser pointer across the Moon fast enough, you can create spots that actually move faster than light

At a meeting of the American Astronomical Society in Seattle Thursday Jan. 8, Robert Nemiroff, a physics professor at Michigan Technological University, reported that this theoretical curiosity may turn out to 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.

Flashes, dubbed “photonic booms” because they are directly analogous to sonic booms, may be detectable 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, said Nemiroff, author of a study accepted for publication by the Publications of the Astronomical Society of Australia (preliminary open-access version available online on arXiv). “And if detected, we could learn more about all of these objects,” said Nemiroff.

How to remotely image asteroids in 3D

To reveal the size and surface features of asteroids passing near the Earth, a laser beam might be swept across the rock’s surface thousands of times a second, with each sweep forcing a harmless but telling photonic boom. The flashes could be recorded with high-speed cameras attached to large telescopes, potentially mapping out major features on the asteroid.

Photonic booms could also be seen much farther out in the universe. An example occurs in Hubble’s Variable Nebula in the constellation of Monoceros. There, shadows cast by clouds moving between the bright star “R Mon” and reflecting dust move so fast that they might create photonics booms visible even for days or weeks.

The physics that creates the photonic boom is tied to the faster-than-light sweep speeds of the illuminating spots and cast shadows. Specifically, a flash is seen by an observer when the speed of the scattered spot toward the observer drops from above the speed of light to below the speed of light.

The phenomenon is possible only because the spots contain no mass and so cannot only move faster than light, but decelerate past the speed of light without violating Einstein’s theory of special relativity.

Details of the effect hinge 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. So measuring photonic booms gives information about the depth of the scatterer. Were the Moon just a flat disk on the sky, for example, no photonics boom would occur.

“Photonic booms happen around us quite frequently, but they are always too brief to notice,” says Nemiroff. “Out in the cosmos they last long enough to notice, but nobody has thought to look for them!”

Abstract of Superluminal Spot Pair Events in Astronomical Settings: Sweeping Beams

Sweeping beams of light can cast spots moving with superluminal speeds across scattering surfaces. Such faster-than-light speeds are well-known phenomena that do not violate special relativity. It is shown here that under certain circumstances, superluminal spot pair creation and annihilation events can occur that provide unique information to observers. These spot pair events are not particle pair events — they are the sudden creation or annihilation of a pair of relatively illuminated spots on a scattering surface. Real spot pair illumination events occur unambiguously on the scattering surface when spot speeds diverge, while virtual spot pair events are observer dependent and perceived only when real spot radial speeds cross the speed of light. Specifically, a virtual spot pair creation event will be observed when a real spot’s speed toward the observer drops below c, while a virtual spot pair annihilation event will be observed when a real spot’s radial speed away from the observer rises above c. Superluminal spot pair events might be found angularly, photometrically, or polarimetrically, and might carry useful geometry or distance information. Two example scenarios are briefly considered. The first is a beam swept across a scattering spherical object, exemplified by spots of light moving across Earth’s Moon and pulsar companions. The second is a beam swept across a scattering planar wall or linear filament, exemplified by spots of light moving across variable nebulae including Hubble’s Variable Nebula. In local cases where the sweeping beam can be controlled and repeated, a three-dimensional map of a target object can be constructed. Used tomographically, this imaging technique is fundamentally different from lens photography, radar, and conventional lidar.