Nanophotonic light sails may travel at relativistic speeds

One day in the not-so-distant future, light sails may hurtle through space at speeds of around 20% of the speed of light (or 60,000 km/sec), propelled not by fuel but rather by the radiation pressure from high-power lasers on Earth. Traveling at these relativistic speeds, laser-powered light sails could reach our nearest neighboring star (other than the Sun), Alpha Centauri, or the nearest known potentially habitable planet, Proxima Centauri b, in about 20 years. Both objects are a little more than four light-years away.

Designing light sails is a major engineering challenge, however, requiring conflicting features that sound nearly impossible: an ideal light sail should be several meters wide and mechanically robust enough to withstand intense radiation pressure, yet be merely 100 nanometers or so thick and weigh just a few grams.

Further requirements arise from the mechanism by which light sails work. According to Maxwell's equations, light has momentum and asaresult can exert pressure on objects. However, light sails are not simply pushed by radiation pressure like a sailboat is pushed by the wind. Instead, the push results from the light sail reflecting the radiation. As a result, an optimal sail should reflect the majority of the radiation in the laser beam's near-infrared spectrum, while simultaneously emitting radiation in the mid-infrared range for efficient radiative cooling.