Relativistic effects may turn scattered photons into drag for fast solar sails

In a recent arXiv paper, Chao Shen and Jiaze Li of the Harbin Institute of Technology examined how laser-propelled solar sails behave at relativistic speeds. They categorized photon forces into incident momentum, specular reflection, and diffuse scattering, noting that the latter is weakest at low velocities. As the sail accelerates, Doppler shifting reduces the thrust from all three components, making further acceleration harder.

Crucially, when the sail approaches about 75% of the speed of light, relativistic light aberration causes diffusely scattered photons to be emitted forward, producing a drag force opposite the direction of travel. Although the laser continues to push the sail, this drag markedly lowers propulsion efficiency. The study abstracts away non-radiative factors such as interstellar dust, gas drag, and thermal limits of real materials, which would pose additional challenges for practical interstellar missions.