Radiation pressure

Radiation pressure is the mechanical force per unit area that an electromagnetic wave exerts on a surface. It follows directly from the field momentum: the wave carries momentum at density S/c², and when the wave is absorbed or reflected, that momentum is transferred to the surface. For a wave of intensity I (W/m²) incident normally on a perfect absorber, the pressure is I/c; on a perfect reflector, the pressure doubles to 2I/c because the photons reverse direction (momentum change of 2p per photon instead of p).

Maxwell predicted radiation pressure in 1871 as a direct consequence of his field-momentum bookkeeping. The predicted magnitude was tiny and hard to measure experimentally because thermal effects — tiny currents of gas molecules heated by absorbed light and pushing back on the surface — often dominate in poor-vacuum conditions. Pyotr Lebedev at Moscow University demonstrated radiation pressure cleanly in 1901 using a torsion balance with polished and blackened vanes in the best vacuum then available; Ernest Fox Nichols and Gordon Hull at Dartmouth did similar experiments in 1903. Both confirmed Maxwell's predictions quantitatively, settling the question.

At Earth-orbit sunlight intensity (1361 W/m², the solar constant), radiation pressure on a perfectly reflecting surface is about 9.1 µPa — a millionth of atmospheric pressure. Over a 10 m × 10 m solar sail this is 0.91 mN, roughly the weight of 90 mg on Earth. Continuous over months, this is enough to change spacecraft velocity by several metres per second — sufficient for mission-altering manoeuvres. Japan's IKAROS (2010) and LightSail 2 (2019) demonstrated solar-sail propulsion in interplanetary space. More extreme cases: inside a high-Q laser cavity, radiation pressure of ~10³ Pa has been achieved; ultra-intense laser-matter experiments reach petapascals. Radiation pressure is also responsible for the outward push on massive stars from radiation, the Poynting–Robertson dragging of dust grains toward the sun, and the optical tweezer technology that won Ashkin the 2018 Nobel Prize.

James Clerk Maxwell predicted radiation pressure in A Treatise on Electricity and Magnetism (1873). Pyotr Lebedev measured it experimentally in 1901 with a sensitive torsion balance; Nichols and Hull confirmed the result independently in 1903. Einstein's 1905 light-quanta paper reinterpreted radiation pressure as photon-momentum transfer, a view fully consistent with Maxwell's classical derivation.