Pressia

Meet Pressia, a beam of light that gives everything it touches a gentle push.

Light carries energy, and it also carries momentum. When a surface absorbs light, that momentum is handed over, which means a force and therefore a pressure. James Clerk Maxwell's theory of electromagnetism predicted this, and Pyotr Lebedev and, separately, Ernest Nichols and Gordon Hull measured it around 1900. For a fully absorbing surface the pressure is the intensity divided by the speed of light, P = I/c. A perfect mirror gets twice that, because the light bounces back.

Sunlight near Earth has an intensity of about 1361 W/m². Dividing by c = 299,792,458 m/s gives P = 1361 / 299,792,458 ≈ 4.54 × 10⁻⁶ N/m², about 4.54 micronewtons on every square metre.

That is tiny, but in space nothing cancels it. Solar sails use it for thrust, as Japan's IKAROS spacecraft showed in 2010, and mission planners must include it when predicting spacecraft orbits.

On Earth, air currents and friction swamp it. The old radiometer, a glass bulb with spinning vanes, is a famous misconception: it turns mainly because of heated gas, not radiation pressure.