Radiation pressure

Lore & Background

Johannes Kepler put forward the concept of radiation pressure in 1619 to explain the observation that a tail of a comet always points away from the Sun. The assertion that light, as electromagnetic radiation, has the property of momentum and thus exerts a pressure upon any surface that is exposed to it was published by James Clerk Maxwell in 1862, and proven experimentally by Russian physicist Pyotr Lebedev in 1900 and by Ernest Fox Nichols and Gordon Ferrie Hull in 1901. The pressure is very small, but can be detected by allowing the radiation to fall upon a delicately poised vane of reflective metal in a Nichols radiometer (this should not be confused with the Crookes radiometer, whose characteristic motion is not caused by radiation pressure but by air flow caused by temperature differentials).

Reader's Guide

Radiation pressure forces are the bedrock of laser technology and the branches of science that rely heavily on lasers and other optical technologies. That includes, but is not limited to, biomicroscopy, quantum optics, and optomechanics. Direct applications of the radiation pressure force in these fields are, for example, laser cooling (the subject of the 1997 Nobel Prize in Physics), quantum control of macroscopic objects and atoms (2012 Nobel Prize in Physics), interferometry (2017 Nobel Prize in Physics) and optical tweezers (2018 Nobel Prize in Physics). The forces generated by radiation pressure are generally too small to be noticed under everyday circumstances; however, they are important in some physical processes and technologies. This particularly includes objects in outer space, where it is usually the main force acting on objects besides gravity, and where the net effect of a tiny force may have a large cumulative effect over long periods of time. For example, had the effects of the Sun's radiation pressure on the spacecraft of the Viking program been ignored, the spacecraft would have missed Mars orbit by about 15,000 km (9,300 mi). Radiation pressure from starlight is crucial in a number of astrophysical processes as well. The significance of radiation pressure increases rapidly at extremely high temperatures and can sometimes dwarf the usual gas pressure, for instance, in stellar interiors and thermonuclear weapons.

Did You Know?

• Johannes Kepler proposed the concept in 1619 to explain why a comet's tail always points away from the Sun.
• The pressure is so small that a Nichols radiometer, not a Crookes radiometer, is needed to detect it.
• If ignored, the Sun's radiation pressure would have caused the Viking program spacecraft to miss Mars orbit by about 15,000 km.
• Radiation pressure can be accounted for by considering the momentum of a classical electromagnetic field or in terms of the momenta of photons.

From Comet Tails to the Laboratory Bench

The idea that light pushes on matter has a surprisingly long intellectual history. In 1619, Johannes Kepler proposed what we now recognize as radiation pressure to account for a curious astronomical observation: the tail of a comet consistently points away from the Sun rather than trailing behind it. More than two centuries later, James Clerk Maxwell formalized the claim in 1862, arguing that electromagnetic radiation carries momentum and therefore must exert a measurable force on any surface it strikes. The theoretical prediction, however, remained unverified for decades. It was not until 1900 that Russian physicist Pyotr Lebedev demonstrated the effect experimentally, followed almost immediately by Ernest Fox Nichols and Gordon Ferrie Hull in 1901. Their apparatus, the Nichols radiometer, relied on a finely balanced vane of reflective metal suspended so delicately that the minuscule push of light could be detected. It is worth noting that this device is entirely distinct from the Crookes radiometer, whose spinning vanes are driven by air currents produced by temperature differences rather than by the momentum of photons.

The Physics Behind the Push

At its core, radiation pressure is a direct consequence of momentum conservation. When electromagnetic radiation—whether described as a classical wave or as a stream of individual photons—interacts with matter, the total momentum of the combined system must remain constant. Any change in the radiation's momentum is therefore balanced by an equal and opposite change in the momentum of the material body, a relationship that mirrors Newton's third law. The classical picture reveals a subtle division of labor between the two components of the electromagnetic field. The electric field acts first on a stationary charge, doing work and transferring energy while setting the charge into motion. Once the charge is moving, the magnetic component of the radiation field exerts an additional force directed along the propagation direction. In this framework, the electric field is responsible for energy exchange, while the magnetic field is the agent that imparts net momentum to the charge. Remarkably, whether one calculates the pressure from the fields of classical electrodynamics or sums the momenta of individual photons, the numerical result is identical, underscoring the deep unity between the wave and particle descriptions of light.

A Force That Shapes the Cosmos

Under ordinary terrestrial conditions, the push of light on a surface is so feeble that it goes entirely unnoticed in daily life. In the vacuum of outer space, however, the situation changes dramatically. With no atmosphere to provide drag or other competing forces, radiation pressure becomes one of the principal influences acting on any object, second only to gravity. Because the force acts continuously, even a vanishingly small acceleration accumulates over the months or years of a spacecraft's journey. A striking illustration comes from the Viking program: had the effects of the Sun's radiation pressure been neglected in trajectory planning, the spacecraft would have missed Mars orbit by roughly fifteen thousand kilometres. The same principle operates on far grander scales. In the searing interiors of stars and inside thermonuclear weapons, temperatures are so extreme that radiation pressure can overwhelm the ordinary gas pressure that supports matter. Looking forward, large lasers operating in space have been suggested as a means of propelling light-sail craft, a concept known as beam-powered propulsion, in which the cumulative momentum transfer from photons provides the thrust for the vehicle.

The Engine of Modern Optical Science

Radiation pressure is not merely a curiosity of classical physics; it is the foundational mechanism underpinning an entire family of modern technologies and scientific disciplines. In biomicroscopy, light is used to irradiate and observe microbes, cells, and molecules, with the radiation pressure force playing a direct role in these interactions. In quantum optics and optomechanics, the same force is harnessed to probe and control objects ranging from atoms and qubits to macroscopic quantum systems. The practical impact of this principle has been recognized at the highest level of scientific achievement. The 1997 Nobel Prize in Physics was awarded for laser cooling, a direct application of the radiation pressure force. The 2012 Nobel Prize honored quantum control of macroscopic objects and atoms, again built on the momentum transfer between light and matter. The 2017 prize recognized interferometry, and the 2018 prize went to optical tweezers, another field where the radiation pressure force is a direct application. Together, these four Nobel Prizes within a single two-decade span demonstrate that the gentle push of light is one of the most versatile and transformative tools in contemporary physics.