Radiation pressure

Light can push on things.

This push is very small. You cannot feel it at home. But light can move objects in space. It can push a spacecraft. It can even move a comet.

This happens when light hits a surface. The light can be soaked up. Or the light can bounce off. Both ways give the object a push. This push can add up over a long time. It is a very useful force in the stars.

Light can push on things. This is called radiation pressure.

It happens when light hits a surface. Light is made of tiny particles called photons.

These photons carry momentum. Momentum is a way to describe how much motion something has. When photons hit an object, they give that object a push. This happens if the object soaks up the light. It also happens if the light bounces off. A surface that reflects light gets a bigger push. This is because the light hits it and then pushes back as it leaves.

This push is very small. You cannot feel it in your daily life. But in space, it is very important. It can move comets away from the Sun.

It can even move spacecraft. Scientists think we could use large lasers to push sails in space. Inside hot stars, this pressure is very big. It can be even stronger than the pressure from gas. This helps hold the stars up against gravity.

Radiation pressure is a real force that light can exert on objects. Even though we cannot feel it, light carries momentum. This means light has a certain amount of motion. When light hits a surface, it can be absorbed or reflected. This exchange of momentum creates a physical push called radiation pressure.

This force is very small in our daily lives. However, it becomes very important in the vastness of outer space. In space, it is often the main force acting on objects besides gravity.

How does this push actually work? You can think of it as a tiny exchange of motion. When a particle of light, called a photon, hits an object, it transfers its momentum. If the object is black and absorbs the light, it receives one push. If the surface is a perfect reflector, the light bounces back. This bounce creates an extra recoil, which doubles the total pressure.

A surface can also feel pressure if it emits its own radiation. This is called black-body radiation, and it happens with all materials. As things get hotter, this pressure grows very quickly.

People have wondered about this for a long time. In 1619, Johannes Kepler suggested a connection to explain comet tails. He noticed comet tails always point away from the Sun. Later, James Clerk Maxwell wrote about light's momentum in 1862. In 1900, the Russian physicist Pyotr Lebedev proved it with an experiment.

Other scientists, Ernest Fox Nichols and Gordon Ferrie Hull, also proved it that same year. They used a device called a Nichols radiometer to detect the tiny force.

There are many important numbers and facts regarding this force. Near Earth, the Sun's light has a strength called the solar constant. In 2011, this value was measured at 1361 W/m2. This light pushes on all spacecraft unless they are in a shadow. For example, the Viking program spacecraft had to account for this pressure. If they had ignored it, they would have missed Mars by a large amount.

Over long periods, these tiny pushes can even change orbits.

Today, we use radiation pressure in amazing new technologies. It is the foundation for many types of laser science. Scientists use light to control tiny things like atoms and molecules. This is called optomechanics, where light is used to probe objects.

We also use it for laser cooling and optical tweezers. Some people even suggest using large lasers to push sails through space. This could allow us to travel to far-off places using light alone.

Radiation pressure, often called light pressure, is a mechanical force exerted on a surface. This force happens because of an exchange of momentum between an object and an electromagnetic field. This includes any wavelength of light or electromagnetic radiation that is absorbed, reflected, or emitted by matter. This phenomenon occurs on many scales, from massive macroscopic objects to tiny dust particles and gas molecules.

The mechanism behind this force is rooted in the laws of physics. According to the law of conservation of momentum, any change in the momentum of light must result in an equal and opposite change in the momentum of the matter it hits. This is a direct application of Newton's third law of motion. You can view this through two different lenses. One way is by considering the momentum of a classical electromagnetic wave. Another way is to look at the combined momenta of photons, which are the particles of light.

When light interacts with a surface, the resulting pressure depends on how the surface handles the light. If a surface is perfectly absorbing, like a black body, it receives one unit of pressure from the incoming radiation. However, if the surface is a perfect reflector, the light bounces back. This reflection creates an extra recoil, which doubles the net radiation pressure. For surfaces that are only partially reflective, the pressure will be somewhere between these two extremes. Additionally, a body can experience radiation pressure if it emits its own radiation, such as black-body radiation. This emission can be significant in very hot environments, such as stellar interiors.

History shows that humans have observed these effects for centuries. In 1619, Johannes Kepler proposed the concept to explain why comet tails always point away from the Sun. Later, in 1862, James Clerk Maxwell published the idea that light has momentum. The theory was eventually proven experimentally in 1900. Russian physicist Pyotr Lebedev provided proof, as did Ernest Fox Nichols and Gordon Ferrie Hull.

To detect these tiny forces, scientists used a Nichols radiometer, which uses a delicately poised reflective metal vane.

While radiation pressure is usually too small to feel, it has massive significance in space. In the vacuum of space, it is often the primary force acting on objects besides gravity. For example, the solar constant near Earth was measured at 1361 W/m2 in 2011. This solar radiation pressure affects all spacecraft unless they are in a shadow. If the Viking program spacecraft had ignored this force, it would have missed its Mars orbit by a significant margin. Over extremely long periods, these tiny cumulative effects can even influence the orbits of the Earth-Moon system.

Radiation pressure is also vital in high-temperature environments. In the interiors of stars or within thermonuclear weapons, the pressure from radiation can dwarf the usual pressure from gas. This makes it a fundamental part of how stars function and how energy moves through them.

Understanding these forces helps astrophysicists model the life cycles of stars and the behavior of cosmic dust.

Today, the study of radiation pressure is the bedrock of many modern optical technologies. It is essential to fields like quantum optics and optomechanics, where light is used to control atoms and macroscopic quantum objects.

Scientists use these principles for laser cooling, which earned a Nobel Prize in 1997, and for optical tweezers, which won a Nobel Prize in 2018. Looking forward, researchers have even suggested using large space-based lasers to power sail craft through beam-powered propulsion. This could change how we explore the solar system and beyond.