Radiation pressure

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.

Furthermore, large lasers operating in space have been suggested as a means of propelling sail craft in beam-powered propulsion.

[8] Radiation pressure can equally well be accounted for by considering the momentum of a classical electromagnetic field or in terms of the momenta of photons, particles of light.

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.

[9] 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[10] and by Ernest Fox Nichols and Gordon Ferrie Hull in 1901.

[11] 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.)

That momentum can be equally well calculated on the basis of electromagnetic theory or from the combined momenta of a stream of photons, giving identical results as is shown below.

So, dimensionally, the Poynting vector is S = ⁠power/area⁠ = ⁠rate of doing work/area⁠ = ⁠⁠ΔF/Δt⁠ Δx/area⁠, which is the speed of light, c = Δx / Δt, times pressure, ΔF / area.

[12] The above treatment for an incident wave accounts for the radiation pressure experienced by a black (totally absorbing) body.

Combining this with the above expression for the momentum of a single photon, results in the same relationships between irradiance and radiation pressure described above using classical electromagnetics.

From that it can be shown that the resulting pressure is equal to one third of the total radiant energy per unit volume in the surrounding space.

While it acts on all objects, its net effect is generally greater on smaller bodies, since they have a larger ratio of surface area to mass.

This distribution must be taken into account when calculating the radiation pressure or identifying reflector materials for optimizing a solar sail, for instance.

However these pressures persist over eons, such that cumulatively having produced a measureable movement on the Earth-Moon system's orbit.

Note, however, that in order to account for the net effect of solar radiation on a spacecraft for instance, one would need to consider the total force (in the direction away from the Sun) given by the preceding equation, rather than just the component normal to the surface that we identify as "pressure".

The solar constant is defined for the Sun's radiation at the distance to the Earth, also known as one astronomical unit (au).

Spacecraft are affected along with natural bodies (comets, asteroids, dust grains, gas molecules).

The radiation pressure results in forces and torques on the bodies that can change their translational and rotational motions.

As a consequence of light pressure, Einstein[21] in 1909 predicted the existence of "radiation friction", which would oppose the movement of matter.

Solar sailing, an experimental method of spacecraft propulsion, uses radiation pressure from the Sun as a motive force.

The Japan Aerospace Exploration Agency (JAXA) has successfully unfurled a solar sail in space, which has already succeeded in propelling its payload with the IKAROS project.

Radiation pressure has had a major effect on the development of the cosmos, from the birth of the universe to ongoing formation of stars and shaping of clouds of dust and gasses on a wide range of scales.

[24] The gravitational compression of clouds of dust and gases is strongly influenced by radiation pressure, especially when the condensations lead to star births.

Radiation pressure from the member stars eventually disperses the clouds, which can have a profound effect on the evolution of the cluster.

In many cases, the stripping away of the gas from which the cluster formed by the radiation pressure of the hot young stars reduces the cluster mass enough to allow rapid dispersal.Star formation is the process by which dense regions within molecular clouds in interstellar space collapse to form stars.

Solar heating causes gases to be released from the comet nucleus, which also carry away dust grains.

This serves the purpose of gravely enhancing the power of the light, and the radiation pressure it can exert on objects and materials.

Optical control (that is, manipulation of the motion) of a plethora of objects has been realized: from kilometers long beams (such as in the LIGO interferometer)[28] to clouds of atoms,[29] and from micro-engineered trampolines[30] to superfluids.

Atoms traveling towards a laser light source perceive a doppler effect tuned to the absorption frequency of the target element.

[34] An other active research area of laser–matter interaction is the radiation pressure acceleration of ions or protons from thin–foil targets.