Poynting-Robertson Effect

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The Poynting-Robertson effect (after John Henry Poynting and Howard P. Robertson , who theoretically predicted it in the first half of the 20th century) results from the radiation pressure that solar radiation exerts on interplanetary matter and causes the Orbits of small particles are getting closer and closer to the sun.

Explanation

Poynting-Robertson effect seen from the moving particle (drawn greatly exaggerated).
S: sun
v : direction of movement of the particle

Because of their own movement on their orbit around the sun , all bodies see the radiation arriving from the sun at a small angle from the front at an angle (see also aberration ) and are therefore slowed down by the radiation pressure, i.e. That is, their specific orbital angular momentum and their specific orbital energy decrease.

It is assumed that the release of energy through thermal radiation in the rest system of the body is isotropic , i.e. the same in all directions; then the emitted radiation does not affect the movement (in contrast to the Jarkowski effect ).

The radiation pressure in the radial direction, i.e. transversely to the direction of movement of the body, is significantly higher than the component counteracting the movement. Averaged over an orbit, however, the radial component has no effect , because it does not involve any change in the specific orbital angular momentum. Only when the radial radiation pressure can be compared with the strength of the gravitational force , i.e. the attraction by the sun, will there be slight modifications of the orbit, which however is static and does not affect the dynamics of the Poynting-Robertson effect. As soon as the radial radiation pressure overcomes the attraction by the sun, there is no orbit around the sun and the particle is driven out of the solar system by the radiation pressure. This affects extremely small particles.

Effects on interplanetary matter

While the Poynting-Robertson effect is negligible for larger bodies, the small particles of interplanetary dust (approx. 1 millimeter and smaller) in particular are influenced by it. Due to the loss of specific orbital energy, their orbits around the sun become increasingly narrow and approach it in a spiral path. A particle of 1  micrometer radius needs e.g. B. from the belt of the minor planets around 10,000 years.

The particles evaporate or disintegrate into smaller particles before they reach the sun, then become too small for the PR effect and are pushed radially away from the sun by the radiation pressure. The permanent source of supply for interplanetary matter is seen in the dissolution of comets and the decay of minor planets.

Historical

Poynting explained the effect on the basis of the ether theory that is now recognized as wrong ('friction' of the heat radiation isotropic from the particle on the ether). Robertson treated the effect in a general- relativistically correct manner , but the non-general-relativistic derivation also leads to the same result within the framework of the approximation v ≪ c . The only decisive factor is the momentum transfer of the photons of solar radiation to the dust particle, namely - as already mentioned above - the component acting against the direction of the path.

Point of view of a resting observer

Poynting-Robertson effect seen from a resting observer.

To explain the braking effect, the explanation appears again and again that the photons radiated by the dust particle in the direction of movement (viewed from a stationary observer) have higher energy than those radiated against the direction of movement, and this would slow down the particle. In this form the explanation, which is presumably based on Poynting's wrong derivation (1903), is wrong. The correct explanation when viewing from the point of view of a resting observer, i.e. H. an observer who does not move in relation to the sun is more complicated:

When absorption of a photon will impulse of the photon transmitted on the particle. This impulse has the direction of the photon, i.e. with a circular path exactly perpendicular to the direction of movement. This means that the momentum of the photon does not change the momentum of the particle in its direction of movement. However, the photon not only gives off its momentum, but also its energy to the particle. According to the mass-energy equivalence in the special theory of relativity , this increases the mass of the particle. Because of the conservation of momentum , the speed of the particle decreases.

In the emission of energy as heat radiation in the system of the observer at rest is due to the Doppler effect actually forward more radiant energy emitted and therefore more impulse than backwards. This reduces the momentum of the particle, but at the same time the mass decreases due to the release of energy. The speed of the particle remains unchanged. In this reference system , too, one comes to the conclusion that the speed decreases when the photon is absorbed and not when the energy is emitted.

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