Dynamical friction

Dynamical friction is a term in astrophysics related to loss of momentum and kinetic energy of moving bodies through a gravitational interaction with surrounding matter in space. It is sometimes referred to as gravitational drag.

The effect must exist if the principle of conservation of energy and momentum is valid since any gravitational interaction between two or more bodies corresponds to elastic collisions between those bodies.

E.g. when a heavy body B is moving quickly through a cloud of relatively slowly moving lighter bodies not hitting directly any of them the gravitational interaction between B and the light bodies causes the light bodies to accelerate and gain momentum and kinetic energy (see sling effect). Since energy and momentum is conserved, B has to lose a part of its momentum and energy equal to the sums of all momenta and energies gained by the light bodies. Because of the loss of momentum and kinetic energy of the body under consideration the effect is called dynamical friction.

Of course the mechanism works the same for all masses of interacting bodies and for any relative velocities between them. However while in the above case the most probable outcome is the loss of momentum and energy by the body under consideration, in the general case it might be either loss or gain (obviously when one body is losing momentum and energy in an elastic collision the other one is gaining them). In a case when the body under consideration is gaining momentum and energy the same physical mechanism is called sling effect.

An interesting case is the gravitational interaction of photons with the surrounding matter. The effect of gravitational friction of photons and therefore their redshift (that is proportional to the loss of their energy through Planck's law) would be seen by us as accelerating expansion of space. This is so because the photons moving fast among relatively slowly moving matter of the universe would (as predicted by Einstein's theory of gravitation) have their redshift changing exponentially with distance. Such pattern of redshift corresponds to accelerating recession of sources of light in space, like galaxies, when one assumes that the redshift in their light (called Hubble redshift) isn't caused by dynamical friction but by Doppler effect: by distant galaxies moving away from us.

It is assumed in the mainstream cosmologies that distant galaxies are really moving away from us and the whole observed Hubble redshift is attributed in those cosmologies to the movements of galaxies. Consequently the dynamical friction of photons isn't taken under consideration in those cosmologies. It is assumed instead that the principle of conservation of energy isn't valid for cosmological applications.

It should be noted that the dynamical friction for photons shouldn't be confused with tired light effect. There are subtle differences between the two: In the tired light effect the energy of the photon is supposed to drop along its way (through some unknown mechanism like "resitance of space") which obviously can't influence the time rate at the source of the light. In dynamical friction the effect is purely gravitational and so it is controlled by laws of Einstein's gravitation. Therefore the only possible source of the redshift corresponding to the dynamical friction of photons is time dilation at the source of light (as there are no forces acting on bodies in a free fall, like e.g. photons, to change their energy). So the dynamical friction for photons differs from tired light effect by this time dilation (that is confirmed by observations) at the source of light. So one may say that dynamical friction for photons simulates tired light effect but not precisely enough to simulate also its features that are not observed.