Dust (relativity)

In special and general relativity, dust is the name conventionally given to a configuration of matter which can be interpreted as small bodies ("dust particles") which interact only gravitationally.

The number density n of dust is defined as the number of particles per unit volume in the (unique) inertial frame in which the particles are at rest.

Dust possesses a number flux four vector \vec{N} which defines the fluxes across coordinate planes defined by

\vec{N} =n \, \vec{U}

where \vec{U} is the four velocity of the particles.

See also