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 <math>n</math> 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 <math>\vec{N}</math> which defines the fluxes across coordinate planes defined by
- <math>\vec{N} =n \, \vec{U}</math>
where <math>\vec{U}</math> is the four velocity of the particles.
See also
- Dust solution, for more about exact dust solutions in general relativity
