Dust (relativity)
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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 
which defines the fluxes across coordinate planes defined by
where 
is the four velocity of the particles.
[edit] See also
- Dust solution, for more about exact dust solutions in general relativity
