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.

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