  In this work we present a general derivation of relativistic fluid dynamics
from the Boltzmann equation using the method of moments. The main difference
between our approach and the traditional 14-moment approximation is that we
will not close the fluid-dynamical equations of motion by truncating the
expansion of the distribution function. Instead, we keep all terms in the
moment expansion. The reduction of the degrees of freedom is done by
identifying the microscopic time scales of the Boltzmann equation and
considering only the slowest ones. In addition, the equations of motion for the
dissipative quantities are truncated according to a systematic power-counting
scheme in Knudsen and inverse Reynolds number. We conclude that the equations
of motion can be closed in terms of only 14 dynamical variables, as long as we
only keep terms of second order in Knudsen and/or inverse Reynolds number. We
show that, even though the equations of motion are closed in terms of these 14
fields, the transport coefficients carry information about all the moments of
the distribution function. In this way, we can show that the particle-diffusion
and shear-viscosity coefficients agree with the values given by the
Chapman-Enskog expansion.
