  We present an exact solution of the relativistic Boltzmann equation for a
system undergoing boost-invariant longitudinal and azimuthally symmetric
transverse flow ("Gubser flow"). The resulting exact non-equilibrium dynamics
is compared to 1st- and 2nd-order relativistic hydrodynamic approximations for
various shear viscosity to entropy density ratios. This novel solution can be
used to test the validity and accuracy of different hydrodynamic approximations
in conditions similar to those generated in relativistic heavy-ion collisions.
