  The conventional derivation of the gamma-ray burst afterglow jet break time
uses only the blast wave fluid Lorentz factor and therefore leads to an
achromatic break. We show that in general gamma-ray burst afterglow jet breaks
are chromatic across the self-absorption break. Depending on circumstances, the
radio jet break may be postponed significantly. Using high-accuracy adaptive
mesh fluid simulations in one dimension, coupled to a detailed synchrotron
radiation code, we demonstrate that this is true even for the standard fireball
model and hard-edged jets. We confirm these effects with a simulation in two
dimensions. The frequency dependence of the jet break is a result of the angle
dependence of the emission, the changing optical depth in the self-absorbed
regime and the shape of the synchrotron spectrum in general. In the optically
thin case the conventional analysis systematically overestimates the jet break
time, leading to inferred opening angles that are underestimated by a factor
1.32 and explosion energies that are underestimated by a factor 1.73, for
explosions in a homogeneous environment.
