  The energy budget of all known optical jets is discussed. It is found that to
power the extended radio lobes of radio galaxies, the jet bulk motion on kpc
scales must be relativistic, on average. Based on various constraints, a ``most
probable'' region centered around bulk Lorentz factor ~7.5 and jet inclination
\~20 degrees is found. Because of the consequent relativistic beaming, the rest
frame magnetic field is lower and electron lifetimes longer. Combining the
effects of time dilation and lower emission rate, the electron diffusion length
becomes fully consistent with the deprojected jet length, without the need for
reacceleration.
