  We present HST images at 622 nm and 300 nm of the jet in 3C273 and determine
the run of the optical spectral index at 0.2" along the jet. The smoothness of
spectral index changes shows that the physical conditions are varying smoothly
across the jet. There is no correlation between the optical flux and spectral
index, as would be expected for relativistic electrons suffering strong cooling
due to synchrotron emission. We find no evidence for localized acceleration or
loss sites. This suggests that the spectral shape is not changing much
throughout the jet. We show that relativistic beaming and/or sub-equipartition
magnetic fields cannot remove the discrepancy between light-travel time along
the jet and the lifetime of electrons emitting optical synchrotron radiation.
We consider this further evidence in favour of a distributed electron
acceleration process.

Astrophysics