  Nature succeeds in accelerating extended and massive objects to relativistic
velocities. Jets in Active Galactic Nuclei and in galactic superluminal sources
and gamma-ray bursts fireballs have bulk Lorentz factors from a few to several
hundreds. A variety of effects then arises, such as the beaming of the produced
radiation, light aberration, time contraction and the Doppler frequency shift.
I will emphasize that special relativity applied to real (i.e. extended)
observed objects inevitably must take into account that any piece of
information is carried by photons. Being created in different parts of the
source, they travel different paths to reach the observer, depending on the
viewing angle. The object is seen rotated, not contracted, and at small viewing
angles time intervals are observed shorter than intrinsic ones.
