Special Relativity at Action in the Universe

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.