Superluminal motion

In astronomy, superluminal motion is the apparently faster-than-light motion seen in someradio galaxies, BL Lac objects, quasars, blazars and recently also in some galactic sources called microquasars. All of these sources are thought to contain a black hole, responsible for the ejection of mass at high velocities. Light echoes can also produce apparent superluminal motion.Schilizzi ... presented maps of arc-second resolution ... which ... have revealed outer double structure in all but one (3C 273) of the known superluminal sources. An embarrassment is that the average projected size of the outer structure is no smaller than that of the normal radio-source population.The first indications of changes in the structure of some sources were obtained by an American-Australian team in a series of transpacific VLBI observations between 1968 and 1970 (Gubbay et al. 1969). Following the early experiments, they had realised the potential of the NASA tracking antennas for VLBI measurements and set up an interferometer operating between California and Australia. The change in the source visibility that they measured for 3C 279, combined with changes in total flux density, indicated that a component first seen in 1969 had reached a diameter of about 1 milliarcsecond, implying expansion at an apparent velocity of at least twice the speed of light. Aware of Rees's model, (Moffet et al. 1972) concluded that their measurement presented evidence for relativistic expansion of this component. This interpretation, although by no means unique, was later confirmed, and in hindsight it seems fair to say that their experiment was the first interferometric measurement of superluminal expansion. In astronomy, superluminal motion is the apparently faster-than-light motion seen in someradio galaxies, BL Lac objects, quasars, blazars and recently also in some galactic sources called microquasars. All of these sources are thought to contain a black hole, responsible for the ejection of mass at high velocities. Light echoes can also produce apparent superluminal motion. This phenomenon is caused by the jets traveling very near the speed of light towards the observer. The angle is not necessarily very small with the line-of-sight as is commonly asserted. Because the high-velocity jets are emitting light at every point of their path, the light they emit does not approach the observer much more quickly than the jet itself. This causes the light emitted over hundreds of years of the jet's travel to not have hundreds of light-years of distance between its front end (the earliest light emitted) and its back end (the latest light emitted); the complete 'light-train' thus arrives at the observer over a much smaller time period (ten or twenty years), giving the illusion of faster-than-light travel. This explanation depends on the jet making a sufficiently narrow angle with the observer's line-of-sight to explain the degree of superluminal motion seen in a particular case. Superluminal motion is often seen in two opposing jets, one moving away and one toward Earth. If Doppler shifts are observed in both sources, the velocity and the distance can be determined independently of other observations. As early as 1983, at the 'superluminal workshop' held at Jodrell Bank Observatory, referring to the seven then-known superluminal jets,