Superluminal motion

In astronomy, superluminal motion is the apparently faster-than-light motion seen in some radio galaxies, BL Lac objects, quasars 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.

When first observed in the early 1970s, superluminal motion was taken to be a piece of evidence against quasars having cosmological distances. Although a few astrophysicists still argue in favor of this view, most believe that apparent velocities greater than the velocity of light are optical illusions and involve no physics incompatible with the theory of special relativity.

This phenomenon is caused because the jets are travelling very near the speed of light at a very small angle towards the observer. Because at every point of their path the high-velocity jets are emitting light, 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,

Schilizzi ... presented maps of arc-second resolution [showing the large-scale outer jets] ... 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 [on the sky] of the outer structure is no smaller than that of the normal radio-source population.

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