Superluminal motion
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In astronomy, superluminal motion is the apparently faster-than-light motion seen in some radio galaxies, 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 for 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.
[edit] Explanation
The explanation can be given in a fairly straightforward way as a light travel time effect. Imagine a body of matter starting at the center of a galaxy and moving quickly towards the observer, nearly head-on but not exactly.
When the body is at the center of the galaxy, it emits some light towards the observer. After it has moved, and again emits light towards the observer, this light will have a shorter travel time since the object is now closer to Earth. An observer ignoring the movement towards Earth and only noticing the perpendicular movement will underestimate the true time interval (for their inertial reference frame), and so will overestimate the object's speed; this overestimated speed can be many times the speed of light.
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.
[edit] History
In 1966 Martin Rees predicted (Nature 211, 468) that "an object moving relativistically in suitable directions may appear to a distant observer to have a transverse velocity much greater than the velocity of light".
A few years later (in 1970) such sources were indeed discovered as very distant astronomical radio sources, such as radio galaxies and quasars. They were called superluminal (lit. "above light") sources. The discovery was a spectacular result of a new technique called Very Long Baseline Interferometry, which allowed astronomers to determine positions better than milli-arcseconds and in particular to determine the change in positions on the sky, called proper motions in a timespan of typically years. The apparent velocity is obtained by multiplying the observed proper motion by the distance and could be up to 6 times the speed of light.
In 1994 a galactic speed record was obtained with the discovery of a superluminal source in our own galaxy, the cosmic x-ray source GRS1915+105. The expansion occurred on a much shorter timescale. Several separate blobs were seen (I.F. Mirabel and L.F. Rodriguez, Nature 371, 48, "A superluminal source in the Galaxy") to expand in pairs within weeks by typically 0.5 arcsec. Because of the analogy with quasars, this source was called a microquasar.
