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 which would not be compatible with the theory of special relativity.

A pair of plasma clouds expelled from the microquasar GRS1915+105. From March 18 1994 to April 16 it moves about 1 arcsec. The source is in the general direction of the galactic center at a distance of 12 kiloparsec =3.7×10²⁰m. At this distance, 1 arcsec corresponds to 1.8×10¹⁵ m. The expansion takes place in a month or 2.5×10⁶ s, so one observes an apparent expansion of about 10⁹ m/s, about three times the speed of light. (From the cover of Nature, 1994, vol. 392; Mirabel and Rodriguez, Nature, 392, 673)

The explanation can be given in a fairly straightforward way as a light travel time effect. Imagine a blob of matter starting at the center of a galaxy and moving towards you very fast, nearly head-on towards you, but not exactly.

When that blob is at the center of the galaxy, it emits some light towards you. After it has moved towards you (and slightly to the side), and again emits light towards you, this light will take a shorter time to travel toward you, because it is closer to you. If you ignore this fact, then you will underestimate the true time interval (for your inertial reference frame), and so you will overestimate the speed.

In other words, if you calculate how fast that blob is moving, assuming that it is moving perpendicular to the line between you and the galaxy, and you underestimate the time interval by ignoring the fact that it is also moving towards you, then you will get a speed which can be many times the speed of light.

They are often seen in two opposing jets, one moving away and one moving toward us. If in both sources Doppler shifts are observed, the velocity and the distance can be determined independent of other observations.

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. "faster than light") sources. The discovery was a spectacular result of a new technique called Very Long Baseline Interferometry, which allowed 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.