Like an audience bemused by a clever magician, astronomers are challenged by a kind of cosmic sleight of hand. It makes certain distant astronomical material appear to move faster than light.
Astronomers don't think the motion actually exceeds the speed of light - Einstein's universal speed limit. But a dozen years after their discovery, the so-called superluminal sources remain a major cosmic puzzle.
So far, such speedy matter has been associated with seven radio sources. These are energetic objects observed mainly by the radio noise they emit. The appearance of matter moving away from them at speeds up to 10 times that of light is widely thought to be a trick of perspective. If matter, moving at a substantial fraction of the speed of light, heads toward an observer so that its course makes only a small angle with the observer's line of sight, there is an illusion of faster-than-light motion.
One popular notion is that there is nothing especially unusual about the superluminal sources. They generally are quasars (quasi-stellar radio sources). These are objects so compact they appear starlike in photographs. Yet they are more powerful than an entire ordinary galaxy full of stars. Quasars often appear to shoot out jets of material. Some astronomers now suggest this matter can move at speeds near that of light. They then identify the superluminal sources as those objects whose high-speed jets happen to point our way.
Picking up this concept recently in Nature, R.T. Schilizzi and A.G. Bruyn of Holland's Westerbrok radio astronomy facility suggest the superluminal illusion may be temporary. They report observations which indicate that such jets may wobble or precess. The seven we now see may have only wandered close to our line of sight temporarily. Within a few thousand to a few million years, they may no longer point this way and the superluminal effect will be gone.
A related concept, which some astronomers favor, conceives of radio quasars as all being members of a single class of astronomical objects. The rather substantial differences among quasars then are considered to be due mainly to the angles at which Earth-based instruments view them. Again, the faster-than-light motion associated with seven of them is considered an illusion of perspective.
Such schemes seeking an essential unity in these objects are scientifically controversial. But they illustrate that the discovery of superluminal motion has given central importance to the question of family relationships among the powerful matter-ejecting radio sources. It has raised related questions as to whether or not there are large masses of cosmic material moving at near-light speeds.
As Richard Procas of West Germany's Max Planck Institute for Radio Astronomy observed recently in reviewing the subject in Nature, superluminal radio sources haven't violated existing physical law. But ''12 years of intensive study (of them) have raised more questions than they have answered.''