The notion that the speed of light is the highest attainable velocity is central to modern physics. Scores of predictions made assuming this fact to be true have been verified to high accuracy since Einstein first published his theory of relativity early in the twentieth century (More Precisely 22-1). Astronomers almost universally agree that some reinterpretation of these apparent superluminal (that is, faster than light) quasar motions is needed.

One alternative assumes that quasars are not at cosmological distances from us. If quasars were relatively local, the angular motion of the blobs would not imply high physical velocities, and the problem of explaining how the individual blobs move faster than light would not exist. But if the quasars are local and not distant, the observed redshifts of their spectral lines cannot be a distance indicator, and we are forced to find some other explanation for the large quasar

Several alternative solutions have been proposed to account for the apparent superluminal motion of the blobs without requiring the quasars to be local or the speeds to be truly faster than light. The most straightforward model suggests that the observed changes in quasar structure are not caused by actual motions of the interior blobs at all but rather by variations of the radio intensity of stationary blobs. In other words, the interiors of quasars may resemble the blinking lights that sometimes appear on movie theater marquees. These marquee bulbs blink on and off in a programmed sequence that gives the illusion of motion, but the individual bulbs are of course stationary. Likewise, blinking radio sources within the quasar may suggest motion where none really exists. A variation on this model explains the motions as a different kind of illusion—a projection effect, produced by blobs moving almost precisely along our line of sight at slightly less than the speed of light. Calculations of how these blobs would look from Earth indicate that they could in fact appear to be moving faster than light.

None of the alternative models is simple. They all require peculiar geometries, and no one model is agreed upon by all researchers. We still lack a complete explanation of the puzzling phenomenon of superluminal velocities. However, the existing models demonstrate that although the interiors of quasars are exceedingly complex and not terribly well understood, there is no compelling need to discard the laws of physics in order to explain them.