DR STEPHEN UNWIN / SCIENCE PHOTO LIBRARY DR STEPHEN UNWIN / SCIENCE PHOTO LIBRARY
Sequence of false-colour radio images of the quasar 3C 273, showing the superluminal expansion of its jet. The images show the movement of a 'knot' in the jet (right) away from the nucleus at left (red). The four images were made in Oct 85, May 86, Nov 86 & May 87 (top to bottom). The apparent speed of the knot is 0.9 milliarcseconds per year, which correponds to about six times the speed of light. This can be explained if the jet is pointing almost along our line of sight. In this case the knot can appear to travel faster than light without actually doing so, an effect of the same theory of relativity which prohibits real motions faster than the speed of light.
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