Superluminal Velocities

Some discrete (defined in the next chapter) sources within quasars have been observed to change positions over a brief period. Their motion generally appears to the observer to be radially outward from the center of the quasar image. The apparent velocities of these objects have been measured, and if the red shifts actually do represent the distance and recession velocities of the quasar, then these discrete objects are moving at speeds greater than the speed of light! We call these apparent speeds superluminal velocities or superluminal expansion.

Well, we know this is impossible, right? So astronomers had to come up with a more reasonable explanation. The most widely accepted explanation is that the radiation emitted from the object at the first position (A in the diagram below) has travelled farther and thus taken longer to reach Earth than the radiation emitted from the second position (B), 5 LY from A.

Suppose A is 4 light years (LY) farther from Earth than B (that is, AC is 4 LY). Moving just a bit under the speed of light, the object takes just over 5 LY to travel from A to B. However, the radiation it emitted at A reaches C in 4 years. As that radiation continues toward Earth, it is one year ahead of the radiation emitted toward us by the object when it arrived at B. When it finally (after several billion years) reaches Earth, the radiation from A is still one year ahead of the radiation from B. It appears to us that the object has moved tangentially out from the center of the quasar, from C to B and (from the Pythagorean theorem) has gone 3 LY in just over one year! That the object appears to travel at nearly three times light speed is only because of the projection effect, with its radiation travelling from A to C in 4 years, while the object itself went from A to B in 5 years.