Explanation
This phenomenon is caused because the jets are travelling very near the speed of light and at a very small angle towards the observer. Because at every point of their path the high-velocity jets are emitting light, the light they emit does not approach the observer much more quickly than the jet itself. To be more clear, the jet is essentially "chasing" the light it emits. This causes the light emitted over hundreds of years of travel to not have hundreds of lightyears of distance between it, the light thus arrives at the observer over a much smaller time period (ten or twenty years) giving the illusion of faster than light travel.
This explanation depends on the jet making a sufficiently narrow angle with the observer's line-of-sight to explain the degree of superluminal motion seen in a particular case.
Superluminal motion is often seen in two opposing jets, one moving away and one toward Earth. If Doppler shifts are observed in both sources, the velocity and the distance can be determined independently of other observations.
Read more about this topic: Superluminal Motion
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