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superluminal motion

(faster-than-light movement)

Superluminal motion refers to faster-than-light motion, a term used in astronomy for discussing observations that apparently indicate such motion. The motion in question is of knots (fat parts) in AGN jets (as per radio observations), which observations over time sometimes show to be moving across the celestial sphere at a rate, which, with the determined distance to the galaxy, indicate the knot is moving faster than the speed of light (c).

The general explanation is that the jet is relativistic and beamed toward Earth with a small angle between the jet and the line of sight, and the effect is due to EMR from the knot being emitted significantly later than from the galaxy. With the right geometry, the motion across the sky looks faster than c. Some observations suggest some of the jet angles don't fit with this explanation, so possibly there is more to it, though actual "faster than light" travel is not considered a reasonable explanation.


Referenced by:
light echo