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How fast can quasar jets travel?

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six quasar host galaxies — Quasar Six quasar host galaxies, as observed by the Hubble Space Telescope.
Shown are apparently normal, solitary galaxies (left), colliding galaxies (centre), and merging galaxies (right).

Quasar jets are highly collimated beams of plasma that are propelled out along the rotation axis of a central accretion disk at speeds often approaching that of light.1

While these jets move at relativistic speeds—meaning they travel at a significant fraction of the speed of light—astronomers sometimes observe a phenomenon known as "superluminal expansion," where components within the jets appear to move at velocities exceeding the speed of light when viewed from Earth. This is an optical illusion caused by the jet being directed at a small angle toward the observer; because the material is moving close to the speed of light, the time between emission events is systematically underestimated by the observer, making the apparent velocity across the line of sight seem much faster than the actual physical velocity.2