"Superluminal" Motion

Although faster than light is convenient for science fiction shows, we are reluctant to accept it as a physical reality. Animation from Kenneth Jones We think it is a sophisticated optical illusion in the active nucleus jets.

This animation (by G. Rieke) illustrates how it can happen.

At the start, we watch a bright blob moving away from a red ball nearly at the speed of light, "from the top" (the eyeball is ours!). We show the wavefronts emitted by all the flashes. Next, we rotate around and watch from the side. Then we move around and watch the same event from the top but with the blob coming almost toward us (the eyeball again!). Again, we show all the wavefronts from all flashes. Finally, we rotate down and see how it would look from the side. See how the wavefronts "pile up" because the blob nearly keeps up with the light from previous flashes.

As a result, when the blob is coming almost toward us, all the flashes arrive piled up, one immediately on top of the other, and the impression we get is that there is motion faster than light.