What's faster than light

Faster than light

When a loudspeaker moves faster than sound to a listener, a fascinating effect occurs. In the ears of the listener, the piece of music coming from the loudspeaker plays backwards. Physicists have now set up a sophisticated experiment in order to be able to measure the analog effect for light as well. In fact, with a very fast detector, they were able to detect a kind of time reversal. They emphasize that their results, published in the journal “Science Advances”, do not contradict Albert Einstein's theories. Neither matter nor information was transmitted faster than light.

Superluminar light source

"But light sources can move faster than the speed of light," says Matteo Clerici from the University of Glasgow. An important condition for this: The faster than light propagation is not linked to the movement of matter. Together with his colleagues, Clerici succeeded in using a rapidly clocked laser to construct a light source that, from the point of view of a detector, was superluminar, i.e. moved at faster than light. For this experiment, the physicists directed the very short light pulses from the laser onto a reflective surface. The reflections now served as the source of light that was supposed to move faster than light. Simple geometric relationships were sufficient for the effect. With a high-speed camera, the researchers recorded the movement of the reflections over the reflector screen. At an angle of 65 degrees between the camera and the reflector screen, the reflections spread from the view of the camera at almost half the speed of light. The camera detected a movement of the reflexes from left to right on the reflector screen. However, if the viewing angle was reduced to 25 degrees, the reflections from the view of the camera moved at about twice the speed of light. Light pulses emitted later could overtake the previously emitted and reached the camera first. Correspondingly, the camera images showed a propagation of the reflexes in the opposite direction from right to left.

With their experiments, the researchers confirm an interesting thought model for light sources that can move faster than light even relative to an observer. According to Clerici, these phenomena could have practical applications for evaluating seismograms. Because it cannot be ruled out that earthquake waves within the earth would also be scattered on diffracted surfaces and thus lead to strange measurement results on the earth's surface.