The fact that the speed of light is constant is one of the cornerstones of physics. Nothing carrying information can travel faster than the speed of light, but by playing with geometries physicists can create faster-than-light motion. And that’s when the fun starts.
An international team of scientists set up an experiment to simulate what a stationary observer would see when looking at a superluminal (faster than light) event. In their paper, published in Science Advances, they demonstrated that if a light source approaches the observer faster than the speed of light, then the images appear to move backwards in time.
“The existence of an absolute limit, the speed of light, is the natural source of the question: what would happen if we cross this limit?” lead author Mattero Clerici told IFLScience. “Light sources, however, may move faster than the speed of light when their speed is not associated with the physical motion of matter. Following this line of thought, we devised a way to experimentally investigate the [effects] of superluminal motion.”
To tackle the problem, the researchers looked back at work by Lord Rayleigh over 100 years ago, who performed a similar thought experiment with the speed of sound. He posited that if a supersonic jet was passing overhead blasting some music very loudly, you'd hear the song playing in reverse, as well as the sonic boom and the bits of the song emitted earlier. In a very similar, but theoretical scenario, a spaceship moving faster than the speed of light would appear to move backwards to the casual stationary observer.
“If a source of light approaches an observer at superluminal speeds, the temporal ordering of events is inverted and its image appears to propagate backward,” said Dr. Clerici.
The set up of the experiment, and the detection first with subluminal speed (on the left) and then at superluminal. Once the source is faster than the speed of light, the image is propagated backwards in time.
This may all sound like science fiction, but it's very real, and doesn't violate the laws of physics. Essentially, what the team are did was shine extremely quick laser pulses at different points on a screen. At each point, the laser light is scattered out, captured by a high-speed camera that can photograph events a few trillionths of a second apart.
This experimental setup doesn't actually send anything material faster than light. Rather, the scattering events simply happen at two distinct regions on the surface, so the superluminal event photographed does not transfer superluminal information.
But as the pulse shines at different points on the screen, it gives the illusion of superluminal motion, because the scattering events occur at different points before the light should be able to travel the small distance. By observing these scattering events, the team noticed the same effect posited by Rayleigh; namely, the images appeared to go “backwards” in time.
True superluminal motion, though, cannot be achieved due to the laws of relativity. If particles don't have mass they can travel at the speed of light, but if they do have mass they need an infinite amount of energy to get to the speed of light, and they can't go beyond.
“Faster than light propagation of objects or information has never been observed, and is also forbidden by the accepted physical understanding,” said Dr. Clerici. “Yet there are partial answers that can be provided, such as how would we see a superluminal object. We experimentally show what to expect, that is, an object propagating back in time.”
The team believes that this might have applications both in theoretical physics, as a testing ground for superluminal ideas, and in geology and mechanics. Sounds waves and mechanical vibration may give rise to detectable temporal inversions. Seismic waves, for example, could hit different surfaces and have time reversal effects, giving backwards information. So understanding time-reversal could be important to better work out what goes on in Earth's interior.