Wednesday, June 01, 2016 by Greg White
Using a phenomenon where particles of light appear to travel quicker than light speed, researchers have demonstrated that light waves can appear to travel backwards in time.
It is a common known law in physics that nothing can travel quicker than light speed. This isn’t entirely true, however. While it is impossible for any object to accelerate quicker than light speed, theoretically, it is possible for particles to move faster than the speed of light. Tachyons, for example, are hypothetical particles that move faster than light speed, but have never been directly observed.
Although particles moving faster than light have never been observed, a team of researchers devised an experiment that simulated what an observer would see if something was approaching them quicker than light speed. This is known as a superluminal event. In the paper, published in the journal Science Advances, researchers claim they demonstrated that whenever something approaches an observer faster than the speed of light, it will 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 of the study, 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 address the challenge at hand, the team referenced a similar experiment purported by Lord Rayleigh more than 100 years ago. He hypothesized that if a supersonic jet passed overhead playing loud music, then the song would sound like it was being played backwards. By the same token, the researchers hypothesized that in a scenario where a spaceship was traveling faster than light speed, it would appear to move backwards in time from the standpoint of a 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,” Dr. Clerici added.
While this sounds impossible, it is consistent with the known laws of physics. The researchers flashed quick laser pulses on different coordinates on a screen. At every point, the laser light is dispersed out and captured by a high-speed camera that takes photographs just trillionths of a second apart.
The experiment doesn’t actually make anything move quicker than light speed. Instead, the scattering events occur at two different spots on the surface, meaning the the superluminal event captured does not transfer superluminal data.
Since the pulse shines on different spots on the screen, it gives the appearance of superluminal motion. The illusion is stirred by scattering events occurring at different spots before the light can commute minute distances. Upon observing these scattering events, the researchers bared witness to the same effect hypothesized by Rayleigh: the image appeared to project backwards in time.
“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 scientists believe their research may have implications in theoretical particle physics, geology and mechanics. It could even shed light on the internal working of Earth’s interior. The lesson to be learned? In order to make progress in science, you sometimes have to move backwards – or at least pretend to.