Scottish Scientists Create Images Moving Faster Than Light, Indicating That Images Can Travel Back in Time
British physicist Lord Rayleigh predicted the retrograde propagation of sound waves nearly a century ago. Rayleigh argued that since the speed of sound is constant, when a sound source moves faster than the speed of sound, the sound waves would propagate towards the source (instead of radiating outward as usual). As a result, sound appears to be reversed in the direction of time.
Illustration of a laser scanning the surface of the Moon. (Photo: Sabine Hossenfelder).
However, researchers have yet to find a straightforward way to test Rayleigh’s argument. Sound travels at 1,225 km/h, meaning that to hear a three-second audio clip playing backward, a supersonic jet would need to fly at Mach 2 (twice the speed of sound). At that point, the sound would begin to replay more than a kilometer away from the listener. However, scattering and absorption of sound waves in the air would render the sound inaudible, making it difficult to verify Rayleigh’s experiment.
According to Live Science, physicist Daniele Faccio and his colleagues at Heriot-Watt University in Scotland realized that if Rayleigh’s predictions about sound were correct, light could also be reversed. Light propagates at a speed of 1.1 billion km/h, and its wavelengths are much smaller compared to sound waves. This means scientists do not need a supersonic jet or kilometers of distance; they can conduct their experiments in a regular room.
The biggest challenge they faced was that nothing can travel faster than the speed of light. To create an object moving faster than the speed of light, Faccio employed a bizarre phenomenon known as “photonic boom”, described through a series of fascinating thought experiments.
The secret behind “photonic boom” is that the reflection of an object (not the object itself and its photons) can move faster than light.
Imagine a laser pointer powerful enough to point at the Moon. With a flick of your wrist, you can make the projection of the laser pointer sweep from one side of the Moon to the other. While the photons from the laser pointer travel at normal light speed, their image on the Moon moves much faster.
The projection of an object on a surface may not adhere to Einstein’s equations. (Photo: Engadget).
To capture the reversed time in their experiment, the research team projected a beam of light onto a screen and moved that beam across the screen faster than the speed of light. At the same time, they recorded the moving light beam on the screen using a high-speed camera, capturing frames at about a few picoseconds each.
As a result, the camera recorded the beam of light moving in the opposite direction to how they scanned the light beam. Thus, the image on the screen was traveling backward in time.
In a second experiment, the research team confirmed an even stranger phenomenon known as pair creation-annihilation. This phenomenon was predicted by Robert Nemiroff, a physicist at Michigan Technological University, for astronomical objects in a preprint published on arXiv in May 2015.
Faccio and his colleagues repeated the experiment on a curved screen. When the speed of the light beams exceeded the speed of light, a pair of light beams moving away from each other was created. When using a screen with a different curvature, the pairs of light beams moved towards each other, merged, and then annihilated each other.
These findings still hold many mysteries and require further analysis and understanding. Faccio’s research and his colleagues were published in the journal Science Advances on April 15.
