For the first time, researchers have observed “voids” moving faster than light, without violating the theory of relativity. These areas of darkness, devoid of matter and energy, travel at speeds that exceed the famous threshold of 300,000 km/s, while remaining compatible with Einstein’s laws, explains an article in Live Science.
This feat is based on advances in ultrafast electron microscopy. The team studied so-called phonon-polariton waves inside a thin strip of boron nitride. These phonon-polaritons are hybrid quasi-particles, born from the coupling between photons (the grains of light) and tiny vibrations of the material: they behave both like light waves and like sound waves. A sound and light show, if you will allow us this slight popularization.
To understand what it is, researchers compare these waves to the surface of a lake. On its surface, a multitude of waves and ripples intersect, add up or compensate for each other. When two ripples meet at the top, they form a higher wave; when they cross each other in their hollows, they can on the contrary cancel each other out and dig a deeper hole.
In some cases, this interference results in points where the amplitude of the wave drops to zero: these are singularities, tiny swirls of “vacuum” at the heart of the wave. In a lake, it would look like small, ephemeral, moving whirlpools. In physics, these singularities (or “dark spots”) have been known since the 1970s and theorists had previously suggested that they could move faster than light in certain configurations.
Nothing moves very quickly
Special relativity states that nothing can transport information, matter or energy faster than light in a vacuum. How can these singularities then cross this fateful threshold? Well quite simply because they are… nothing: they are points of absence, devoid of particles and energy. They do not carry any information on their own, which allows them to escape the limitation imposed on physical objects.
The researchers show that these voids don’t just go slightly faster than the speed of light. When interacting, two singularities can attract each other and accelerate exponentially to theoretically infinite speeds, just before annihilating each other. The faster they go, the more difficult it becomes to observe them, which makes the experimental demonstration particularly delicate.
In their study, published on March 25 in the journal Nature, scientists from the Technion (Israeli Institute of Technology) explain that they managed to track these dark spots thanks to a combination of nanometric spatial resolution and temporal resolution of the order of a femtosecond (one millionth of a billionth of a second). They deduce that, beyond a certain speed, these singularities cease to behave like particles, since real particles remain subject to the limit of the speed of light.
According to Ido Kaminer, professor at the Technion and co-author of the study, this discovery highlights universal laws that apply to all types of waves, from sound waves to fluid flows to complex systems like superconductors. The new microscopy methods developed could thus open a window on hitherto invisible processes in many areas of physics, chemistry and biology.
