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Coherence and superradiance from a quasi-particle accelerator

"Nothing can move faster than light. Absolutely nothing, except perhaps rumors." This belief comes from the British writer and humorist Douglas Adams, who decided to challenge one of the fundamental ideas in Einstein's theory of relativity. But what if there were tiny particles that could? These super-speedy particles, known as tachyons, were first suggested by a German physicist named Gerald Feinberg back in 1967. Since then, the idea of breaking the speed limit of light has captured the imagination of scientists and thinkers alike.

A group of international scientists led by researchers from EuPRAXIA partner Instituto Superior Técnico (Laser and Plasma Group, Institute of Plasma and Nuclear Fusion) and including experts from the University of Rochester and University of California Los Angeles in the United States and the Applied Optics Laboratory in France, have discovered something fascinating. If these particles exist, they might be the key to creating a new kind of super-bright light source, just as powerful as the most advanced ones we have today, but much smaller. This groundbreaking research has been published in a paper in Nature Photonics today.

Instead of focusing on individual particles (which can't go faster than light), they looked at something called "quasi-particles." These quasi-particles are the result of electrons moving together in sync, similar to the Mexican wave that goes around football stadiums even though every person stays put. The fascinating part is that these quasi-particles can travel at any speed, even faster than light, and can withstand intense forces.

Representation of super-intense light cones (yellow). These light cones form from a quasi-particle located at the centre of each cone. Quasi-particles are a result of a plasma electron (spheres) accumulation at the back of each plasma wave (blue). Credits: Bernardo Malaca/IST (OSIRIS simulation).

According to Jorge Vieira, a professor at the Instituto Superior Técnico, and coordinator of this study, "these special quasi-particles provide an exciting new way to explore and suggest extremely powerful sources of light that nobody had thought of before.” This approach is simple enough that it can be tried in dozens or even hundreds of labs around the world, bringing the theoretical concept a step closer to becoming a reality.

Bernardo Malaca, a doctoral student at IST and the study's primary author, said: "The flexibility is enormous. Even though each electron is performing relatively simple movements, the total radiation from all the electrons can mimic that of a particle moving faster than light or an oscillating particle, even though there isn't a single electron locally that's faster than light or an oscillating electron."

Light sources have a huge impact on our lives, from science and technology to everyday applications. For example, they play a crucial role in non-destructive imaging (like scanning for viruses or checking product quality), understanding biological processes (like photosynthesis), manufacturing computer chips, and exploring the behavior of matter in planets and stars.

Bernardo Malaca added: "We started with the basics - what conditions make multiple particles emit light as if they were one? - and then applied that to the most intense sources of light." The most powerful sources of light, like free electron lasers, are rare and massive, making them impractical for most laboratories, hospitals, and businesses. But with the theory proposed here, these quasi-particles could produce incredibly bright light with just a tiny distance to travel, potentially sparking a scientific and technological revolution in labs everywhere.

The researchers explored quasi-particles in plasma waves, using intense laser and electron beams. To study the behavior of these quasi-particles and their light emissions, the researchers ran advanced computer simulations on supercomputers available in Europe through the EuroHPC consortium.

According to the latest theory, these almost-particles only have to travel a tiny fraction of an inch to create extremely bright light. So, if this theory turns out to be true in experiments, it could lead to a small but significant scientific and social revolution.

Further reading:

Malaca, B., Pardal, M., Ramsey, D. et al. Coherence and superradiance from a plasma-based quasiparticle accelerator. Nat. Photon. (2023).

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