The quantum world is a funny place. If you want to boost an electron a little, you can simply bump it with a photon – a little bit like billiard balls on a pool table. But if you want to stop an electron completely, you can expose it to so much light that it begins to emit so much radiation that it ceases any movement. Good luck doing that with billiard balls.
The curious phenomenon is known as “radiation reaction” and it was achieved in the lab for the first time. This is reported in Physical Review X. The team, led by researchers at Imperial College London, was able to create the intense beam of light by focusing on a spot a few millionths of a meter across for a mindboggling short time, about 40 quadrillionths of a second.
"One thing I always find so fascinating about this is that the electrons are stopped as effectively by this sheet of light, a fraction of a hair's breadth thick, as by something like a millimeter of lead. That is extraordinary," study co-author Professor Alec Thomas, from Lancaster University and the University of Michigan, said in a statement.
The laser beam was thin and quick but extremely powerful, a million billion times the brightness of the light on the surface of the Sun. This was necessary to try to stop the high-energy beam of electrons. The laser was made of photons at visible light wavelengths, but the interaction with the electron bumped them up to much shorter wavelengths (and higher energies) – all the way into the gamma-ray domain.
“We knew we had been successful in colliding the two beams when we detected very bright high-energy gamma-ray radiation,” senior author Dr Stuart Mangles, from the Department of Physics at Imperial, explained. “The real result then came when we compared this detection with the energy in the electron beam after the collision. We found that these successful collisions had a lower than expected electron energy, which is clear evidence of radiation reaction.”
This phenomenon cannot be explained by classical physics and instead requires quantum models. The lab analysis allows for insights into this new phenomenon.
“Testing our theoretical predictions is of central importance for us at Chalmers, especially in new regimes where there is much to learn,” co-author Professor Mattias Marklund of Chalmers University of Technology, added. "Paired with theory, these experiments are a foundation for high-intensity laser research in the quantum domain."
Radiation reaction is expected to happen in extreme space environments like stellar black holes and quasars. Measuring what happens in the lab can actually help us solve some of the big mysteries of the universe.