Published in Physical Review Letters X, the team's results shed light on a fundamental problem in modern physics, known as radiation reaction. An accurate understanding of such a process is not only of interest for fundamental physics but is also crucial for our understanding of massive and exotic astrophysical objects, such as black holes and quasars, which are permeated by electromagnetic fields of comparable strength.
Radiation reaction is the back-action on an accelerated electron from the radiation it emits. Whilst this phenomenon is well understood in the classical realm, a consensus has not yet been reached in regimes of ultra-high intensities, where a quantum approach must be adopted.
In order to provide a first direct experimental evidence of the phenomenon, the team took one of the twin laser beams of Astra-Gemini and focused it at the entrance of a helium-filled gas cell (Figure 1). Through the principle of laser wakefield acceleration - a technique used to create an electron plasma wave from a laser pulse - the group were able to accelerate a multi-GeV electron beam. The electron beam then collided with the second laser beam of Astra-Gemini, focussed down to a size comparable to the breadth of a human hair. During collision, the electrons were observed to lose a significant fraction of their energy, in a way that defies the laws of classical physics.
Upon completion of numerical simulations of the phenomenon, the team confirmed that when electrons were propagated through the short burst of intense laser light, up to 30% of their energy was lost. This effectively indicated that - at these high intensities - this tiny layer of light, the width of a human hair, was as efficient in stopping particles as half a centimetre of iron.
Figure 1: The experimental set-up of the Astra-Gemini laser (Poder et al, 2018)
Frontlined by Dr. Gianluca Sarri and Prof. M. Zepf at Queen’s University Belfast, the group found results that will help towards the construction of the next generation of large-scale laser facilities, where similar fields will be routinely generated to advance cutting-edge technology in particle acceleration, medicine, and industrial applications. As Dr Gianluca Sarri explains, these results are ground-breaking;
"This experiment is an absolute first in the area, and provides confirmation of advanced physics theories that lied untested since the 70's. It is breath-taking to see that unifying two of the most fascinating theories of modern physics, relativity and quantum mechanics, results in such an exotic behaviour of particles. Not only light acts as an iron wall, but the electrons show peculiar behaviour, which clashes with our classical understanding of Nature."
The research was supported by EPSRC and STFC. The full publication is available to view in Physical Review X.
For further information about the research, please contact Dr Gianluca Sarri (firstname.lastname@example.org)
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 K. Poder, M. Tamburini, G. Sarri, A. Di Piazza, S. Kuschel, C. D. Baird, K. Behm, S. Bohlen, J. M. Cole, D. J. Corvan, M. Duff, E. Gerstmayr, C. H. Keitel, K. Krushelnick, S. P. D. Mangles, P. McKenna, C. D. Murphy, Z. Najmudin, C. P. Ridgers, G. M. Samarin, D. R. Symes, A. G. R. Thomas, J. Warwick, and M. Zepf “Experimental Signatures of the Quantum Nature of Radiation Reaction in the Field of an Ultraintense Laser" Phys. Rev. X 8, 031004 (2018)