Physicists have captured the “surfing” of protons in shock waves.
Protons can surf some truly gnarly waves.
A new experiment suggests that the subatomic particles can be accelerated by a process akin to surfers catching waves. The protons get a speed boost not from ocean swells, but from shock waves within the plasma, a mixture of electrically charged particles. Such shock waves are sonic boom–like disturbances marked by an abrupt increase in density, temperature and pressure.
The research could help scientists better understand some of the high-energy particles that zip through the cosmos. Shock waves in space are thought to propel charged particles, but it’s still not fully understood how particles get their pep.
In the experiment, which mimicked certain types of cosmic shock waves, protons reached energies up to 80,000 electron volts, physicists report August 19 in Nature Physics. In space, similar shock waves occur where the outflow of charged particles from the sun meets the Earth’s magnetic field, for example, and also where those particles slow down dramatically as they approach the edge of the solar system, at what’s called the termination shock
Scientistshave used powerful lasers to physically reproduce such cosmic shock waves on a smaller scale. In the experiment, a laser explosion vaporized the target, creating a plasma explosion in a cloud of hydrogen gas. Measurements showed that a shock wave was formed and the protons in the gas accelerated.
Scientists predict that protons can be accelerated by a process called shock acceleration, which occurs in the presence of a magnetic field. The particle is repelled by the electric field of the shock wave, and the magnetic field keeps the particle at rest. If a particle slips out of the shock wave, the magnetic field will redirect the particle back to the wave so that the proton can redirect it back. Julien Fuchs of CNRS and the High-Intensity Laser Laboratory in Palaiseau, France, said:
Of course, there is no such automatic return for human navigators. Unfortunately, he mused, “I think they’ll love this.”
However, measurements alone do not indicate whether shockwaves are responsible for the acceleration of protons.” The challenge has always been to explain what exactly causes acceleration,” said plasma physicist Frederico Fiusa of the SLAC National Accelerator Laboratory in Menlo Park, California, who was not involved in the study.
So Fuchs and his colleagues created a computer simulation of the experiment. Comparison of simulations and real-world data shows that protons drive the shockwave.
“This is definitely an interesting result,” says plasma physicist Caroline Kurantz of the University of Michigan at Ann Arbor, who hopes further research will uncover more direct evidence not based on simulation. “This is very promising for future work.”
NB: Images are for illustrative only.