Talk about getting something for nothing. Physicists predict that just by shooting charged particles through an electromagnetic field, it should be possible to generate light from the empty vacuum. In principle, the effect could provide a new way to test the fundamental theory of electricity and magnetism, known as quantum electrodynamics, the most precise theory in all of science. In practice, spotting the effect would require lasers and particle accelerators far more powerful than any that exist now.
“I’m quite confident about [the prediction] simply because it combines effects that we understand pretty well,” says Ben King, a laser particle physicist at the University of Plymouth in the United Kingdom, who was not involved in the new analysis. Still, he says, an experimental demonstration “is something for the future.”
Physicists have long known that energetic charged particles can radiate light when they zip through a transparent medium such as water or a gas. In the medium, light travels slower than it does in empty space, allowing a particle such as an electron or proton to potentially fly faster than light. When that happens, the particle generates an electromagnetic shock wave, just as a supersonic jet creates a shock wave in air. But whereas the jet’s shock wave creates a sonic boom, the electromagnetic shock wave creates light called Cherenkov radiation. That effect causes the water in the cores of nuclear reactors to glow blue, and it’s been used to make particle detectors.
However, it should be possible to ditch the material and produce Cherenkov light straight from the vacuum, predict Dino Jaroszynski, a physicist at the University of Strathclyde in Glasgow, U.K., and colleagues. The trick is to shoot the particles through an extremely intense electromagnetic field instead.
According to quantum theory, the vacuum roils with particle-antiparticle pairs flitting in and out of existence too quickly to observe directly. The application of a strong electromagnetic field can polarize those pairs, however, pushing positive and negative particles in opposite directions. Passing photons then interact with the not-quite-there pairs so that the polarized vacuum acts a bit like a transparent medium in which light travels slightly slower than in an ordinary vacuum, Jaroszynski and colleagues calculate.
Putting two and two together, an energetic charged particle passing through a sufficiently strong electromagnetic field should produce Cherenkov radiation, the team reports in a paper in press at Physical Review Letters. Others had suggested vacuum Cherenkov radiation should exist in certain situations, but the new work takes a more fundamental and all-encompassing approach, says Adam Noble, a physicist at Strathclyde.
Spotting vacuum Cherenkov radiation would be tough. First, the polarized vacuum slows light by a tiny amount. The electromagnetic fields in the strongest pulses of laser light reduce light’s speed by about a millionth of a percent, Noble estimates. In comparison, water reduces light’s speed by 25%. Second, charged particles in an electromagnetic field spiral and emit another kind of light called synchroton radiation that, in most circumstances, should swamp the Cherenkov radiation.
Still, in principle, it should be possible to produce vacuum Cherenkov radiation by firing high-energy electrons or protons through overlapping pulses from the world’s highest intensity lasers, which can pack a petawatt, or 1015 watts, of power. However, Jaroszynski and colleagues calculate that in such fields, even particles from the world’s highest energy accelerators would produce much more synchrotron radiation than Cherenkov radiation.
Space could be another place to look for the effect. Extremely high energy protons passing through the intense magnetic field of a spinning neutron star—also known as a pulsar—should produce more Cherenkov radiation than synchrotron radiation, the researchers predict. However, pulsars don’t produce many high-energy protons, says Alice Harding, an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and the particles that do enter a pulsar’s magnetic field should quickly lose energy and spiral instead of zipping across it. “I’m not terribly excited about the prospect for pulsars,” she says.
Nevertheless, King says, experimenters might see the effect someday. Physicists in Europe are building a trio of 10-petawatt lasers in Romania, Hungary, and the Czech Republic, and their counterparts in China are developing a 100-petawatt laser. Scientists are also trying to create compact laser-driven accelerators that might produce highly energetic particle beams far more cheaply. If those things come together, physicists might be able to spot vacuum Cherenkov radiation, King says.
Others are devising different ways to use high-power lasers to probe the polarized vacuum. The ultimate aim of such work is to test quantum electrodynamics in new ways, King says. Experimenters have confirmed the theory’s predictions are accurate to within a few parts in a billion. But the theory has never been tested in the realm of extremely strong fields, King says, and such tests are now becoming possible. “The future of this field is quite exciting.”