New work at the University of Michigan, now written up in Physical Review Letters, discusses the possibility of producing matter and antimatter from the vacuum. The idea is that a high-energy electron beam combined with an intense laser pulse can pull matter and antimatter components out of the vacuum, creating a cascade of additional particles and anti-particles. UM Engineering research scientist Igor Sokolov has this to say about the theoretical study:
“We can now calculate how, from a single electron, several hundred particles can be produced. We believe this happens in nature near pulsars and neutron stars…”
That would make the vacuum a lively place indeed, as Sokolov acknowledges:
“It is better to say, following theoretical physicist Paul Dirac, that a vacuum, or nothing, is the combination of matter and antimatter—particles and antiparticles.Their density is tremendous, but we cannot perceive any of them because their observable effects entirely cancel each other out.”
Of course, to produce these hundreds of particles from a single electron, we need not only an ultra-high-intensity laser beam but a two-mile long particle accelerator. But the model is intriguing. We’re deep in the realm of quantum electrodynamics (QED), which describes how electrically charged particles interact by exchanging photons. Richard Feynman called QED ‘the jewel of physics’ because of its predictive capabilities. Feynman’s QED: The Strange Theory of Light and Matter (1986) is based on a lecture series designed for the general public, and without it, those of us who are mathematically challenged wouldn’t have a chance with QED ideas.
Reading the Sokolov paper, I can see that it’s time for me to return to Feynman, or maybe this Wikipedia entry, which calls QED ‘a perturbation theory of the electromagnetic quantum vacuum,’ and goes on to provide a useful backgrounder on the theory’s evolution. But let’s say this: Normally, matter and antimatter destroy each other when they come into contact. The UM team is interested in how this annihilation may be averted under certain conditions. Thus Sokolov colleague John Nees:
“…in a strong electromagnetic field, this annihilation, which is typically a sink mechanism, can be the source of new particles. In the course of the annihilation, gamma photons appear, which can produce additional electrons and positrons.”
One experiment in the 1990s produced effects that the Sokolov paper calls ‘weak and barely observable,’ creating gamma photons and electron/positron pairs, but the new work suggests that ramping up the laser pulse intensity should produce a more definitive result. The UM team’s equations model how a sufficiently strong laser could cause the creation of more particles than were initially injected into the experiment through a particle accelerator. Sokolov again:
“If an electron has a capability to become three particles within a very short time, this means it’s not an electron any longer. The theory of the electron is based on the fact that it will be an electron forever. But in our calculations, each of the charged particles becomes a combination of three particles plus some number of photons.”
The HERCULES laser at the University of Michigan is one way to test this work in a small-scale laboratory setting, but a similar laser would have to be built at a particle accelerator like the SLAC National Accelerator Laboratory at Stanford to work out all its implications, and because no such construction is planned, it may be some time before we can take these ideas forward. The team speculates that their theory may have applications for inertial confinement fusion — I bring this up particularly because of the interest of the Project Icarus team in ICF — but it’s a case of intriguing theory awaiting the experimental infrastructure to test it adequately.
The paper is Sokolov et al., “Pair Creation in QED-Strong Pulsed Laser Fields Interacting with Electron Beams,” Physical Review Letters Vol. 105, 195005 (2010) Abstract/Preprint. The paper is so dense in mathematics that I want to be sure to give you this news release as well.
