How to make matter from light? Physicists propose ingenious tool

Scientists have long known the process by which light can be turned into matter, but a new collider proposed by a team of physicists could lead to a new range of experiments.

Converting light into matter may sound like alchemy, but it's a natural outcome of physics – one that scientists have been demonstrating to varying degrees for decades. Now, a team of European physicists is proposing a way to do it much more simply.

If the approach works as the researcher's initial calculations suggest, the results are unlikely to immediately answer any vexing question, physicists say. The fundamental science behind the process of turning light to matter is already well understood. But it would be a new tool in physicists' toolkit.

Currently, the process of getting packets of light, known as photons, to collide and make particles can be a complicated business, requiring a few tricks. But the new technology might enable a range of new experiments, which could lead to unexpected answers or uses.

"Often when people first get an idea for some big thing, it turns out to be fantastically useful for something else," says Chris Quigg, a senior theorist at the Fermi National Accelerator Laboratory in Batavia, Ill., who was not involved in the study.

The tool is a collider for photons, the subatomic particles associated with visible light and other forms of electromagnetic radiation, such as radio waves and gamma rays. By crashing them head-on, the collider would turn the photons into electrons and their antimatter counterparts, positrons. Calculations suggesting how this might work appear in the current issue of the journal Nature Photonics.

The theory behind this was first proposed in 1934 by two American physicists, Gregory Breit and John Wheeler. But it wasn't until 1997 that scientists working at the Stanford Linear Accelerator Center in Stanford, Calif., were able to successfully carry out the first true photon-to-photon collision and create the two particles.

They did it by using electrons to ricochet light back into itself. The team accelerated a beam of electrons to high energies, then blasted the beam with a laser. Some of the photons in the laser scattered off these electrons, traveling back toward the laser beam and picking up energy in the process. When these higher-energy photons collided with the additional photons the laser was sending out, the collisions produced pairs of electrons and their antimatter counterparts, positrons.

While the research team saw positrons, as Drs. Breit and Wheeler predicted, it had to fall back on modeling to piece together the events that led to the generation of the positrons.

With their new plans, a quartet of physicists from Britain and Germany are proposing a collider that performs the experiment in a simpler, more direct fashion and lays bare the entire process.

"The experimental design we propose can be carried out with relative ease and with existing technology," said Oliver Pike, a physicist at Imperial College in London who led the team, in a prepared statement.

A lower-energy electron beam would smack into a gold target, generating a beam that includes electron-positron pairs as well as high-energy gamma ray photons. Magnets would deflect the electrically charged electrons and positrons, leaving only gamma ray beam. That beam would head into a tube smaller than a thimble and kept in a high vacuum. Another laser would heat the tube's interior, generating additional photons. The gamma rays would hit photons the heated tube emits, generating electron-positron pairs, which would be detected as they escape from the other end of the tube.

The hotter the tube, the more electron-positron pairs the collisions would create, the researchers estimate. By boosting the energy of the electron beam, the team suggests that the collisions could produce heavier particles than electrons or positrons. These heavier particles are known collectively as hadrons, and are built from combinations of smaller particles known as quarks. Hadrons include the familiar proton and neutron, as well as a range of other particles from the particle-physics zoo.

Indeed, high-energy photon-to-photon colliders could help scientists study some of these particles. Researchers have been exploring ways to use photon-to-photon colliders to create vast numbers of Higgs bosons in a next-generation linear accelerator. The Higgs boson, whose discovery was announced in 2012, is a particle associated with a quantum field that imparts mass to other subatomic particles. But to study the particle's properties in detail, researchers must generate large quantities of them.

The immediate scientific payoff for using the newly proposed approach to make matter from light could well be small, suggests Dr. Quigg at Fermilab.

Out of the seven interactions between matter and light physicists have identified and verified, making two beams of light collide "has not been done," he says. "On the other hand, the reverse reaction – electrons and positrons annihilating to make two photons – is done every day at every laboratory. We know all about the fundamental science about that."

That said, if researchers can produce a sufficiently dense collection of photons in the tube to make them an easy target to hit, some interesting physics might emerge form such experiments, Quigg says. "You could imagine shooting all sorts of other particles at them." 

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