Colliding photons were spotted making matter. But are the photons ‘real’?

By | August 9, 2021

Collide light with light, and poof, you get matter and antimatter. It sounds like a simple idea, but it turns out to be surprisingly hard to prove.

A team of physicists is now claiming the first direct observation of the long-sought Breit-Wheeler process, in which two particles of light, or photons, crash into one another and produce an electron and its antimatter counterpart, a positron. But like a discussion from an introductory philosophy course, the detection’s significance hinges on the definition of the word “real.” Some physicists argue the photons don’t qualify as real, raising questions about the observation’s implications.

Predicted more than 80 years ago, the Breit-Wheeler process had never been directly observed, although scientists have seen related processes, such as light scattering off of light (SN: 8/14/17). New measurements from the STAR experiment at Brookhaven National Laboratory’s Relativistic Heavy Ion Collider match predictions for the elusive transformation, Brookhaven physicist Daniel Brandenburg and colleagues report in the July 30 Physical Review Letters.

“The idea that you can create matter from light smashing together is an interesting concept,” says Brandenburg. It’s a striking demonstration of the physics immortalized in Einstein’s equation E=mc2, which revealed that energy and mass are two sides of the same coin.

Whether the observation truly qualifies depends on whether the photons are considered “real,” as demanded by the Breit-Wheeler process, or “virtual.” In particle physics, virtual particles are ones that appear only for brief instants and don’t carry their normal masses.  

Photons from a commonplace source of light, like a lightbulb or a laser, are real, physicists agree. But the bona fides of Brandenburg and colleagues’ photons are up for debate. That’s because the light the team is colliding comes from an unusual source.

In the Relativistic Heavy Ion Collider, atomic nuclei travel at nearly the speed of light before ramming into one another. Those speedy nuclei are surrounded by electromagnetic fields, and those fields have photons associated with them. Normally, such photons from electromagnetic fields are virtual. But in the experiment, the photons act as if they are real due to the high speeds at which the two nuclei are zipping along.

The new evidence for the Breit-Wheeler process comes from collisions where the nuclei just missed one another. In those cases, the electromagnetic fields of the two nuclei overlap, and two photons from those fields can collide. So the researchers looked for near-misses that spit out one electron and one positron.

When two atomic nuclei (red) pass close by, two particles of light (yellow) collide, producing a negatively charged electron ( e) and a positively charged positron (e+).BNL

But, says study coauthor Zhangbu Xu, a physicist also at Brookhaven in Upton, N.Y., “the issue is how you actually say these are from [real] photons, not from other processes.” To shore up the case that the particles came from real photons, the researchers studied the angles between those particles, which differ depending on whether real or virtual photons collided. The angles matched expectations for real photons, suggesting that the team had seen the legit Breit-Wheeler process.

Still, “strictly speaking,” says particle physicist Lucian Harland-Lang of the University of Oxford, the experiment is “kind of one step removed” from the true Breit-Wheeler process. Although the photons behave almost as if real, they are technically virtual.

Brandenburg and colleagues take a different view, akin to a physics version of the classic duck test: If it walks like a duck and quacks like a duck, then it probably is a duck. If the reality of a photon is based only on how it behaves then these would be real photons.

And the scientists’ measurements back that up, says laser plasma physicist Stuart Mangles of Imperial College London, who was not involved with the new study: “Everything they’re measuring about it makes it look like a real photon.” However, Mangles notes that the photons are still virtual by some definitions: Unlike normal photons, which have no mass, these photons do have mass.

One way to skirt thorny questions about the definition of reality would be to perform this experiment with indisputably real photons. Mangles and others are working toward detecting the Breit-Wheeler process with lasers, which produce light that’s as real as the light allowing you to read this article. That, physicists are hoping, will clinch the case for colliding light making matter.