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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.

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