Ok, so, if you accelerate an electron in a magnetic field, you can make it generate light – radio light – but that is still light. This is your casual reminder that mass and energy are all interrelated. Thanks to the popularity of human-made facilities like the CERN accelerator in Europe and the stunning images of black hole-driven jets like those in active galaxies, we’re used to thinking about how accelerated particles can generate light, and in their collisions, sometimes even generate new particles. Slam two protons together hard enough and you can even find Higgs bosons in the resulting mess.

While slamming particles together to generate light and different particles is pretty standard, it should also be possible to slam beams of light together to get particles and different kinds of light. This possibility is called the Breit-Wheeler effect, and in 1934 it was predicted that collisions of photons could generate matter and antimatter. Unfortunately, testing this idea isn’t as simple as pointing two standard lasers into one another and watching for matter/anti-matter reactions where they collide.

It turns out that, to collide light, you have to first have a near-miss between heavy, charged atoms – ions – like ionized gold. 

In a new paper in Physics Review Letters, researchers led by J Adam describe how photons associated with gold ions accelerated to near light speed were observed to produce electrons and positrons. This experiment was performed at Brookhaven National Lab and is an amazing demonstration of the precision of these instruments. Researchers literally accelerated streams of gold ions, moving in opposite directions, so that they grazed past each other in a way that allowed their surrounding photons to collide, and in the process, proved a 90ish-year-old theory.

Now, I want to go back to the firing lasers at each other idea. The reason that doesn’t work is that the photons in lasers just don’t have the needed energy. This kind of implies that if we someday do have lasers with photons of enough energy, we could try this experiment again with just the photons, but the photons that miss each other would be very dangerous, so let’s maybe not do that experiment.

More Information

U.S. DoE press release

“Measurement of e+e− Momentum and Angular Distributions from Linearly Polarized Photon Collisions,” J. Adam et al., 2021 July 27, Physical Review Letters