Physicists have figured out something that stumped two fictional characters across three episodes of "The Big Bang Theory": how fusion reactors might produce axions, the ghostly particles that could explain what dark matter actually is.
The catch? It took a team of real theoretical physicists from the University of Cincinnati, Fermi National Laboratory, MIT, and Technion-Israel Institute of Technology to work it out.
Dark matter is the universe's most abundant substance — it makes up roughly 85% of all matter — yet we've never directly detected it. We know it exists only because we can see its gravitational effects: the way it warps galaxies and holds stars in their orbits. Axions are one leading candidate for what dark matter might be made of. They're theorized to be incredibly light particles that barely interact with normal matter, which is why they've been so hard to pin down.
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In a study published in the Journal of High Energy Physics, Professor Jure Zupan and his colleagues propose that deuterium-tritium fusion reactors — the kind being built in southern France as part of an international collaboration — could actually produce axions as a byproduct of generating energy.
Here's how: fusion reactors generate enormous numbers of neutrons. When those neutrons slam into the lithium-lined walls of the reactor, they trigger nuclear reactions that can create new particles. Additionally, when neutrons slow down after bouncing off other particles, they release energy through a process called bremsstrahlung ("braking radiation"), which can also spawn these elusive particles.
The sun produces axions through similar processes, but the sheer size and power of the sun means it would generate far more of them than a reactor could. Zupan's team found a different pathway — one where reactors could actually win the comparison.
"The sun is a huge object producing a lot of power," Zupan explained. "The chance of having new particles produced from the sun that would stream to Earth is larger than having them produced in fusion reactors using the same processes as in the Sun. However, one can still produce them in reactors using a different set of processes."
Why This Matters Now
The timing is significant. We're on the cusp of building fusion reactors that actually generate more energy than they consume. If those same reactors can also help us detect — or even produce — dark matter particles, we'd be solving two of physics' biggest puzzles simultaneously.
It's also worth noting the small moment of vindication here: in "The Big Bang Theory," Sheldon and Leonard sketched out the sun-based approach on a whiteboard and left a sad face beneath it, marking their failure. The fictional physicists were on the right track; they just didn't know there was another path forward.
The next step is experimental. As fusion reactor technology matures in the coming years, physicists will be watching closely to see whether these theoretical predictions hold up in practice.
