A new theoretical study suggests fusion reactors could do more than generate energy, they might also produce particles linked to dark matter.
Researchers at the University of Cincinnati say they have worked out, at least on paper, how fusion reactors could produce subatomic particles known as axions, a challenge that stumped two of America’s most famous fictional physicists.
In the CBS sitcom “The Big Bang Theory,” particle physicists Sheldon Cooper and Leonard Hofstadter, who share an apartment, grapple with the same idea across three episodes in Season 5 but never solve it.
UC physics Professor Jure Zupan and his co-authors, all theoretical physicists from the Fermi National Laboratory, MIT and Technion-Israel Institute of Technology, now report what they consider a workable approach. Their study appears in the Journal of High Energy Physics.
What Are Axions and Why Do They Matter?
Axions are particles that have not yet been confirmed, but many physicists think they could be connected to dark matter. Dark matter matters because it helps scientists explain how the universe developed after the Big Bang nearly 14 billion years ago.
Although dark matter has never been detected directly, researchers believe it makes up most of the universe’s matter, with ordinary, visible matter accounting for only a smaller share. It is called dark matter because, unlike normal matter, it does not absorb or reflect light.
University of Cincinnati Profesor Jure Zupan is a theoretical physicist who studies topics such as dark matter. Credit: Joseph Fuqua II
Even so, scientists infer that dark matter exists because of the way its gravity shapes what we can see. Its pull changes how galaxies move through space and how stars travel within those galaxies. One leading idea is that dark matter could consist of an extremely light particle known as the axion.
Fusion Reactors as Particle Factories
In their study, Zupan and his co-authors looked at a deuterium and tritium fusion reactor design that uses a lithium-lined vessel and is being developed by an international collaboration in southern France. Along with generating energy, this kind of reactor could also create particles from the dark sector because fusion produces a very large number of neutrons.
“Neutrons interact with material in the walls. The resulting nuclear reactions can then create new particles,” he said.
The second way the new particles can get generated is when neutrons bounce off other particles and slow down, releasing energy in a process physicists call bremsstrahlung or “braking radiation.”
The new particles could be axions, or at least axion-like particles. And that’s where the show’s fictional physicists failed, Zupan said.
Where Fiction Fell Short
“The Big Bang Theory” ran from 2007 to 2019 and earned seven Emmys. It remains among the most-watched shows of any streaming service, according to Nielsen.
“The general idea from our paper was discussed in ‘The Big Bang Theory’ years ago, but Sheldon and Leonard couldn’t make it work,” Zupan said.
In one episode, a whiteboard features an equation and diagram that Zupan said describes how axions are generated from the sun. In a subsequent episode, another equation appears on a different board. Below the calculations in a different marker color is an unmistakable sad face — a symbol of failure.
Jure Zupan. Credit: Joseph Fuqua II
Zupan said Leonard and Sheldon’s equation estimates the likelihood of detecting axions from their proposed fusion reactor compared to the sun — with discouraging results, which explains the sad face.
“The sun is a huge object producing a lot of power. 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,” he said.
Science as an Inside Joke
The characters in the show never talk about axions or the whiteboards in the episodes. They’re just an Easter egg for physicists in a show famous for incorporating scientific concepts like Schrodinger’s cat and the Doppler effect into its storylines, along with cameos by Nobel laureates and “Star Trek” alumni alike.
“That’s why it’s fantastic to watch as a scientist,” Zupan said. “There are many layers to the jokes.”
Reference: “Searching for exotic scalars at fusion reactors” by Chaja Baruch, Patrick J. Fitzpatrick, Tony Menzo, Yotam Soreq, Sokratis Trifinopoulos and Jure Zupan, 27 October 2025, Journal of High Energy Physics.
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