Ghostly particles called neutrinos pose some of the biggest mysteries of modern physics, but researchers thought they could get a handle on them by corralling them into a laser beam. Two new analyses show that neutrinos are more slippery than that, making the proposed neutrino laser design impossible.

Researchers have been studying neutrinos since the 1940s. They are among the most abundant particles in the universe, but much about them remains unknown. Notably, they are extremely light, mere ghosts compared with more massive particles like neutrons, yet their exact mass is unclear.��

In 2025,  at the University of Manchester in the UK and  at the Massachusetts Institute of Technology (MIT) suggested a novel and surprising way to gain clarity –  using thousands of extremely cold radioactive atoms to create a laser beam of neutrinos.

, also at MIT, heard a lecture about the idea and immediately worried about it being too good to be true. He and his colleagues have now confirmed that hunch with two rigorous mathematical investigations.

Neutrinos are produced when radioactive atoms undergo nuclear decay. Jones and Formaggio theorised that if many such decaying atoms were pushed into a quantum phase of matter called a “Bose-Einstein condensate” (BEC), where they all share a quantum state, then their respective neutrino emissions would be amplified, forming a laser-like beam. Creating such a BEC would require making thousands of radioactive atoms extremely cold so they could behave quantumly, which is a big technical challenge.��

But Ketterle, who received the Nobel prize for creating some of the first ever BECs in the 1990s, and his team uncovered a more fundamental obstacle.

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The key to the neutrino laser proposal was a memory effect: when an atom in the BEC emitted a neutrino, it would be more likely to continue emitting more neutrinos in the same direction, thus pushing them into a beam, because the quantum state that all the ultracold atoms share would retain a trace of that first emission. Ketterle and his colleagues showed that this memory, although present, would be about 10,000 billion times too brief to affect the neutrinos as intended. Even more troublingly, the team uncovered that the memory would actually have the opposite effect from that intended, which Ketterle calls an anti-memory.��

“If I am an atom and I have emitted a neutrino, I am not allowed to [immediately] emit a neutrino again,” he says. This effect’s origin is subtle, stemming from neutrinos being a type of particle called fermions, which fundamentally behave differently than particles of light that our ideas about lasers tend to be built upon.��

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“I think these papers sharpen where the real difficulty lies,” says  at Queen’s University in Canada. “For nuclear-scale energies [pertaining to atoms’ decay], these requirements become extraordinarily demanding.” 

He says the new analysis does not categorically rule out every possible way to build a neutrino laser, but shows that the most conventional scenario where each atom emits one neutrino cannot work. If each atom emitted two neutrinos at a time, the analysis may be different, he says. “I think the interesting scientific question now becomes more precise rather than disappearing: what kinds of nuclear or neutrino processes, if any, avoid the limitations they have identified?” says Leach. In his view, this question is most likely to be definitively answered through experiments.��

Formaggio and Jones didn’t respond �鶹��ý’s request for comment.

For Ketterle, the lesson of the neutrino laser that couldn’t be is that science can correct itself even when doing so means abandoning inspiring and creative ideas. Getting clarity required hours of discussion, including with Jones and Formaggio. But that is ultimately how physics ought to work, he says.

Quantum mechanics and general relativity don’t fit together, and a big part of the issue comes down to gravity. For decades, the accepted route to an ultimate theory of everything has involved taking our best theory of gravity and squeezing it into the frame of quantum mechanics. Yet, almost a century later, scientists still haven’t managed to make gravity fit. Ivette Fuentes is a professor of quantum mechanics who conducts experiments at the scales where quantum theory and general relativity interplay. Fuentes sat down with �鶹��ý features editor Thomas Lewton to discuss the issues and fascinating theories that pop out when we try to fit classical and quantum mechanics together.

Read more: The experiments that could finally explain gravity