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聽neutrinos聽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聽exact mass is unclear.听
In 2025,聽聽at the University of Manchester in the UK and聽聽at the Massachusetts Institute of Technology (MIT) suggested a novel and surprising way to gain clarity 鈥撀犅爑sing thousands of extremely cold radioactive atoms to create a laser beam of neutrinos.
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, 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聽nuclear decay. Jones and Formaggio theorised that if many such decaying atoms were pushed into a quantum phase of matter called a 鈥�Bose-Einstein condensate鈥� (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聽extremely cold聽so they could behave quantumly, which is a big technical challenge.听
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.
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聽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.听
鈥淚f I am an atom and I have emitted a neutrino, I am not allowed to [immediately] emit a neutrino again,鈥澛�he says. This effect鈥檚 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.听
鈥淚 think these papers sharpen where the real difficulty lies,鈥� says聽聽at Queen鈥檚 University in Canada. 鈥淔or聽nuclear-scale energies聽[pertaining to atoms鈥� decay], these requirements become extraordinarily demanding.鈥澛�
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. 鈥淚 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?鈥� says Leach. In his view, this question is most likely to be definitively answered through experiments.听
Formaggio and Jones didn鈥檛 respond聽New 女生小视频鈥檚聽request for comment.
For Ketterle, the lesson of the neutrino laser that couldn鈥檛 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.
Physical Review Letters
Physical Review Letters
