Physicists have shown that a proposed laser made of neutrinos, the ghostly particles that pervade the universe, is impossible to build under the most conventional design. Two new analyses reveal that the quantum effect the proposal relied upon is about 10,000 billion times too short-lived to work, and in fact acts in the opposite direction to what its proponents intended.
The idea, put forward in 2025 by Ben Jones at the University of Manchester and Joseph Formaggio at the Massachusetts Institute of Technology, suggested that thousands of extremely cold radioactive atoms could be coaxed into a quantum state called a Bose-Einstein condensate, or BEC. In such a state, all atoms share a single quantum identity, and the researchers theorised that neutrinos emitted during nuclear decay would be amplified into a coherent beam, much like photons in an ordinary laser.
Wolfgang Ketterle, also at MIT, heard a lecture about the concept and immediately suspected it was too good to be true. Ketterle, who shared the Nobel Prize for creating some of the first BECs in the 1990s, and his colleagues have now confirmed that suspicion with two rigorous mathematical investigations published in Physical Review Letters.
The key to the original proposal was a memory effect. When an atom in the BEC emitted a neutrino, the shared quantum state would retain a trace of that emission, making it more likely that subsequent neutrinos would be emitted in the same direction, thus forming a beam. Ketterle and his team calculated that this memory, although present, lasts only about 10,000 billion times too briefly to influence the neutrinos as intended.
More troublingly, the team found that the memory effect would actually produce the opposite of what was hoped for, an effect Ketterle calls an anti-memory. If an atom has emitted a neutrino, it is not allowed to immediately emit another one, he explains. This behaviour stems from the fact that neutrinos are fermions, a class of particles that fundamentally differs from the photons that ordinary lasers are built upon.
Kyle Leach at Queen's University in Canada said the new papers sharpen the understanding of where the real difficulty lies. For nuclear-scale energies, the requirements become extraordinarily demanding, he said. Leach noted that the analysis does not categorically rule out every possible neutrino laser design, but it shows that the most conventional scenario, in which each atom emits a single neutrino, cannot work. If each atom emitted two neutrinos at a time, the analysis might be different, he added.
The interesting scientific question now becomes more precise rather than disappearing, Leach said, asking what kinds of nuclear or neutrino processes, if any, could avoid the limitations identified. He suggested that experiments are most likely to provide a definitive answer.
For Ketterle, the episode illustrates how science can correct itself even when that means abandoning inspiring and creative ideas. Reaching clarity required hours of discussion, including with Jones and Formaggio, but that is ultimately how physics ought to work, he said. Jones and Formaggio did not respond to requests for comment.