Physicists proposed a way to focus ghostly neutrinos into a laser-like beam, but new MIT calculations show that violent atomic recoil and the particles’ own quantum nature prevent the effect from taking hold.
Trillions of subatomic particles stream through every square inch of Earth, passing through solid rock, massive stars, and human bodies every second without leaving a trace. These particles, known as neutrinos, carry near-zero mass and interact so weakly with normal matter that physicists have struggled to detect them, much less control them into a focused beam, since their discovery in 1956.
Last year, MIT physics professor Joe Formaggio and Ben Jones, then an associate professor at the University of Texas at Arlington and now at the University of Manchester, proposed a theoretical way around that physical barrier.
They theorized that cooling a cloud of radioactive atoms down to nanokelvin temperatures, one-billionth the temperature of interstellar space, would bring atomic motion to a standstill governed purely by quantum uncertainty rather than thermal heat. At that extreme cold, the atoms form a Bose-Einstein condensate, behaving as a single, quantumly correlated whole.
If radioactive atoms could be brought into that state, Formaggio and Jones reasoned, their decay might become synchronized through a quantum effect known as superradiance. Rather than releasing neutrinos independently in random directions, the atoms could potentially accelerate one another’s decay and concentrate the particles into a laser-like beam. In one proposed scenario, radioactive rubidium atoms would go from a half-life of 86 days to just one minute. No one has yet produced a Bose-Einstein condensate from radioactive atoms.
Two barriers block a neutrino laser
MIT physicists have now concluded that the scheme cannot work. In two companion papers published in Physical Review Letters, Wolfgang Ketterle, the John D. MacArthur Professor of Physics at MIT, and postdocs Hanzhen Lin and Yu-Kun Lu identified two separate barriers. One comes from the violent recoil produced when a neutrino leaves an atom. The other comes from neutrinos themselves, which belong to a class of particles called fermions.
“These two papers are sort of punch one and punch two,” Ketterle says. “Each paper would have killed the proposal.”
Superradiance had previously been demonstrated with photons. When a laser shines into a Bose-Einstein condensate, the ultracold atoms can synchronize their scattering so that photons emerge in the same direction. At room temperature, those photons would instead scatter randomly and produce little more than a diffuse glow.
Each scattering event also pushes an atom backward through recoil. Inside a condensate, those recoils can remain synchronized, reinforcing further scattering and causing the process to grow exponentially. The result is a superradiant beam of photons. Formaggio and Jones proposed that radioactive atoms might produce an analogous effect as they released neutrinos during decay.
Neutrino recoil erases quantum memory
Ketterle, who co-discovered Bose-Einstein condensates in 1995 and shared the 2001 Nobel Prize in Physics for related work, was skeptical that the delicate behavior of ultracold atoms could survive the energy released in nuclear decay.
“My experience has always been that the condensate can do marvelous things at low energy — superfluidity, vortices — and if you were to speak in a room filled with condensate, it would take one hour for you to hear my voice. That’s how slow the condensate is,” Ketterle says. “And I had always come to the conclusion that for anything violent, like nuclear reactions, the condensate would not do anything.”
Visible photons carry about 1 electron volt of energy. Neutrinos emitted during radioactive decay can carry roughly a million times more. That enormous difference means an atom releasing a neutrino should recoil about a million times more strongly than an atom interacting with visible light.
“As long as the recoil atom stays in the condensate, it can make the condensate superradiant,” Ketterle says. “But when a neutrino is emitted at a million electronvolts, the atom recoils at velocities equivalent to Mach 10, faster than a fighter jet. This is so fast that the atom would almost instantly disappear.”
For superradiance to build, the condensate needs to retain a quantum memory of earlier emissions. The neutrino laser proposal assumed that even after a recoiling atom escaped, it would leave behind an imprint that encouraged later atoms to emit neutrinos in the same direction.
Ketterle and his colleagues tested that assumption using a theoretical model that describes the conditions needed for superradiance. They adapted it to radioactive atoms and neutrinos while accounting for the range of neutrino energies, the recoil of the decaying atoms, and the changing dynamics of the condensate.
Every scenario they analyzed failed. The recoiling atom left the condensate too quickly for the required quantum imprint to develop. Without that memory, radioactive atoms would continue emitting neutrinos normally rather than amplifying one another into a beam.
Fermions reverse the memory effect
Even eliminating that recoil problem would not rescue the concept. The second paper found that neutrinos would produce the wrong kind of quantum memory for superradiance. Instead of encouraging the condensate to emit another neutrino in the same direction, an emitted neutrino would make subsequent emission in that direction less likely.
That reversal comes from the neutrino’s identity as a fermion. Fermions and bosons are the two fundamental classes of particles that make up matter. Photons are bosons with whole-integer spin, while particles including electrons and neutrinos are fermions with half-integer spin. Those different quantum properties determine how the particles behave in relation to one another.
“In superradiance, it is about a memory effect, or quantum correlations in the condensate. And in that context, people had thought that whatever is emitted from the condensate, it doesn’t matter if it is a boson or a fermion,” Ketterle explains. “But we analyzed it, and if you describe it correctly for emitted fermions, you get an anti-memory, which makes the condensate not accelerate in a superradiant form. It rather has the memory to not do it.”
An experiment could still test it
Formaggio, who helped develop the original proposal, views the challenge as part of the normal process of testing new ideas.
“When a new idea — such as the one we proposed — is shared, it is the duty of the community to scrutinize it. Such is the scientific process,” Formaggio says. “Indeed, it was great to see how our paper generated a lot of thinking outside of our original concept. We suspect that will continue.”
Ketterle, Formaggio, and Jones have met multiple times to work through the original proposal and the objections raised by the new analyses. Formaggio still leaves room for an experimental test.
“I suspect that someday, someone will do the experiment,” Formaggio says. “Nature, as always, is the final arbiter of such things. And here I would be remiss to not point out that every prior prediction about neutrinos has been wrong. The one thing about neutrinos that never surprises physicists is that they never fail to surprise.”
“Creative ideas and discussions among scientists are needed to uncover nature’s surprises,” Ketterle says. “But in the case of neutrino lasers, the surprise was too good to be true.”
Reference:
“Fundamental Impossibility of a Superradiant Neutrino Laser” by Yu-Kun Lu, Hanzhen Lin and Wolfgang Ketterle, 2 September 2026, Physical Review Letters.
DOI: 10.1103/8x7k-rwx2
“To Lase or Not to Lase: The Question of Neutrino Superradiance” by Ana Maria Rey, James K. Thompson and Haoqing Zhang, 2 September 2026, Physics.
This research is supported, in part, by the National Science Foundation, the Center for Ultracold Atoms, the Vannevar-Bush Faculty Fellowship, the Gordon and Betty Moore Foundation, and the U.S. Army Research Office.
Never miss a breakthrough: Join the SciTechDaily newsletter.
Follow us on Google and Google News.