A beam of neutrinos concentrated like the light from a laser seemed possible on paper, but two physical obstacles have now closed off this path.
Neutrinos are extremely lightweight particles that interact very little with matter, which is why they are sometimes called "ghost particles." They thus pass through our bodies and even the Earth almost without leaving a trace. This elusiveness makes them difficult to detect, but it had also given rise to an astonishing idea: creating a kind of laser capable of emitting a concentrated beam of them.
Classic laser beam
Unsplash illustration
The proposal, published in 2025, relied on radioactive atoms cooled almost to absolute zero, or about -273.15 °C. At this temperature, atoms can adopt collective behavior. In this state, known as a Bose-Einstein condensate, they no longer behave merely as a multitude of independent atoms.
With light, this collective behavior can produce enhanced emission in the same direction. The researchers behind the project thought that a comparable mechanism could accelerate the decay of radioactive atoms. The neutrinos produced would then be emitted collectively, forming the desired beam.
A team from MIT analyzed this proposal in detail. First obstacle: when an atom emits a neutrino, it experiences recoil, like an object that would move in the opposite direction after a throw. But a neutrino produced by a decay carries much more energy than a visible-light photon. The corresponding recoil therefore becomes very significant.
In practice, the emitting atom would be ejected from the group of ultracold atoms almost immediately. The calculations by Hanzhen Lin, Yu-Kun Lu, and Wolfgang Ketterle indicate that it would move too quickly to preserve the collective effect required. The group would then lose the “memory” of the direction taken by the previous neutrino.
But even assuming that this first problem disappeared, a second mechanism would still block the laser. Neutrinos belong to a family of particles called fermions, which also includes electrons. These particles obey quantum rules different from those of photons, the particles that make up light.
This difference changes precisely the collective behavior being sought. Instead of encouraging the next emission in the same direction, the presence of an already-emitted neutrino makes this repetition more random. The effect therefore prevents the gradual accumulation of identical emissions needed to form a beam comparable to that of a laser.
The results were published on September 2, 2026, in two complementary articles in Physical Review Letters. The authors of the initial proposal consider these analyses a convincing refutation, although experimental verification may still be conceivable. No Bose-Einstein condensate made up of radioactive atoms has yet been produced in a laboratory.