New Analyses Show Proposed Neutrino Laser Is Physically Impossible

A Ghostly Beam That Cannot Be

Neutrinos are among the most abundant and elusive particles in the universe, streaming through planets and bodies by the trillions each second. Since their discovery in 1956, they have continued to surprise physicists with their unexpected properties, including their ability to change flavor and their extremely weak interactions with matter.

Last year, scientists proposed a concept for a neutrino laser: a concentrated beam of neutrinos produced by cooling a cloud of radioactive atoms to nanokelvin temperatures, one-billionth the temperature of interstellar space. The atoms would form a Bose-Einstein condensate (BEC), a quantum state in which they act as a single coherent whole, potentially amplifying their radioactive decay and emitting a laser-like beam of neutrinos.

Now, two new analyses from MIT physicists show that this concept is physically impossible. In two companion papers published in Physical Review Letters, Wolfgang Ketterle, the John D. MacArthur Professor of Physics at MIT, together with postdocs Hanzhen Lin and Yu-Kun Lu, present a two-part analysis demonstrating that the neutrino laser concept is flawed due to "recoil"—the kinetic energy created by the reaction—and the fundamental fermionic nature of neutrinos.

The Proposal and Its Doubts

The neutrino laser idea was proposed by Ben Jones at the University of Manchester in the UK and Joseph Formaggio at the Massachusetts Institute of Technology. They suggested using thousands of extremely cold radioactive atoms to create a laser beam of neutrinos, based on the principle of superradiance. In their scenario, radioactive rubidium atoms with a half-life of 86 days would be accelerated to decay within one minute.

Wolfgang Ketterle, who co-discovered Bose-Einstein condensates in 1995 and shared the Nobel Prize in Physics in 2001, heard a lecture about the idea and immediately worried that it was too good to be true. His team's new mathematical investigations confirmed that hunch.

"When a new idea—such as the one we proposed—is shared, it is the duty of the community to scrutinize it," Formaggio said, as reported by Mirage News.

The Fundamental Obstacles

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. However, 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." 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.

"If I am an atom and I have emitted a neutrino, I am not allowed to [immediately] emit a neutrino again," Ketterle said, as quoted by New Scientist.

The recoil effect also plays a role: the kinetic energy from emitting a neutrino would cause the atom to move so fast that it would almost instantly disappear from the condensate. "This is so fast that the atom would almost instantly disappear," Ketterle said, as reported by Mirage News.

Reactions from the Community

Kyle Leach at Queen's University in Canada commented on the findings, saying, "I think these papers sharpen where the real difficulty lies. For nuclear-scale energies [pertaining to atoms' decay], these requirements become extraordinarily demanding."

Leach added that 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.

Formaggio and Jones did not respond to New Scientist's request for comment.

A Continuing Mystery

Despite the setback, the researchers remain philosophical about the nature of neutrinos. "The one thing about neutrinos that never surprises physicists is that they never fail to surprise," Formaggio said, as reported by Mirage News.

Ketterle, reflecting on the capabilities of Bose-Einstein condensates, noted, "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."

The new papers sharpen the understanding of why the neutrino laser concept fails, but the quest to harness these ghostly particles continues.