A proposed device that would use radioactive atoms in a Bose-Einstein condensate (BEC) to produce a coherent beam of neutrinos — a so-called superradiant neutrino laser — is facing serious scientific doubt. In two papers published in a prestigious journal, Nobel laureate Wolfgang Ketterle and colleagues at MIT argue that the scheme is fundamentally impossible.
The original proposal, put forward last year by Benjamin Jones and Joseph Formaggio, also of MIT, suggested that cooling radioactive atoms to near absolute zero to form a BEC could trigger a rapid, superradiant emission of neutrinos. Superradiance is a phenomenon in which a dense ensemble of excited atoms collectively emits radiation in a short, intense burst, with the emission rate proportional to the square of the number of atoms. Jones and Formaggio argued that an analogous process could occur with neutrinos instead of photons.
But Ketterle, along with Yu-Kun Lu and Hanzhen Lin, now say that three key obstacles rule out the idea. First, the de Broglie wavelength of the emitted neutrinos is far too short for superradiance to occur — a concern previously raised by BEC expert James Thompson of NIST, JILA and the University of Colorado Boulder. Second, the atoms left behind after neutrino emission are fermions, and fermionic statistics preclude the collective quantum behavior required for superradiance. Third, the recoil of the atoms after emitting a neutrino would disrupt the coherence needed for the effect.
Superradiance was originally theorized for ensembles of excited atoms emitting photons. In such systems, if the photon wavelength is much longer than the spacing between atoms, the emission becomes a collective process involving all atoms, leading to a burst of coherent radiation. This coherence has led some to call such systems «superradiant lasers», though the physics differs from conventional lasers.
The debate is notable because all three papers — the original proposal and the two rebuttals — appear in a highly prestigious journal, underscoring the seriousness with which the physics community treats the question. Thompson had already expressed skepticism about the viability of the device, and Ketterle's papers now provide a detailed theoretical refutation.
Jones and Formaggio have not yet responded publicly to the new criticisms. The proposal's fate remains uncertain, but the burden of proof now lies with its authors to address the fundamental objections raised by Ketterle's team. A BEC-based neutrino laser, if it could be built, would be a remarkable tool for studying neutrinos and fundamental physics. For now, however, it appears to be an impossibility.