What are neutrino lasers and black hole bombs, and how do I get one?

few weeks ago, two physicists started talking about creating a laser that emits neutrinos instead of photons. It conjured up the terrifying picture of a powerful beam of nearly undetectable, nearly massless particles that could blast through the Earth at near light speed and do absolutely nothing to anybody. The enemy might even be able to detect them if they built a sufficiently sensitive detector.

Of course, they’d be useful for studying neutrinos, which is important. The important term here is ‘superradiant,’ which is different way of producing coherent light like that from an ordinary laser.

Unfortunately, science news websites just breathlessly and uncritically repeated the authors’ claims in their press release. They missed the interesting scientific part and the controversy about whether a neutrino laser is actually possible.

Superradiance (SR)

A laser works, as its name implies, by stimulated emission of radiation. To produce a laser, you create a cavity where all the atoms are in excited states. This is called a population inversion because normally most atoms are in a low energy state. When a photon of the right wavelength comes along, it interacts individually with the excited atoms which release more photons in a controlled, coherent manner, getting large amplification.

By contrast, super­radiance occurs when a large number of excited atoms are confined in a tiny space of a cube less than one DeBroglie wavelength on a side. Although the effect is similar to a laser, it works on an entirely different principle. A SR ‘laser’ is a type of collective spontaneous emission. Though amplification still occurs and is a criterion of successful SR, SR doesn’t require a cavity. The original idea from Dicke ^[1] way back in 1954—well before the laser was invented—was that atoms in close proximity are coupled together by their common radiation fields. If one atom spontane­ously radiates by giving off a photon (thereby lowering its energy level), all the other atoms will feel the disturbance in the force (so to speak) and emit their photons at the same time, creating a fast, short, intense, coherent light pulse like a laser.

This high density of atoms needed to induce superradiance immediately suggested that it might occur in Kerr (rotating) black holes, where such high densities would naturally occur.^[2] But super­radiance can also occur in more mundane situations, including the well-known Vavilov-Cherenkov effect, where a charged particle hits a medium while traveling faster than the speed of light in the medium. Cherenkov radiation is not really super­radiance because amplifica­tion doesn’t occur, but it illustrates the principle.^[3] Another application is super­radiant organic light-emitting diodes that use Fabry-Pérot micro­cavities. They’re claimed to provide purer colors and brighter emission at lower voltages than ordinary OLEDs.^[4]

Thus, superradiance can be thought of as a sort of anti-absorption, as it occurs only in a dielectric medium (like air), never in a vacuum. Bekenstein and Schiffer ^[3] say super­radiance can happen any time atoms or molecules interact collectively. They say it explains emission of phonons that produce the loud boom in a supersonic shock wave and the emission of quasipar­ticles in superfluids. They describe a gadget that could make super­radiance in the lab using an ordinary rotating cylinder, even one rotating non-relativ­ist­ically at lower than six trillion rpm implied by the theory, perhaps as low as 1000 rpm.^[3]

Astrophysical lasers

Astrophysicists, who have studied naturally occurring microwave lasers (masers) for decades, now have evidence for optical lasers in gas condensates near Eta Carinae. The criteria for these lasers are somewhat relaxed over those in the lab, as there are no highly collimated mirrors in space; but according to the authors of a recent book they do show population inversions and some modest amount of amplification.^[5] So far, most astrophysical lasers are in stellar atmospheres, which aren’t dense enough for superradiance to occur.

Neutrino superradiant lasers

Because superradiance is a collective phenomenon, it’s supposed to occur only with bosons because many bosons can be in the same state. Unfortunately, neutrinos are not bosons, but fermions. In a new paper speculating about a neutrino laser, Jones and Formaggio ^[6] say SR depends on the gain medium, so fermions ought to be able to exhibit it if we could make a Bose-Einstein condensate (BEC) with certain trapped nuclear isotopes. But of course a BEC only works with bosons. So they started out with bosons first. The atom they selected was rubidium-83 (⁸³Rb), which normally produces neutrinos at a slow rate through a type of radiactive decay called electron capture.

A boson is a particle with an integer spin. A fermion is a particle with a half-integer spin. ⁸³Rb has 37 protons + 37 electrons + 46 neutrons. The ½ spin of the protons and electrons always add up to an integer and 46 is an even number. That makes ⁸³Rb a boson (assuming it’s not ionized). (Pay no attention to those AI answerbots, which mangle this question.) The authors calculate that their hypothetical device containing only a million atoms would reduce the half-life from 86.2 days to one minute and, they claim, produce a neutrino laser.^[6]

Electron capture is also known as inverse beta decay, which is   0−1e + 11p → 10n + 00ν_e . It changes a proton into a neutron and emits a neutrino. So what Jones and Formaggio are really suggesting is that this nuclear reaction can be triggered by a neutrino. If we had a good source of neutrinos, we could test it. But nobody in the science media seemed to appreciate how far-fetched it was.

Controversy

It sounds like a magic trick, but in fact it’s plausible: in a BEC the atoms are delocalized, so you wouldn’t need a high pressure. It sounds like a very clever way of generating coherent neutrinos (which are fermions). The question is: can you really amplify radioactive decay this way?

It sounds crazy: radioactive decay is not an optical phenomenon. Why would an isotope that decays by electron capture, as ⁸³Rb does, care whether a neutrino is around? And almost immediately after Jones and Formaggio’s paper came out, other scientists rushed a paper into print saying emphatically that no, a neutrino laser that runs by accelerating nuclear decay is not possible. The reason is simple: unlike photons, neutrinos aren’t bosons. The authors say that even though ⁸³Rb is indeed a boson, a neutrino has a spin of ½ and emitting one would leave the atom with a non-integer spin, turning it into a fermion in an excited state, so the emission would stop after emitting a single neutrino.^[7] This is called Pauli blocking from the Pauli Exclusion Principle, which states that unlike bosons, fermions can’t occupy the same quantum state on an atom.

This is obviously true: each rubidium atom would decay into something else, specifically krypton-83m (^83mKr), so it would only be able to emit a neutrino once whether it was a fermion or not. But the critics didn’t mention the real problem: why would putting ⁸³Rb in a BEC do anything at all? The authors just sort of assumed by analogy that radioactive decay involving electron capture by ⁸³Rb was comparable to decay of an electrically excited atom by emission of a photon and so it could be triggered by a neutrino.

That’s not a realistic way to get rid of nuclear waste as some science websites are claiming. The authors also recognize that a million neutrinos is an insignificant number. Trading one radioactive atom for one neutrino would be highly impractical, but even an inefficient way of generating neutrinos would be an important advance.

Accelerating nuclear decay, no creationists needed

It is theoretically possible to accelerate some types of radioactive decay. For example, by fully ionizing radioactive rhenium 187  75Re to 187  75Re⁷⁵⁺, which decays to osmium-187 by beta-decay, you can reduce the half-life from 42 billion years to 32.9 years. For fissionable high-Z atoms, it’s even easier: just blast them with neutrons and change them into something bigger that decays faster. But the best way is to dump the whole mess into a supernova, where it gets broken down into alpha particles. That’s what they’re there for.

Black hole bombs

A black hole bomb is a black hole surrounded by a perfectly reflecting mirror. This would reflect the superradiant field and amplify it until the black hole became unstable and exploded.^[8][9] You would probably use ionized matter, which is a good reflector of low frequency EM waves, not an actual metal mirror.^[2] Even so, they are impossible and no, you can’t have one.

nov 18 2025, 3:55 am