Neutrino detectors, like the one used in the BOREXINO collaboration here, generally have an enormous tank that serves as the target for the experiment, where a neutrino interaction will produce fast-moving charged particles that can then be detected by the surrounding photomultiplier tubes at the ends. However, slow-moving neutrinos cannot produce a detectable signal in this fashion. (INFN / BOREXINO COLLABORATION)


The neutrino has long been one of the most perplexing and elusive cosmic particles. It took more than two decades from the time it was originally predicted to the time it was eventually discovered, and it arrived with a slew of surprises that set it apart from all the other particles we know about. They have the ability to “change flavor” from one kind to another (electron, mu, tau). All neutrinos have a left-handed spin, but all anti-neutrinos have a right-handed spin. And every neutrino we’ve ever seen travels at speeds that are almost identical to the speed of light. Is this, however, necessary? Laird Whitehill, a Patreon patron, is curious about this, asking:

“I’m aware that neutrinos move at almost the speed of light. However, since they have mass, they should be able to move at any speed. But, as you’ve hinted, their bulk forces them to move at almost the speed of light.

Light, on the other hand, travels at a constant speed. Anything with mass, on the other hand, may go at any speed.”

So why do we only detect neutrinos flying at speeds comparable to the speed of light? It’s an enthralling question. Let’s get started.

The neutrino was initially hypothesized in 1930, when a particular form of decay — beta decay — seemed to defy two of the most fundamental conservation laws: energy conservation and momentum conservation. When an atomic nucleus disintegrated in this way, it did the following:

  • raised by one atomic number


  • emitted an electron, emitted an electron, emitted an electron,


  • and lost a little amount of rest mass

The rest mass energy of the electron and the energy of the post-decay nucleus, when combined together, was always somewhat less than the rest mass of the original nucleus. Furthermore, the initial momentum of the pre-decay nucleus did not match the momentum of the electron and the post-decay nucleus. Either energy and momentum were being wasted, invalidating these ostensibly basic conservation principles, or a previously undetectable extra particle was being generated, carrying the excess energy and momentum away.

That particle, the elusive neutrino, would take around 26 years to discover. We couldn’t see these neutrinos directly — and still can’t — but we can detect the particles with which they hit or react, proving their existence and informing us about their characteristics and interactions. The neutrino has shown itself in a variety of ways, each of which gives us with an independent measurement and constraint on its attributes.

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In nuclear reactors, neutrinos and antineutrinos have been measured.

The Sun’s neutrinos have been measured.

We’ve detected neutrinos and antineutrinos emitted by cosmic rays colliding with the Earth’s atmosphere.

Particle accelerator experiments have created neutrinos and antineutrinos, which we have measured.

We’ve detected neutrinos released by SN 1987A, the nearest supernova in the last century.

We’ve even recorded a neutrino coming from the core of an active galaxy — a blazar — from under the ice in Antarctica in recent years.

We’ve learnt a lot about these phantom neutrinos thanks to all of this information. The following are some particularly pertinent facts:

Every neutrino and antineutrino we’ve ever seen travels at speeds so close to the speed of light that they’re indistinguishable.

Neutrinos and antineutrinos are classified into three types: electron, mu, and tau.

Every neutrino we’ve ever seen is left-handed (if you point your thumb in the direction of motion, your left hand’s fingers “curl” in the direction of its intrinsic angular momentum, or spin), while every anti-neutrino is right-handed.

When neutrinos and antineutrinos move through matter, they may oscillate, or change flavor from one kind to another.

Despite seeming to travel at the speed of light, neutrinos and antineutrinos must have a non-zero rest mass; otherwise, the “neutrino oscillation” phenomena would not be conceivable.

Neutrinos and antineutrinos have a broad range of energies, and the probability of interacting with a neutrino increases as the neutrino’s energy rises. To put it another way, the higher the energy of your neutrino, the more likely it is to interact with you. It would take around a light-year worth of lead to block about half of the neutrinos fired against it in the current Universe, which are produced by stars, supernovae, and other natural nuclear events.

We’ve been able to make certain inferences regarding the rest mass of neutrinos and antineutrinos based on all of our data. For starters, they can’t be zero. The masses of the three kinds of neutrinos are probably likely different, with the heaviest neutrino being around 1/4,000,000th the mass of an electron, the next-lightest particle. And, based on two sets of data — the large-scale structure of the Universe and the leftover light left over from the Big Bang — we can estimate that for every proton in the Universe today, one billion neutrinos and antineutrinos were generated in the Big Bang.

This is where the gap between theory and experiment is found. Because neutrinos have a non-zero rest mass, they should be able to slow down to non-relativistic speeds in principle. In principle, the neutrinos left over from the Big Bang should have slowed down to these speeds by now, and they should only be travelling at a few hundred km/s today, slow enough to have dropped into galaxies and galaxy clusters by now, accounting for around 1% of all dark matter in the Universe.

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However, we simply do not have the experimental capability to detect these slow-moving neutrinos directly. Their cross-section is millions of times too small for us to perceive them, since these minuscule energy would not cause recoils detectable by our present technology. These low-energy neutrinos, which are the only ones that should exist at non-relativistic speeds, will remain undetected until we can accelerate a current neutrino detector to speeds very near to the speed of light.

And it’s a shame, since finding these low-energy neutrinos — those that travel at a slower rate than light — would allow us to run a crucial test that we’ve never done before. Assume you have a neutrino in your possession and are traveling behind it. If you look at this neutrino, you’ll see that it’s travelling straight ahead of you: forward. If you try to measure the neutrino’s angular momentum, it would act as if it’s spinning counterclockwise, similar to how your fingers would curl around your thumb if you pointed your left hand forward.

It would be impossible to go faster than the neutrino if it constantly went at the speed of light. You’d never be able to overcome that, no matter how much effort you put in. However, if the neutrino’s rest mass is more than zero, you should be able to boost yourself to go faster than the neutrino. Instead of moving away from you, it would come closer to you. Despite this, its angular momentum would have to be the same in the counterclockwise direction, which would require you to depict it with your right hand rather than your left.

This is an intriguing contradiction. It seems to suggest that just altering your speed relative to a matter particle (a neutrino) may change it into an antimatter particle (an antineutrino). Alternatively, it’s plausible that right-handed neutrinos and left-handed antineutrinos do exist, but that we’ve never observed them for some reason. It’s one of the most pressing concerns regarding neutrinos, and the capacity to detect low-energy neutrinos — those that travel at a slower rate than light — would provide an answer.

But in actuality, we won’t be able to do so. The lowest-energy neutrinos we’ve yet observed have such a large amount of energy that their speed must be at least 99.99999999995 percent of the speed of light, which implies they can only travel at 299,792,457.99985 meters per second. When we’ve witnessed neutrinos coming from galaxies other than the Milky Way at cosmic distances, we’ve found no difference between their speed and the speed of light.

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Despite the complexity, there is a tantalizing prospect that we will be able to overcome this dilemma. It is conceivable to have an unstable atomic nucleus that not only undergoes beta decay, but also double beta decay, in which two neutrons in the nucleus suffer beta decay at the same time. We’ve seen this happen: a nucleus alters its atomic number by two, emits two electrons, and loses both energy and momentum, resulting in the emission of two (anti)neutrinos.

However, if you can turn a neutrino into an antineutrino merely by shifting your frame of reference, then means neutrinos are a new sort of particle that has only been discovered in theory: the Majorana fermion. It means that the antineutrino released by one nucleus might theoretically be absorbed (as a neutrino) by the other nucleus, resulting in a decay in which:

The nucleus’ atomic number changed by two.

There are two electrons emitted.

However, no neutrinos or antineutrinos are produced.

Several experiments, including the MAJORANA experiment, are now seeking for this neutrinoless double beta decay. It will profoundly shift our outlook on the elusive neutrino if we are able to witness it.

But, for the time being, the only neutrinos (and antineutrinos) we can detect through their interactions travel at speeds that are indistinguishable from the speed of light, thanks to existing technology. Neutrinos have mass, but it is so little that out of all the ways the Universe may manufacture them, only the neutrinos produced during the Big Bang should be travelling slower than the speed of light today. Those neutrinos may be all around us as an unavoidable element of the cosmos, but we are unable to detect them directly.

Neutrinos, on the other hand, may theoretically move at any speed as long as it is less than the cosmic speed limit, which is the speed of light in a vacuum. The problem we’re dealing with is two-fold:

slow paced Interaction probability for neutrinos are very low.

And the interactions that do happen are so low-energy that we can’t see them right now.

The only neutrino interactions we observe are those caused by neutrinos traveling at a speed that is indistinguishable from that of light. This will continue to be the case unless a breakthrough new technology or experimental approach emerges, which is regrettable.

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