FAQ

Answers to some of the most commonly asked questions about neutrinos and neutrino science.

The basics

What are neutrinos, and how do they work?

Neutrinos are elementary particles, which means they are one of the many sorts of small particles that make up the universe. Neutrinos are the most prevalent mass-carrying particles in the universe. In the time it took you to blink while holding out your thumb, about 100 billion neutrinos traveled through your thumbnail. Neutrinos, on the other hand, have another crucial property: they don’t like to interact much. Even though millions of neutrinos pass through your body every second, just a few will interact with you over your lifetime.

Neutrinos are extremely light particles that come in three flavors (named for the particles they make when they collide). Neutrinos include electrons, muons, and taus. Neutrinos are unique among fundamental particles in that their flavors change as they move, which is one of the reasons physicists are fascinated by them.

See what exactly is a neutrino for a longer introduction.

Are neutrinos considered to be safe?

Yes! Neutrinos are extremely harmless. The vast majority of neutrinos pass through stuff without ever colliding. Because they are so little and neutral (they don’t have a charge), they don’t come into touch with other particles very often. Neutrinos do not produce radiation or cause damage to the items they pass through.

How many neutrinos pass through me right now?

Roughly 100 trillion every second, most of them from the Sun. Over an entire human lifetime, perhaps one of them will interact with an atom in your body. That combination — overwhelming abundance and almost no interaction — is what makes neutrinos both ubiquitous and extraordinarily hard to study.

Where do neutrinos originate?

Everywhere! When one particle converts into another, neutrinos are produced as a natural byproduct. Neutrinos can originate from the Earth’s core, our sun, far-off star explosions, the Big Bang, particle interactions in our atmosphere, or even reactions within your own body. Neutrinos are produced by even bananas. Scientists can also make them using accelerator beams or nuclear reactors, resulting in a more regulated and studyable source.

Each source is covered separately: the sun, supernovae, the atmosphere, reactors, accelerators, the Earth’s interior and the Big Bang.

Do neutrinos have mass?

Yes, but very little. Neutrino oscillation can only happen if at least two of the three mass states differ from one another, and different masses mean the masses cannot all be zero. Oscillation experiments measure those differences, not the masses themselves — which is why the individual values are still unknown. See masses of neutrinos.

Do neutrinos travel at the speed of light?

They don’t, in fact. Neutrinos are strange, but not that strange.

The source of this misunderstanding is a 2011 result. The findings from the OPERA experiment revealed that neutrinos arrived at the detector remarkably swiftly, reportedly faster than the speed of light. Other tests in the same neutrino beam (and elsewhere) were unable to reproduce the anomaly. The OPERA team eventually discovered that the timing mismatch was caused by a malfunctioning piece of equipment (a cable). After being restored, OPERA measured neutrinos to be very close to, but not exceeding, the speed of light.

Because they carry mass, neutrinos must travel slightly below light speed. The difference is far too small for current experiments to resolve directly.

Discovery and history

How did neutrinos come to be discovered?

In 1930, neutrinos were proposed as a possible explanation for the radioactive process known as beta decay. Clyde Cowan and Frederick Reines, armed with hypotheses, found the neutrino in a reactor experiment in 1956. Different flavors of neutrinos and additional features were discovered in subsequent investigations. In the timeline, you may learn more about these experiments.

The full story is on the history of neutrinos page.

Who proposed the neutrino, and why so reluctantly?

Wolfgang Pauli, in a 1930 letter to a physics conference he did not attend. Beta decay appeared to violate the conservation of energy, momentum and angular momentum, and Pauli suggested an unseen neutral particle was carrying the missing amounts away. He considered proposing a particle nobody could detect a poor thing for a theorist to do — the particle was confirmed 26 years later.

Why did it take 26 years to detect one?

Because a neutrino interacts so rarely that a detector needs either an enormous amount of material, an enormous source, or both. Cowan and Reines solved this by placing their detector next to a nuclear reactor, which produces antineutrinos in vast numbers, and by looking for a two-step signature that ordinary background radiation could not imitate.

Was there a Nobel Prize for neutrinos?

Several. The 1995 prize recognised Reines for the first detection, the 2002 prize went to Davis and Koshiba for detecting cosmic neutrinos, and the 2015 prize went to Kajita and McDonald for the discovery of neutrino oscillation — and with it, neutrino mass.

Flavours and oscillation

What are neutrino oscillations, and how do they work?

The way neutrinos change flavor as they travel is referred to as oscillations. A neutrino that starts off as one flavor (electron, muon, or tau neutrino) will eventually morph into the other flavors, with the likelihood of it appearing as a different flavor varying depending on how far it has traveled. Quantum physics, or the strange way things behave at very small sizes, causes oscillations. The discovery of neutrino oscillations was particularly intriguing because it proved that neutrinos have mass, something the current model could not account for. It’s the first and only proof that particle physics’ current model isn’t complete. That means there will be more fascinating physics to explain this big puzzle.

What are the three flavours?

Electron, muon and tau neutrinos, named after the charged particle each one produces when it interacts. A muon neutrino striking a nucleus can produce a muon; an electron neutrino can produce an electron. Detectors identify the flavour by identifying that partner particle, never the neutrino itself. See flavors of neutrinos.

What was the solar neutrino problem?

From the 1960s onwards, Ray Davis’s detector counted roughly a third of the solar neutrinos that John Bahcall’s models of the Sun predicted. For decades the question was which of the two was wrong. Neither was: the missing neutrinos had changed flavour on the way and the chlorine-based detector could only see electron neutrinos. The Sudbury Neutrino Observatory settled it around 2001 by counting all three flavours and finding the predicted total.

Why does oscillation prove that neutrinos have mass?

A neutrino’s flavour is a mixture of three mass states. Those states travel at slightly different rates only if their masses differ, and it is that difference which shifts the mixture over distance. If every mass were zero, the mixture could never change and no oscillation would occur. Oscillation therefore measures mass differences — the individual masses remain unknown.

Are neutrinos and antineutrinos different?

They behave differently in interactions: a neutrino produces a negatively charged partner, an antineutrino a positively charged one. Whether they are genuinely distinct particles is still open — see the question on Majorana particles below, and the antineutrinos page.

Detecting neutrinos

What method do we use to detect neutrinos?

To detect neutrinos, scientists can utilize a variety of materials, ranging from mineral oil and dry cleaning fluid to Antarctic ice and water. It is unable to detect neutrinos directly due to their neutrality and small size. Instead, depending on the substance, all systems rely on detecting the heavier, charged particles produced when a neutrino interacts, which provide a distinctive track, flash of light, line of bubbles, change in temperature, or other signal. Because neutrinos interact so seldom, detectors must be large and experiments must operate for lengthy periods of time to collect enough data. They’ll also need technologies to filter out interactions from other particles that could contaminate neutrino data.

Why are neutrino detectors built deep underground?

Not to catch the neutrinos — they pass through rock as easily as through air — but to keep everything else out. Cosmic rays constantly strike the atmosphere and produce muons that would swamp the handful of genuine neutrino events. A kilometre of rock absorbs almost all of them while leaving the neutrino flux untouched.

What is Cherenkov radiation, and why does it matter here?

When a charged particle moves through water or ice faster than light does in that medium, it emits a cone of blue light. That cone is what large water and ice detectors actually record. Its direction points back towards where the neutrino came from, and its brightness indicates the energy — which is how a tank of water becomes a telescope.

What is a neutrino beam, and how does it work?

Physicists can examine neutrinos using a neutrino beam. Particle accelerators are used by scientists to create energetic particles that collide with a target and produce additional particles that decay into neutrinos. This results in a concentrated group of neutrinos with a distinct taste and energy level. It’s easier for researchers to perform tests and analyze neutrinos when they have a lot of knowledge on the types of neutrinos that are produced.

Why are neutrinos being sent such a vast distance?

Neutrinos change flavor as they travel, and the amount of change is proportional to the distance traveled. We can learn more about neutrino properties by placing detectors at varied distances from the source. Researchers can learn more about how neutrinos and antineutrinos differ when they travel large distances by comparing how they alter. This can help them understand how neutrinos and antineutrinos have shaped our cosmos.

Why the research matters

Why are neutrinos being studied?

Neutrinos are the universe’s most prevalent massive particle, but we know very little about them. We don’t know how much they weigh—or why they have mass at all—despite the fact that they are one of the universe’s essential building blocks. They wouldn’t, according to our models. Neutrinos are a harbinger of new physics, or methods of explaining the world that we don’t yet understand. They may also possess special features that explain why the universe is made up of matter rather than antimatter. We won’t know some of the secrets of our universe—or how to harness them for more practical purposes—until we learn more about these mystery particles.

What are some of the advantages of neutrino research?

Because neutrinos are still poorly understood, basic research is currently the top priority. This gives us more information about the particles and how they fit into our understanding of the universe. They can also help us better comprehend and test our theories about how things work by assisting us with broader fundamental physics concerns. We don’t always know where basic research will lead us. Consider the electron: early researchers could not have predicted that the discovery of the electron would change the world forever, bringing us electronics, computing, and a more linked globe. No one could have imagined how the World Wide Web, which was created to share physics data, would revolutionize the way we interact, shop, travel, and do a thousand other things.

The same can be said of neutrino studies. We don’t know where the technology—sensitive detectors, powerful particle accelerators, data processors, and other components that allow experiments to run—will be valuable in the future. Neutrinos and neutrino studies have already spawned a slew of intriguing uses. Because neutrinos are so small, deceptive, and difficult to detect, there are numerous practical challenges between where we are now and where we want to go. The use of neutrino detectors to monitor nuclear proliferation for national security is maybe the closest to reality. It might also be used to look for mineral riches in the Earth’s crust or give a new form of communication. We’re still at the start of our neutrino trip; what we do with this technology and information will be left to future scientists to figure out.

Could neutrinos explain why the universe is made of matter?

Possibly. The Big Bang should have produced equal amounts of matter and antimatter, which would have annihilated completely — yet matter remains. If neutrinos and antineutrinos oscillate at measurably different rates, that asymmetry could be part of the explanation. Measuring this difference precisely is a central goal of the current generation of long-baseline experiments.

Can neutrinos be used for communication?

In principle yes, and it has been demonstrated once: in 2012 a group at Fermilab sent a short message through several hundred metres of rock using the NuMI beam and the MINERvA detector. The data rate was extremely low and the equipment filled a laboratory, so the demonstration proved the principle rather than any practical use. The appeal is that a neutrino signal would pass through the entire planet unimpeded.

Can neutrinos be used to generate electricity?

This question comes up often, and the honest answer separates two things.

What is established: neutrinos carry energy, and in 2017 the COHERENT experiment confirmed coherent elastic neutrino-nucleus scattering, a process in which a neutrino recoils off an entire nucleus. The energy transferred in such an event is on the order of a few thousand electron-volts — around a millionth of a millionth of a millionth of a joule — and events are extraordinarily rare. No experiment has demonstrated the conversion of neutrino energy into usable electrical power, and no such result exists in the peer-reviewed literature.

What is claimed: several companies pursue approaches described as harvesting ambient energy, sometimes including neutrinos among several sources alongside thermal and electromagnetic effects. These claims are not currently supported by published, independently reproduced measurements.

We report on this area because readers ask about it. Where a claim has not been demonstrated experimentally, we say so.

Open questions

How much do neutrinos actually weigh?

Nobody knows. Oscillation experiments give the differences between the three mass states, cosmology constrains their sum, and direct measurements such as KATRIN place an upper limit on the electron neutrino mass. Together these narrow the range considerably, but the individual values remain unmeasured. See what is the mass of a neutrino.

Which neutrino is the lightest?

Also unknown. The two possible arrangements of the three mass states are called normal and inverted ordering, and distinguishing between them is one of the main goals of experiments now under construction. See which neutrino has the smallest mass.

Are neutrinos their own antiparticles?

Possibly. A particle identical to its own antiparticle is called a Majorana particle, and the neutrino is the only known candidate among the fundamental fermions. The question is not academic: a Majorana neutrino would allow processes that are otherwise forbidden and would help explain why neutrino masses are so small. See do neutrinos have their own antiparticles.

What is neutrinoless double beta decay?

A hypothetical radioactive decay in which two neutrons convert into two protons and two electrons, with no neutrinos emitted at all. It can only occur if the neutrino is its own antiparticle. Several experiments are searching for it; none has observed it. A confirmed detection would be among the most consequential results in particle physics.

Do sterile neutrinos exist?

Unclear. A sterile neutrino would not interact through the weak force at all, only through gravity, making it even harder to detect than the three known types. Some experiments have reported anomalies consistent with a fourth state; others looking in the same range have found nothing. See neutrinos that are sterile.

Why are neutrinos left-handed?

Every neutrino ever observed spins in one particular direction relative to its motion, and every antineutrino in the other. Nothing in the Standard Model explains why. If right-handed neutrinos exist but interact too weakly to have been seen, they could account for the tiny neutrino masses. See is it true that all neutrinos are left-handed.