Welcome to Neutrinos 101 — a six-part beginner’s guide to one of the strangest and most consequential particles in physics. By the end of these six short parts, you will understand what neutrinos are, how we know they exist, why they matter, and where the field of neutrino physics is going.
No physics background is required. We’ll explain every technical term in plain language and link to deeper coverage on this site if you want to follow specific threads further.
A particle that’s almost not there
Right now, as you read this sentence, about 100 trillion neutrinos are passing through your body every second. Most come from the Sun. A small fraction comes from cosmic rays, the Earth’s interior, distant supernovae, and even the residual radioactivity in bananas in your kitchen.
Almost none of them interact with anything in your body. They sail through your hand, your desk, the planet beneath you, and out the other side. A single neutrino can pass through a wall of solid lead a light-year thick with only about a 50% chance of bumping into something along the way.
This is what makes neutrinos both the most abundant matter particles in the universe and the hardest to study. They are, in the words of one physicist, “the closest thing to nothing we have ever found.”
What makes them so weird
In the Standard Model of particle physics, every particle has a few basic properties: a mass, an electric charge, and a set of interactions it can have with other particles. Neutrinos are unusual in all three.
Mass. Neutrinos have masses so small that physicists initially assumed they were zero. The current best laboratory limit puts the heaviest neutrino mass below about 0.45 eV — about a million times lighter than the next-lightest particle, the electron. Whether they have any mass at all was unknown until 1998. The exact value is still being measured.
Charge. Neutrinos have no electric charge — none, exactly zero. This means they don’t feel the electromagnetic force at all. They don’t interact with light. They cannot ionize atoms. The only forces they feel are the weak nuclear force (extremely short-range) and gravity (extremely feeble).
Interactions. Because they only interact via the weak force and gravity, they are nearly impossible to detect directly. A typical neutrino can pass through the entire Earth without interacting. To catch any meaningful number of them, you need vast detectors — thousands of tons of water, ice, or other material — and you have to wait.
Why physicists care
Three reasons.
Scientifically. Neutrinos carry information from places we cannot otherwise see. They are produced by nuclear fusion in the Sun’s core, by core-collapse supernovae, by cosmic-ray accelerators in distant active galaxies. Because they pass through everything, the neutrinos that reach us from these sources have not been distorted by intervening matter the way light has been. They are uniquely clean cosmic messengers.
Practically. Neutrinos probe the fundamental structure of matter in ways that other particles can’t. Their interactions involve specific configurations of the weak force that are otherwise hard to study. Their tiny masses are sensitive to physics at energy scales (the grand-unification scale, around 10¹⁵ GeV) that no accelerator can reach directly.
Philosophically. Neutrinos were the first clear evidence that the Standard Model of particle physics — the most successful theoretical framework in the history of physics — is incomplete. The model was built assuming neutrinos had zero mass. The 1998 discovery that they actually have mass forced the field to begin extending the model. Understanding how neutrinos fit into a deeper theory is one of the most active research programs in fundamental physics today.
Where neutrinos come from
Neutrinos are produced in many natural and artificial processes. The major sources you will meet again in later parts of this series:
- Stellar fusion. The Sun produces about 2 × 10³⁸ neutrinos per second from the nuclear reactions that power it. Other stars do the same. About 60 billion solar neutrinos pass through every square centimeter of Earth’s surface every second.
- Supernovae. When a massive star runs out of fuel and its core collapses, it releases about 99% of its energy as neutrinos in roughly 10 seconds. The 1987 supernova in the Large Magellanic Cloud is the only one we have caught in neutrinos so far.
- Cosmic rays. High-energy particles from outside the solar system hit Earth’s atmosphere and produce showers that include muons and muon neutrinos. The atmospheric neutrino flux is constant and well-measured.
- The Earth. Radioactive decay deep inside the Earth — primarily uranium and thorium — produces geo-neutrinos. Detecting them lets us measure how much of Earth’s internal heat comes from radioactivity.
- Nuclear reactors. Fission produces enormous numbers of antineutrinos as a byproduct. Reactor antineutrinos have been used for many of the most important experimental measurements.
- The Big Bang. The early universe was full of neutrinos. Many of them are still here — about 336 per cubic centimeter everywhere in space — though they have cooled to tiny energies and have never been directly detected in a laboratory.
What comes next
In Part 2, we will look at how we know neutrinos exist at all — from Wolfgang Pauli’s “desperate remedy” in 1930 to Cowan and Reines’s 1956 telegram to Pauli announcing that they had finally caught one. The 26 years in between were one of the more patient detective stories in modern science.
Frequently asked
What is a neutrino?
A neutrino is an elementary particle — one of the basic building blocks of matter. It has no electric charge, has very little mass (less than a millionth of the electron's mass), and almost never interacts with anything. About 100 trillion of them pass through your body every second from the Sun alone, and you don't notice a single one.
Where do neutrinos come from?
Everywhere. The Sun produces them in fusion reactions in its core. The Earth produces them through radioactive decay deep underground. Cosmic rays produce them when they hit the upper atmosphere. Supernovae produce huge bursts. Nuclear reactors produce them as a byproduct of fission. The Big Bang left a relic background of them everywhere in space.
Why do they matter?
Three reasons. Scientifically: they tell us about processes deep in stars, inside the Earth, in active galaxies billions of light-years away — places we cannot otherwise see. Practically: they probe the fundamental structure of matter at scales no accelerator can reach. Philosophically: they were the first hint that the Standard Model of particle physics is incomplete.
How small are they?
Very. Neutrinos have at most a few hundred meV of mass — millions of times lighter than the next-lightest particle (the electron). Whether they have any mass at all was unknown until 1998. The exact mass is still being measured.
Cite this article 5 formats
APA
Neutrino Times Editorial Team. (2026, January 10). Neutrinos 101 — Part 1: What is a neutrino?. Neutrino Times. https://neutrino-times.com/articles/neutrinos-101-part-1-what-is-a-neutrino/
Chicago
Neutrino Times Editorial Team. "Neutrinos 101 — Part 1: What is a neutrino?." Neutrino Times, January 10, 2026. https://neutrino-times.com/articles/neutrinos-101-part-1-what-is-a-neutrino/.
MLA
Neutrino Times Editorial Team. "Neutrinos 101 — Part 1: What is a neutrino?." Neutrino Times, 10 Jan. 2026, https://neutrino-times.com/articles/neutrinos-101-part-1-what-is-a-neutrino/.
BibTeX
@misc{neutrino-times-neutrinos-101-part-1-what-is-a-neutrino,
author = {Neutrino Times Editorial Team},
title = {Neutrinos 101 — Part 1: What is a neutrino?},
howpublished = {Neutrino Times},
year = {2026},
month = {jan},
url = {https://neutrino-times.com/articles/neutrinos-101-part-1-what-is-a-neutrino/},
note = {Accessed: 2026-01-10}
} RIS
TY - GEN TI - Neutrinos 101 — Part 1: What is a neutrino? AU - Neutrino Times Editorial Team PY - 2026 DA - 2026-01-10 PB - Neutrino Times UR - https://neutrino-times.com/articles/neutrinos-101-part-1-what-is-a-neutrino/ ER -