The detection chamber of the Super-Kamiokande neutrino experiment in Japan
Kamioka Observatory/ICRR (Institute for Cosmic Ray Research)/The University of Tokyo

The Ghost Particle: What Is a Neutrino and Could It Be the Key to Modern Physics?

Scientists have one of astrophysics' most wanted particles in their sights.

It came from deep space, moving at the speed of light, and crashed into Antarctica. Deep below the ice, it met its end. It wasn't an asteroid or alien spacecraft, but a particle that rarely interacts with matter, known as a neutrino.  

Though theorized in the 1930s and first detected in the 1950s, neutrinos maintain a mysterious aura, and are often dubbed "ghost particles" -- they're not haunting or dangerous, but they just zip through the Earth without us even noticing them. Oh, "and it's a cool name," according to astrophysicist Clancy James at Curtin University in Western Australia.

In recent years, ghost particles have been making headlines for all sorts of reasons and not just because they have a cool name. That Antarctic collision was traced to a black hole that shredded a star, for instance, and other neutrinos seem to come via the sun. In early 2022, physicists were able to directly pin down the approximate mass of a neutrino -- a discovery that could help uncover new physics or break the rules of the Standard Model. 

Imagine if we actually captured a ghost and could say the specter was of someone who had died. It would change everything we know about the universe. A ghost particle is pretty much a big deal for the same reason, and that's why astrophysicists are trying to trap them. They're excited, and here's why you should be, too.

The IceCube observatory at the South Pole, surrounded by snow, with a central rectangular unit and two cylindrical towers on either side

The IceCube observatory in Antarctica.

Erik Beiser, IceCube/NSF

What is a neutrino?

In a nutshell, a neutrino is a fundamental, subatomic particle. Under the Standard Model of particle physics it's classified as a "lepton." Other leptons include electrons, the negatively charged particles that make up atoms, with protons and neutrons. But look, if we get into all that, we're going to go real deep on particle physics and it'll explode our brains.

The neutrino is unique because it has a vanishingly small mass and no electrical charge and it's found across the universe. "They are made in the sun, in nuclear reactors, and when high-energy cosmic rays smash into Earth's atmosphere," says Eric Thrane, an astrophysicist at Monash University in Australia. They're also made by some of the most extreme and powerful objects we know of, like supermassive black holes and exploding stars, and they were also produced at the beginning of the universe: the Big Bang. 

Like light, they travel in basically a straight line from where they're created in space. Other charged particles are at the mercy of magnetic fields, but neutrinos just barrel through the cosmos without impediment; a ghostly bullet fired from a monstrous cosmic gun.

And, as you read this, trillions of them are zipping through the Earth and straight through you.

They're crashing into me right now?

Yes, exactly. Every second of every day since the day you were born, neutrinos have been moving through your body. You just don't know it because they interact with hardly anything. They don't smash into the atoms that make you up, and so you don't even know they're there. Just like a shadowy spirit passing through a wall, the neutrino moves right on through. Fortunately, there's no exorcism required.

But why should I care about neutrinos?

Studying them for decades has thrown up a bit of a surprise for scientists. Under the standard model, neutrinos shouldn't have any mass. But they do. "The fact they do points us to new physics to enhance our understanding of the universe," notes James.

The puzzle of the neutrino mass first came to light in the 1960s. Scientists had suggested the sun should be producing what's known as electron neutrinos, a particular type of the subatomic particle. But it wasn't. This "solar neutrino problem" led to a breakthrough discovery: that neutrinos can change flavor. 

Like an almost-empty bag of Mentos, the ghost particle comes in just three distinct flavors -- electron, muon and tau -- and they can change flavor as they move through space (flavor is the actual terminology, I'm not making that up for this analogy). For instance, an electron neutrino might be produced by the sun and then be later detected as a muon neutrino. 

And such a change implies the neutrino does have mass. Physics tells us they couldn't change flavor if they were massless. Now research efforts are focused on elucidating what the mass is.

In a study published in the prestigious journal Nature in February 2022, researchers revealed the mass of a neutrino to be incredibly tiny (but definitely there). Physicists were able to show directly, using a neutrino detector in Germany, that the maximum mass for a neutrino is around eight-tenths of an electron volt (eV). That's an unfathomably tiny mass, more than a million times "lighter" than an electron.

The barrel-shaped detector of the KATRIN is wheeled between houses near Leopoldshafen, southern Germany

The is what a ghost hunter looks like: The main spectrometer of the Karlsruhe Tritium Neutrino Experiment (KATRIN) is manoeuvred through a road in southern Germany.

Michael Latz/Getty

Wait! A neutrino detector? But aren't they... ghost particles? How do you detect neutrinos?

As James notes, "the darn things mostly pass straight through whatever detector you build!"

But there are a number of ways to trap a ghost.

One of the key ingredients you need is space. Physical space, deep underground. For great results, scientists have built their neutrino detectors under meters of ice in Antarctica and, soon, at the bottom of the ocean. This helps keep the data clean from any interference from things like cosmic rays, which would bombard the sensitive detectors at the surface. The detector in Antarctica, known as IceCube, is buried about 8,000 feet straight down.

"Trapping" a ghost particle might not actually be the best terminology for what these detectors are doing. IceCube, for instance, doesn't hold any neutrinos prisoner. The particles mostly blast straight through the detector. But on the way, some very (very!) rarely interact with the Antarctic ice and produce a shower of secondary particles emitting a type of blue light known as Cherenkov radiation. A range of light-sensing spherical modules, vertically arranged like beads on a string, pick up the light those particles emit. A similar detector exists in Japan: Super-Kamiokande. This uses a 55,000 ton tank of water instead of ice and is buried under Mount Ikeno.

Both are able to detect which direction the neutrino came from and its flavor. And so, physicists can see signs the ghost particle was there, but not the ghost particle itself. It's kind of like a poltergeist -- you can see the way it interacts with chairs (throwing them at you) and lights (menacingly switching them on and off), but you can't see the phantom itself. Spooky!

The sun is known to produce a specific type of neutrino


Great. So what can we learn from neutrinos?

Neutrinos are a fundamental particle in our universe, which means they underlie, in some way, everything that exists. Learning more about neutrinos will help unlock some of the mysteries of physics.

"Particle physicists study neutrinos in order to look for clues for physics beyond the Standard Model," says Thrane. He notes that physicists want to understand if neutrinos violate some of the fundamental laws of the Standard Model. "This may shed light on why there's more matter than antimatter in the Universe," Thrane says, noting that the problem has been referred to as one of the great mysteries in physics. 

We also know that extreme cosmic objects and events can produce them. For instance, exploding stars, or supernovas, are known to create neutrinos and shoot them across the universe. So are supermassive black holes chomping on gas, dust and stars. 

"Detecting neutrinos tells us about what is going on in these objects," says James.

Because they hardly interact with the surrounding matter, we could use neutrinos to see these types of objects and understand them in regions of the universe we can't study with other electromagnetic wavelengths (like optical light, UV and radio). For example, scientists could peer into the heart of the Milky Way, which is hard to observe in other electromagnetic wavelengths because our view is interfered with by gas and dust. 

Reliable detection and tracing could stimulate an astronomy revolution akin to the one we're currently seeing with gravitational waves. Essentially, neutrinos can give us a whole new eye on the cosmos, complementing our existing set of telescopes and detectors to reveal what's going on in the void. 

And then there are "sterile" neutrinos which...

Oh god. What are sterile neutrinos? 

I probably should've kept those under wraps, but seeing as you're here, sterile neutrinos are a whole other class of neutrinos. They're entirely theoretical, but scientists think they likely exist because of a feature in physics known as chirality. Essentially, the normal neutrinos we've been discussing are what some call "left-handed." So, some physicists think there may be "right-handed" neutrinos -- sterile neutrinos. 

They give them this name because they don't interact with other particles via the weak force, like normal neutrinos. They interact only through gravity. These types of neutrinos are considered a candidate for dark matter, the stuff that makes up more than a quarter of the universe but that we've never seen. 

That means neutrinos might also help answer another vexing puzzle in physics: What, exactly, is dark matter? There are lots of candidates for dark matter theorized by physicists, and there's still plenty to learn -- it may not be related to neutrinos at all!

A graphic image of the four types of neutrino: electron neutrino, muon neutrino, tau neutrino, sterile neutrino

The three flavors of neutrino and the theoretical "sterile" neutrino.

IceCube Collaboration

Cool. Anything else I need to know about neutrinos?

As Deborah Conway once sang, "It's only the beginning, but I've already gone and lost my mind."

We haven't gotten into some of the more mind-blowing theories about neutrinos, like neutrinoless double beta decay and the idea of the neutrino as a Majorana particle. 

Several new neutrino experiments have been proposed, including the Giant Radio Array for Neutrino Detection, or GRAND, which would see up to 200,000 receivers placed. The total area of the array is designed to be about the same size as Great Britain. The first 10,000 antennas are expected to be placed on the Tibetan plateau, near the city of Dunhuang, in the next few years.

Though we've been able to detect and trace only a few neutrinos so far, the next decade should see neutrino astronomy really take off. The bottom line is that understanding neutrinos, their flavors and masses, will provide a window into the fundamental nature of our universe. 

And it's always cool to chase ghosts. 

Originally published on April 17.