Every moment of your entire life, billions of particles called neutrinos have been flying through your body. You can’t see them and can’t feel them. In fact they are so miniscule that they almost never interact with the atoms that make up your cells and tissues, the atoms that make up the screen you're reading this on, or the mass of the world around you.

Neutrinos aren’t visible, and so rarely interact with visible particles that, despite being the most numerous particles in the universe, we know almost nothing about them. And yet, unlocking the hidden secrets of neutrinos could help us understand the world around us, our sun, distant stars, black holes, and even the beginning of the universe itself. That’s why researchers at the University of Minnesota Duluth (UMD) are hoping to learn more about them.

To help us understand more about neutrinos, we connected with Alec Habig, PhD, one of those researchers, and a professor from the Department of Physics and Astronomy at UMD. Since 1989, Habig has been working with laboratories and collaborators across the country and around the world to study these tiny particles.

"Neutrinos are the most numerous particles out there, and they're the ones we know the least about," says Habig. "Out of all the fundamental particles, they're the hardest to study because they go right through whatever you build to look at them."

Habig and researchers from the University of Minnesota are hoping that by studying these ephemeral particles, they might learn more about where neutrinos come from. That’s where partners like the U.S. Department of Energy’s Fermi National Accelerator Laboratory come into play. From its lab near Chicago, Illinois, Fermi generates the world's strongest lab-created neutrino beam, and aims it directly through the earth at detection sites in northern Minnesota.

Since neutrinos aren’t disturbed by dust, planets, light or dark, they can zoom across the universe (and the earth’s crust) uninhibited, and if we can catch one, we can potentially learn more about it, or point back to where it came from. Studying lab-generated neutrinos coming from Fermi lab helps us better understand what we’re seeing with naturally occurring neutrinos. And as we learn more, we might be able to discover more about the distant black hole, star or supernova where natural neutrinos originate.

If we zoom way in, we can better understand how a universe of stuff can let so much slip through.

Atoms: emptier than you think

An atom is composed of particles that form a nucleus, and around that nucleus spins a series of other, infinitely smaller particles called electrons. We're used to seeing a diagram showing these as being close to each other, but in reality that's not to scale because an atom is mostly empty space. If you picture an atom as a baseball stadium, the nucleus would be like a baseball sitting on the pitcher's mound, while the electrons would be spinning around the outside of the stadium.

In other words, the nucleus is 100,000 times smaller than the size of the total atom.

To every other atom though, and to people interacting with them, that empty space between the electrons and the nucleus is perceived as solid because all atoms are charged and push against one-another like opposing magnets.

The neutrino, however, is another type of infinitely small particle. Unlike the particles in an atom, a neutrino has no charge (it's neutral), so it doesn't have that same push-pull effect as atoms. Because it’s neutral, a neutrino can go right through the cloud of electrons spinning around the atom, and right through all that empty space.

“Unless it happens to hit a miniscule target in that tiny nucleus,” says Habig. A bullseye.

Thinking back to our stadium-sized atom and the baseball-sized nucleus on the pitcher’s mound, “the neutrino has to hit that baseball really dead-on to interact with it,” he says. “The bullseye is a speck of dust 1000 times smaller than the baseball. “

The coin toss and the cosmic cue ball

Because neutrinos are so tiny and have so little mass, it’s incredibly rare that a neutrino bumps into the particles that make up the atoms. It’s so rare in fact, that over an average person’s lifetime, even with billions of them passing through per second, the likelihood of a neutrino bumping into even a single particle in a person’s body is just 50:50.

“If I live to be 80, it's a coin flip that even one tripped over something in my body,” says Habig.

And yet, every once in a while, a neutrino will hit the bullseye and bump directly into a nucleus, and that's when some magic happens. That’s exactly what Habig and his team are watching for in their purpose-built detectors.

"Our detectors have to do two things," he says. "They have to be big, so we have a lot of bullseyes. And they have to be able to see the results of charged particles flying off." Sensors in the detectors track that movement of visible particles, and are able to reconstruct the path of invisible neutrinos.

"We know how things bounce. The same physics governs really tiny balls at the subatomic level," says Habig. Like watching a game of pool, he and his team can reconstruct the ball strikes. "Okay, we saw stuff fly out that way, so the neutrino must have come from over there to hit there, to make them go there,” he says, pointing this way and that. “Now we can say we see the neutrino, even though we never saw it."

Piecing the puzzle

What might Habig and fellow researchers learn from neutrinos? They really don’t know yet. Scientists are still in the early stages of understanding these tiny particles, but they’re a critical piece of the puzzle that makes up the universe. "We're at the stage of the puzzle-building where we’re opening the box and you're rooting around, and you found the corners and the sides," he says. "What shape are the rest of the puzzle pieces? We don't know."

And that’s science. Sometimes there’s a very clear purpose and goal, and other times, it’s less clear.

"A couple hundred years ago, people were studying this electricity thing without knowing what use it would be," Habig says. "It gave you a shock. That's cool. It made lightning go. That's obviously interesting. But then putting the work in to understand this strange new thing that didn't have any immediate practical implications today has paid off, big time, in that everything we do uses what turned out to be electrons," he says.

Habig thinks the same could be true of studying neutrinos.

"There are a billion of them for every electron. And what do they do? Well, let's find out."