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Today, cosmic rays are used to understand things like supernovas, but in the early 1900s, they helped us discover brand-new subatomic particles long before the first accelerators.

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[ ♪ Intro ].

Around 1910, two small-but-mighty experiments popped up in the scientific literature. The first was from a physicist named Domenico Pacini in 1911.

He sent instruments underwater and measured radiation levels at different depths. The second came from another physicist named Victor Hess. In 1912, he jumped in a hot air balloon and traveled about five kilometers above Central Europe to measure the radiation way up there.

These experiments might seem like standard data-collecting stuff, but they helped reveal something that would change the way we understand the universe. See, about ten years earlier, scientists had detected a very low level of radiation in the air. Originally, they assumed it came from rocks, but Pacini and Hess showed them that they were wrong.

They found that there was more radiation at higher altitudes and less lower down. And that meant the radiation couldn't just come from the Earth. Some of it had to be coming from space.

Eventually, this kind of radiation became known as cosmic rays. Today, it's mostly used to understand the objects that produce it, but in the early 1900s, it had a different job:. It helped us discover brand-new subatomic particles long before today's accelerators were a glimmer in any physicist's eye.

So in a way, all those experiments underwater and in balloons didn't just help us find some space radiation:. They helped found modern particle physics. After we first discovered them, it took decades to figure out what cosmic rays were.

But today, we know they're charged particles that are constantly hitting Earth at nearly the speed of light. They're mostly made of individual protons, but have a few electrons and larger atomic nuclei every once in a while, too. And while some come straight from the Sun, most are from big, destructive things, like supernovas or stars falling into black holes.

Recently, astronomers have used these rays to learn about these extreme environments, but many scientists in the early and mid-1900s weren't really focused on the rays themselves. Instead, they studied what happened when cosmic rays reached Earth. And we're glad they did, because these rays ended up being one of our most important early tools for studying the universe.

Since cosmic rays move very close to the speed of light, they carry a lot of energy. When they hit things, that collision is so powerful that the energy itself can turn into subatomic particles. The idea that energy and matter are interchangeable is actually the whole point of “E = m c-squared,” so the “energy becoming particles” thing isn't game-breaking.

But it was game changing, because some of the particles that popped up in those cosmic ray collisions were things we had never seen before. In the early 1900s, scientists were able to study them in devices called cloud chambers, where water or alcohol vapor condensed into a cloud around anything that passed through it. They watched how the particles moved in these chambers, and then they could use that data to figure out their properties.

For instance, if the cloud chamber sat in a magnetic field, electrically-charged particles would make curved paths, and the shape and length of that path would reveal things like the particle's speed, mass, and how long it lived. Which sounds like magic, but it's really a pretty straightforward research method and it revealed so much. In 1932, for example, a postdoctoral researcher named Carl Anderson was using these sorts of methods to watch cosmic rays hit a lead plate.

He found that the particles coming out of his collisions had the same properties as electrons, except that they had the opposite electric charge. They had a positive charge instead of the usual negative one, but everything else seemed weirdly the same. As it turned out, Anderson had discovered positrons and the first kind of antimatter ever observed.

All matter has an antimatter counterpart, so Anderson effectively doubled the number of known particles with a single discovery. And if you know anyone who's ever had a Positron Emission Tomography, or PET scan, you can thank Carl Anderson. But that wasn't his last cosmic ray discovery.

A few years later, he and others found another key particle: the muon. It's a heavier version of the electron and in the eighty years since this discovery, scientists have used muons to conduct hundreds of tests of Einstein's theory of special relativity, the theory that talks about how things behave when they're going really fast. We use Einstein's ideas to understand the universe, so making sure he was actually right is kind of important.

And muons help with that. But in the 1940s, the most important thing about muons was that they existed. Physicists had predicted there'd be positrons a few years before their discovery.

But no one thought there'd be heavy electrons out there. They were completely unexpected. And the closer physicists watched cosmic ray collisions, the more of these sorts of surprises popped up.

They even put scientists on a path that would lead to the discovery of quarks, the tiny building blocks that make up things like protons and neutrons. Ultimately, the torrent of discoveries coming out cosmic ray research was amazing. Like, who would have thought that space radiation would turn out to be such an accessible way to understand the subatomic world?

But also, all these discoveries kind of sent everyone scrambling. So many things were being discovered that particle physics, which was now becoming a proper scientific field, was in chaos. Nobody knew how all these pieces fit together, and scientists were often unsure if what they were seeing was something completely new, or was just a known particle acting in a different way.

To make things more complicated, cosmic rays aren't exactly reliable. They're free and powerful, and you can't control how often they come into your chamber, or how strong they'll be. And if physics needs anything, it's a lot of consistent data.

Eventually, this led researchers to start building particle accelerators to test their ideas. And by the 1960s, they were mostly moving on from cosmic rays. Although we still use this radiation for other studies, this was the end of kind of a sweet chapter in physics.

This space radiation let us see a universe that had been completely hidden from us. Thanks for watching this episode of Scishow Space which we couldn't make without our Patrons on Patreon. If you want to learn more, go to [ ♪ Outro ].