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Cosmic rays, explained

Cosmic rays are particles from outer space that travel across the universe. They can be made by the sun, by other stars exploding, and even black holes. They move extremely fast, nearly the speed of light, but when they hit the Earth they are generally blocked by our atmosphere and made harmless to humans.

Scientists are fascinated by cosmic rays because they can tell us about space—where they came from and what they encountered along the way—as well as the makeup of the galaxy and the universe.

The name “cosmic ray” dates back to 1925, when Nobel laureate and former UChicago faculty member Robert Millikan coined the term in a paper.

What are cosmic rays?

Cosmic rays are actually particles from space that travel across the universe. They started out as atoms that had their outer layers stripped away and are now just nuclei. They move extremely fast—nearly the speed of light.

Trillions upon trillions of cosmic rays hit the Earth every day. The vast majority of them are blocked by Earth’s atmosphere and magnetic field, but sometimes they will strike particles in the atmosphere and create a shower of secondary particles that make it to the ground.

Most of the cosmic rays we get here on Earth come from the sun, but we’ve also picked up particles that we think are coming from other galaxies.

Cosmic rays can tell us about space and the universe. In the 20th century, cosmic rays helped scientists discover antimatter and the muon—the first evidence for subatomic particles beyond the proton, neutron and electron. Cosmic rays can also tell us about the chemical and physical makeup of the universe; about how the universe has changed over time; and what happens around supermassive black holes and in the hearts of exploding stars.

The name “cosmic ray” dates back to 1925, when Nobel laureate and former UChicago scientist Robert Millikan coined the term in a paper in Science. (We’ve since learned that they are particles, not rays, but the name stuck.)

Where do cosmic rays come from?

Cosmic rays are constantly traveling across the universe in all directions. It’s difficult to tell where they came from, because cosmic rays are charged particles, so they can be pulled around by magnetic fields as they travel across space. But we can measure their energies and use that to calculate what kind of forces would be needed to accelerate the particles, which gives us clues about where they came from.

We can measure that we get bursts of cosmic rays when the sun flares, so some of them come from the sun. But many of them seem to come from further away. The majority probably come from somewhere in our galaxy, but a few appear to come from beyond our galaxy!

Scientists believe cosmic rays get spit out by faraway stars exploding (called supernovas). Others could be produced when matter falls into supermassive black holes, from highly magnetized neutron stars, or when galaxies collide.

What do cosmic rays tell us about the universe?

Scientists have long been fascinated with cosmic rays because they can tell us a ton of things about the universe.

These include:

  • What the elementary particles are. The first discovery of subatomic particles beyond the proton, neutron, and electron was when scientists watching cosmic rays in a cloud chamber in 1933 discovered the positron—the first known antimatter—and then the muon. This opened up the field of modern particle physics, which would go on to unravel the entire cast of subatomic particles.
  • The makeup of the universe beyond our solar system. Some cosmic rays are a rare opportunity to study matter that came from beyond our solar system, or even beyond our own galaxy. From them, scientists can estimate the amount of matter in the universe and the amounts of different elements.
  • What happens around exploding stars, black holes, and other extreme places in the universe. Cosmic rays are thought to be made in the aftermath of exploding stars, when supermassive black holes eat their surroundings, and when galaxies collide. Studying cosmic rays can give us a rare window into these processes that are otherwise too distant for scientists to easily access.
  • When meteorites formed, when they fell to Earth, and the history of the solar system. Some cosmic rays are energetic enough to hit an object traveling through space and turn some of its elements into new isotopes. When the meteorite falls to Earth, these isotopes start to decay and aren’t replaced, so scientists can sometimes use this fact to estimate how long ago a meteorite fell. Scientists have also used the same principle to estimate how long some of the grains in meteorites were traveling through the solar system, which can tell us about the history of our solar system.

Finally, cosmic rays can be useful in surprising other ways. For example, cosmic ray neutron sensors are being used to monitor soil moisture for more efficient farming and irrigation practices. Cosmic rays are also key to how we do carbon-14 dating, which can tell us the ages of archaeological artifacts, glaciers, and many other things.

How were cosmic rays discovered?

Beginning in the 1700s, scientists occasionally noticed oddities in their experiments. For example, particles in a completely sealed container would sometimes spontaneously discharge their electricity. In the next century and a half, scientists narrowed this down. They knew there was some source of radiation that occasionally interacted with particles in their experiments, but no one was sure where it was coming from. The Earth’s crust? The atmosphere? The sun?

Then in 1912, Austrian physicist Victor Hess flew a balloon to an altitude of more than 17,000 feet and found that the radiation rate increased significantly. He concluded that the radiation must be coming from above the atmosphere. For this, he was awarded the Nobel Prize in 1936.

The name “cosmic ray” itself dates to 1925, when Nobel laureate and former UChicago faculty member Robert Millikan coined the term in a paper in Science. (We’ve since learned that they are particles, not rays, but the name stuck.)

How do you detect cosmic rays?

There are several ways to detect cosmic rays. You can look for them directly—whether on the ground or above the atmosphere—or you can look for the showers of other particles that are produced when cosmic rays strike molecules in the atmosphere.

Current major cosmic ray experiments include:

  • The Pierre Auger Observatory. This is a large installation located in western Argentina, consisting of water tanks spread over an area the size of Rhode Island, and a detector looking at the sky. The detector catches the UV light created when cosmic rays hit the Earth’s atmosphere, and the water tanks catch the signals when cosmic rays hit them. By combining these two methods, scientists can tell how energetic the cosmic ray was and which direction it came from. The Pierre Auger Observatory was co-founded by the late Nobel laureate and UChicago Prof. James Cronin.
  • The Large High Altitude Air Shower Observatory. A ground-based experiment in Sichuan, China, LHAASO is spread over 300 acres and designed to pick up air showers from cosmic rays and gamma rays. It began operating in 2019.
  • The international neutron monitor network. All over the world, more than 50 instruments take continuous data on neutrons produced from cosmic rays. This helps scientists monitor incoming space radiation. (The U.S. stations are known as the Simpson Neutron Monitor Network, in honor of UChicago Prof. John A. Simpson’s pioneering cosmic ray research.) You can see live readings at the Neutron Monitor Database.
  • Other ground-based observatories. Many smaller experiments look for specific subsets of cosmic rays. For example, the High Altitude Water Cherenkov Observatory (HAWC) in Mexico, looks for cosmic rays with very high energies (between 100 GeV and 100 TeV). Others look for by-products of cosmic rays, such as the Very Energetic Radiation Imaging Telescope Array System (VERITAS) in Arizona, which catches incoming gamma rays, some of which are produced by cosmic rays. Many other experiments and observatories also provide data about cosmic rays; for example, the IceCube observatory in Antarctica looks for neutrinos, which are often produced when cosmic rays hit the atmosphere.
  • Balloons. The atmosphere interferes with cosmic rays, so for the purest measurements, scientists often send instruments aboard balloons—starting with Victor Hess’ famous 1912 flight above Paris. Throughout the 20th century, scientists have launched balloons to various altitudes to make these readings. Today, scientists with the University of Chicago and many other institutions work with NASA to launch cosmic ray detectors aboard very high-altitude balloons—just a few miles from space.
  • Satellites and spacecraft. Spacecraft can get measurements of cosmic rays even further—from Earth’s orbit or even around the solar system. Satellites such as China’s Dark Matter Particle Explorer (DAMPE) telescope can pick up cosmic rays from orbit. The Voyager 1 and 2 spacecraft, which launched in 1977 to venture out to the edges of our solar system, carry instruments to detect cosmic rays. So does NASA’s Parker Solar Probe as it loops the sun.

However, much closer to home, you can actually build your own cloud chamber and see cosmic ray trails yourself for about $100! Here’s instructions.

What do we know about cosmic rays, and what remains a mystery?

After more than a century of intensive research, we know a lot about cosmic rays. But many mysteries remain.

We do know:

  • What they’re made of. About 90% of them are hydrogen, 9% are helium, and 1% are heavier elements, like iron.
  • What their energies are. We can measure the kinetic energy they have coming in. The energy depends on how much mass the particle has and how fast it’s going. This can range from less than 1 MeV to more than 1 EeV. That’s less than the energy of a flying mosquito, all the way up to the energy of a tennis ball whacked at 100 kilometers per hour.

    The vast majority of cosmic rays have very low energies, but there are a few with higher energies. The highest-energy cosmic rays are about a hundred million times more energetic than the particles smashed in human-made colliders. (One particular cosmic ray, detected in 1991 and nicknamed the “Oh-My-God particle,” was going very nearly the speed of light when it smacked into a detector in Utah.)

  • That there are “hot spots” of cosmic rays in the sky. Large-scale surveys of cosmic rays suggest that there are a few hot spots in the sky that seem to produce more cosmic rays than others, but we don’t definitively know which object or objects are producing them.

We don’t definitively know:

  • Where cosmic rays come from. A percentage are almost certainly from the sun (those are now called “solar energetic particles”). But many cosmic rays appear to come from elsewhere in the galaxy or even from other galaxies. It’s difficult to pinpoint their sources because their paths get scrambled by magnetic fields on their way to us.
  • How the highest-energy particles get accelerated. People have suggested they might be created in a type of galaxy known as a starburst galaxy (so called because they produce lots of stars). Another possible source is supermassive black holes—maybe a star or other matter falls into a black hole and cosmic rays are spat out from the debris.
  • How they affect planets and life. So many cosmic rays hit the atmosphere that it’s possible the particles play a role in our climate, including cloud formation and behavior. CERN’s CLOUD experiment creates “artificial cosmic rays” to study the physics behind cosmic rays and ultimately, what their role might be in affecting clouds in our atmosphere.

Are cosmic rays dangerous?

Most cosmic rays are deflected or neutralized by Earth’s atmosphere and magnetic fields. Some do reach the ground, but they’re generally no worse than any other background radiation we are routinely exposed to. You are exposed to more of them at high altitudes and during plane rides.

(Cosmic rays might occasionally be responsible for computers crashing, though.)

However, once you leave Earth’s protective bubble, cosmic rays become an issue. NASA and others are researching how to protect both astronauts and equipment from cosmic radiation during long-term space travel.

How do cosmic rays affect the Earth and the universe?

Scientists think that cosmic rays affect our universe in a number of ways!

Cosmic rays probably play a role in creating both stars and the lighter elements by heating up interstellar matter.

They affect Earth, too. They are the source of a percentage of the background radiation experienced everywhere on Earth. Cosmic rays may also play a role in heating the atmospheres of planets, and scientists are investigating whether they affect cloud formation on Earth.

Some have speculated that cosmic rays may even have played a role in creating life on Earth—which is especially of interest as we find more and more planets around distant stars and want to better understand the factors that play into whether a planet could host life.

What role has the University of Chicago played in cosmic ray research?

The University of Chicago has a rich history in cosmic ray research; many of the things we know about them came from UChicago-led experiments.

Robert A. Millikan, a Nobel laureate and former UChicago faculty member, made hundreds of measurements of cosmic rays beginning in the 1920s (and gave them the name ‘cosmic rays’ in a seminal 1925 paper). He sank instruments 60 feet deep into a Colorado alpine lake and found that some of these rays had extremely high energies, and that they were coming from space in all directions. He wrote, “The most penetrating X-rays which we produce in our hospitals cannot go through half an inch of lead. Here were rays originating somewhere out in space, at least a hundred times more penetrating than these.”

In 1931, fellow Nobel laureate and UChicago Prof. Arthur Compton organized a global survey of cosmic rays—taking instruments himself to mountains in multiple countries and coordinating others in locations around the globe from Alaska to South Africa—and found there were more cosmic rays at the poles of the Earth than at the equator. This is because cosmic rays are charged particles, and so Earth's magnetic field guides them towards the Earth's magnetic poles. (Millikan and Compton would go on to have a series of debates on the nature of cosmic rays, which was covered in the New York Times, including on the front page.)

Nobel laureate and UChicago Prof. Enrico Fermi became interested in how cosmic rays could be accelerated, and suggested in a seminal 1949 paper that cosmic rays might be accelerated by the shockwaves coming off very violent phenomena in space, like exploding stars. A modified version of that theory is widely considered likely, but there is no direct evidence yet.

Meanwhile, Prof. Willard Libby deduced that cosmic radiation would affect the isotopes of carbon on Earth and used the principle to develop carbon-14 dating to tell how long an organism has been dead—a technique which revolutionized the fields of archaeology, anthropology, and climate science, among others, and for which he would receive the Nobel Prize.

In the latter half of the 20th century, UChicago became a center for building instruments to measure cosmic rays. These could be done from the ground or carried aboard balloons, aircraft, and even spacecraft.

In the 1940s, Prof. Marcel Schein launched balloons and conducted other experiments which found, among other things, that most cosmic rays are protons—identical to the protons found in the nucleus of every atom. Prof. John A. Simpson invented a neutron monitor to pick up the particles from cosmic ray showers on the ground, with which he made fundamental discoveries about the conditions in space near Earth and which are still used today around the world to monitor incoming space radiation. His instruments picked up a huge solar flare in 1956, which helped prove that radiation from the sun envelops the solar system and interacts with particles coming from further away.

Simpson, along with Profs. Anthony Tuzzolino, Simon Swordy, Dietrich Müller, and Peter Meyer, also developed experiments such as the “Chicago Egg,” which flew aboard the Challenger spacecraft. Simpson’s instruments are still collecting data aboard several ongoing spacecraft missions, like Pioneer 10.

Readings from all of these experiments contributed to our understanding of cosmic rays; Simpson and Meyer showed how the sunspot cycle influenced all but the highest-energy cosmic rays, and Meyer discovered the ratios of intensities of cosmic ray components—providing clues to understanding the origins of cosmic rays.

Nobel laureate Prof. James (Jim) Cronin also devoted the second half of his career to studying cosmic rays. He conducted an experiment known as the Chicago Air Shower Array (CASA), which ran from 1992-1999 in Utah to study ultra-high-energy cosmic rays. He also co-founded a massive international experiment that would eventually become Pierre Auger Observatory, which has been measuring air showers in Argentina since 2005.

This research continues today. Prof. Scott Wakely leads the development of the HELIX experiment, which will fly aboard a balloon to measure the chemical and isotopic abundances of light cosmic ray nuclei. Prof. Angela Olinto leads the planned Probe of Extreme Multi-Messenger Astrophysics space mission and the launched Extreme Universe Space Observatory on a Super Pressure Balloon missions—designed to discover the origin of the highest-energy cosmic particles, their sources and their interactions.