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Step outside on a clear night and look up. You’re watching light that has traveled billions of years to reach your eyes. But something else is raining down on you constantly, something you can’t see, can’t feel, and have probably never thought about: galactic cosmic rays.
At this very moment, these high-energy particles from distant corners of the galaxy are streaking through your body at nearly the speed of light. They pass through you as though you barely exist. Galactic cosmic rays are one of the most fascinating phenomena in nature.
Despite the name, cosmic rays aren’t rays at all. They’re particles: mostly protons, but also helium nuclei, electrons, and occasionally heavier atomic nuclei stripped bare of their electrons. They travel through interstellar space carrying enormous amounts of energy packed into something far smaller than an atom. When they slam into Earth’s atmosphere, they trigger cascades of secondary particles that shower down to the surface like invisible confetti.
Roughly one cosmic ray particle passes through the palm of your hand every second.
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It’s also part of a theme that science fiction has always loved, and is featured in Project Hail Mary, the film based on Andy Weir’s novel. It follows Ryland Grace, a man who wakes up alone on an interstellar spacecraft, tasked with solving the mystery of a substance that is slowly killing the Sun. The physics at the heart of the story – particles, radiation, energy behaving strangely across interstellar distances – is precisely the kind of thinking that real cosmic-ray physicists do every day.
How cosmic rays were discovered
The story of their discovery is an interesting one. In 1912, Austrian physicist Victor Hess climbed onto a hydrogen balloon and ascended to about 5,300 meters, carrying an electroscope to measure ionizing radiation. The expectation was simple: the higher you go from Earth’s radioactive crust, the weaker the radiation should be.
Hess found the opposite. The radiation grew stronger as he climbed, even during a partial solar eclipse that ruled out the Sun as the source. He concluded, correctly, that the ionizing radiation was extraterrestrial in origin. Hess would win the Nobel Prize for this discovery in 1936, a triumph of someone literally going to extraordinary heights to get the answer.
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So where do cosmic rays come from? For the lower-energy variety, scientists believe the main culprits are supernova remnants, the glowing, expanding debris clouds left behind when massive stars explode. Within these turbulent remnants, magnetic fields accelerate charged particles like a cosmic game of pinball, each bounce adding more energy.
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“Supernovae are the most powerful engines we know of in the galaxy,” one astrophysicist noted in a summary of the field, “and cosmic rays are, in a sense, their exhaust.” Over billions of years, the Milky Way has been quietly filling up with this exhaust, and we swim through it constantly.
Mysterious origins: ultra-high-energy cosmic rays
The highest-energy cosmic rays, however, remain a genuine mystery. These particles arrive carrying energies so extreme that they shouldn’t even exist – at those energies, particles should interact with the cosmic microwave background radiation permeating all of space and lose energy over long distances.
Yet they arrive at Earth apparently intact, suggesting they came from somewhere relatively close, cosmically speaking. Candidates include active galactic nuclei, supermassive black holes actively consuming matter at the hearts of other galaxies, and gamma-ray bursts, some of the most violent explosions in the known universe.
Here’s where it gets wonderfully strange.
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Because cosmic rays are charged particles, they don’t travel in straight lines. Magnetic fields – both inside our galaxy and beyond – bend their paths, scrambling their directions over the vast distances they travel. By the time they reach Earth, it’s nearly impossible to trace most of them back to a source. Astronomers and physicists have to work like detectives piecing together indirect clues rather than simply looking backward along the particle’s path.
X’ray of the Great Pyramid
There’s a remarkable real-world consequence to all this. In 2017, researchers using a technique called cosmic ray muon tomography – essentially using the byproducts of cosmic ray collisions in the atmosphere as a kind of X-ray – scanned Egypt’s Great Pyramid of Giza. The particles, far more penetrating than any human-made scanner, revealed a previously unknown void deep inside the monument, roughly 30 meters long and hidden for 4,500 years.
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The universe’s own radiation, raining down on an ancient stone structure, helped solve an archaeological puzzle that had stumped scholars for decades.
Cosmic rays also have a more personal dimension. Astronauts aboard the International Space Station, partially shielded but far above Earth’s protective magnetic field and atmosphere, report seeing occasional flashes of light even with their eyes closed, a phenomenon caused by cosmic rays passing directly through their retinas. It’s a strange, intimate reminder that the universe is not something happening out there, at a safe remove. It’s passing through us.
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“We are not isolated observers of the cosmos,” the physicist and author Carlo Rovelli once wrote. “We are part of it, immersed in it.” Galactic cosmic rays make that abstract truth feel almost visceral. Every second of every day, high-velocity messengers from exploded stars and distant black holes pass through your body and continue on their way, indifferent and unstoppable, as they have since long before there was anyone here to wonder about them.
Shravan Hanasoge is an astrophysicist at the Tata Institute of Fundamental Research.
