Citizen Science #30 by Jamie Zvirzdin

Solving the Clue of the GZK Cutoff

When I was four years old, my pregnant mother drove my sister and me out to Dugway Proving Grounds in Utah to visit my father, who worked on the original Fly’s Eye detector for the University of Utah. Even though I was so young, I remember standing on a hilltop in the desert staring at what looked like big garbage cans. My father told me they were watching for something invisible and rare: tiny particles called cosmic rays, which crash into Earth’s atmosphere with unimaginable energy.

Those “garbage cans” were the early fluorescence detectors for ultrahigh-energy cosmic rays (UHECRs), which are mysterious messengers from deep space. When one of these particles slams into the atmosphere, it triggers a cascade of secondary particles that emit faint streaks of light. The Fly’s Eye recorded those streaks and tried to trace them backward, like Nancy Drew following muddy footprints in the dark. Where did they come from? What sent them?

We are still trying to figure out this mystery.

Decades later, the Fly’s Eye became the High-Resolution Fly’s Eye, which in 1991 detected the aptly nicknamed Oh-My-God particle—the highest-energy cosmic ray ever recorded. In 2021, the next generation of the Fly’s Eye, the Telescope Array Project in Delta, Utah, detected another with an almost equally jaw-dropping energy: 244 exa–electronvolts. We named it Amaterasu, after the Shinto sun goddess. Its source? A puzzling patch of cosmic emptiness known as the Local Void, an abandoned lair in the cosmic fog while the real hideout lies somewhere else.

But physicists do have one solid clue: a cosmic speed limit known as the Greisen–Zatsepin–Kuzmin (GZK) cutoff. Proposed in the 1960s, it predicts that cosmic rays above a certain energy cannot travel indefinitely through space. The universe is full of a soft bath of ancient light—the cosmic microwave background, the faint afterglow of the Big Bang. A cosmic ray moving near the speed of light eventually collides with one of these photons, losing energy in the process.

The collisions are rare but relentless, like invisible speed bumps scattered through spacetime. Any particle exceeding roughly 57 exa–electronvolts can’t travel more than about 100 megaparsecs, or 330 million light-years, without slowing down. It’s as if the cosmos itself enforces its own traffic laws.

When I lived in Nicaragua, the speed bumps were so high they were called policías acostados—sleeping policemen. Hit one too fast, and you’d scrape the underside of your car (which I did on occasion, even in a 4×4 Subaru Forester!). The GZK limit is the universe’s version of that. No matter how powerful the particle, it eventually hits the cosmic equivalent of a speed bump and loses energy.

Yet every so often, like a rebellious driver in a souped-up car, a cosmic ray seems to barrel past the limit. Events like the Oh-My-God particle and Amaterasu appear to defy the cutoff, arriving from regions too distant or too empty to make sense. Maybe magnetic fields are twisting their paths. Maybe they’re not protons but heavier atomic nuclei, like helium or iron, which interact differently with the cosmic background. Or maybe something truly exotic is going on—physics beyond our current understanding.

That’s why the GZK cutoff feels so much like a Nancy Drew case file. In “The Clue of the Whistling Bagpipes,” Nancy travels to the foggy Scottish Highlands to trace her ancestry, only to uncover a sheep-smuggling ring whose bagpipe signals are cleverly disguised and misdirected. The landscape is full of echoes and illusions, and the clues don’t point straight to their source.

Cosmic detectives face the same challenge. Every ultrahigh-energy particle is a cryptic note from the universe, distorted by invisible forces before it reaches us. When the High-Resolution Fly’s Eye confirmed the GZK effect in 2008, it was a triumph of patience and precision: decades of data revealing a sharp drop in cosmic-ray numbers beyond a certain energy threshold—the universe’s speed limit, written in the language of statistics.

Yet the rule breakers persist. The Oh-My-God particle, Amaterasu, and others like them keep turning up, widening our search fields and forcing us to rethink what each clue might signify. Cosmic rays are not just law-breaking particles but also reminders that nature delights in both order and disorder. Limits are real, but so are the culprits that skirt them.

The GZK cutoff tells us that even the most powerful travelers in the cosmos eventually slow down enough for us to catch them. But it also reminds us that science—like Nancy Drew on the trail of the whistling bagpipes—presses on, clue by clue. Despite setbacks that include lost funding, dwindling public attention, and the creeping fog of misinformation, I’m hopeful that one day, with enough data and statistical certainty, we’ll be able to name hideout headquarters of ultrahigh-energy cosmic rays—most likely the furious hearts of active galactic nuclei in neighboring galaxies, where supermassive black holes spin, feed, and hurl matter across terrific distances at terrible speeds.

Until then, we keep gathering clues, mapping trajectories and following the faintest cosmic trails back to their extragalactic hidden lairs.

Jamie Zvirzdin researches cosmic rays with the Telescope Array Project, teaches science writing at Johns Hopkins University and is the author of “Subatomic Writing.”