The Oh-My-God Particle and Amaterasu

On October 15, 1991, a single proton struck Earth's atmosphere carrying the energy of a baseball thrown at 60 mph — all compressed into a particle smaller than an atom. The physicists who detected it could only say "oh my god." Three decades later, an even stranger particle arrived from an empty void in space.

October 15, 1991: The Oh-My-God Particle

The Fly's Eye cosmic ray detector sat in the Utah desert, watching the night sky with arrays of photomultiplier tubes. On that October night, something extraordinary happened.

A cosmic ray entered the atmosphere and created an air shower so massive that it triggered detectors across the entire array. When scientists analyzed the data, they calculated an energy of approximately 3 × 10²⁰ electron volts — 320 EeV, or 320 exa-electron volts.

To put this in perspective: if you could travel that fast, a trip from the Milky Way to the Andromeda Galaxy — 2.5 million light-years away — would take only 20 minutes from your perspective. The entire observable universe would pass by in a few months of subjective time.

The discovery was so stunning that University of Utah physicist Pierre Sokolsky reportedly exclaimed "oh my god" — and the name stuck.

Why It Shouldn't Exist

The Oh-My-God particle wasn't just remarkable for its energy — it was theoretically impossible.

In 1966, physicists Kenneth Greisen, Georgiy Zatsepin, and Vadim Kuzmin had predicted that cosmic rays above about 5 × 10¹⁹ eV should be absorbed by the cosmic microwave background. At these energies, protons interact with CMB photons, producing pions and losing energy. This "GZK cutoff" should limit the distance such particles can travel to about 100-200 million light-years.

But at 3 × 10²⁰ eV — six times above the GZK threshold — the Oh-My-God particle should have lost most of its energy unless it came from somewhere very close. Within 100 megaparsecs, there are no obvious sources capable of accelerating particles to such energies.

Where did it come from? The arrival direction pointed to a relatively empty region of the northern sky, near the constellation Ursa Major. No obvious source — no active galactic nucleus, no powerful radio galaxy, no known accelerator of any kind.

Three Decades of Mystery

The Oh-My-God particle sparked intense theoretical speculation:

Exotic sources: Perhaps the particle came from the decay of superheavy dark matter, or topological defects from the early universe. These "top-down" models could produce particles locally, avoiding the GZK limit.

Lorentz invariance violation: If special relativity breaks down at extreme energies, the GZK threshold might shift, allowing particles to travel farther than expected.

Z-bursts: Ultra-high-energy neutrinos might interact with cosmic neutrino background particles near Earth, producing hadronic showers that mimic cosmic rays.

Mundane explanations: Perhaps it was a statistical fluke, an instrumental error, or a heavy nucleus that appeared more energetic than it was.

Over the following decades, both Fly's Eye's successor (HiRes) and the Pierre Auger Observatory detected more events above 10²⁰ eV — though none quite as energetic as the original. The phenomenon was real, but its explanation remained elusive.

May 27, 2021: Amaterasu Arrives

Thirty years after the Oh-My-God particle, history repeated — with new mysteries.

At 00:56:57 UTC on May 27, 2021, the Telescope Array detected a cosmic ray with energy 2.4 × 10²⁰ eV — the second-highest ever recorded. The shower triggered 23 surface detectors across 48 km², creating an unmistakable signature.

The Telescope Array collaboration named it "Amaterasu" after the Japanese sun goddess, referencing both the Japanese institutions that built much of the detector and the particle's seemingly impossible origin from emptiness.

Arrival from the Void

When scientists traced back Amaterasu's arrival direction, accounting for uncertainty, they found it pointed toward the Local Void — a region of space notably empty of galaxies.

The Local Void is a roughly spherical region about 150 million light-years across, lying beyond the Virgo Supercluster. It contains far fewer galaxies than average — a genuine emptiness in the cosmic web.

For a cosmic ray to come from this direction, it would need either:

• A source within the void that we haven't detected (unlikely given its emptiness)
• Extreme magnetic deflection bending its path by tens of degrees
• An origin mechanism that doesn't require conventional astrophysical sources

As Telescope Array physicist Toshihiro Fujii put it: "You trace its trajectory to its source and there's nothing high energy enough to have produced it. That's the mystery of this — what the heck is going on?"

Comparing the Giants

The Oh-My-God particle and Amaterasu share striking similarities:

Both particles arrived from directions with no apparent powerful sources. Both had energies well above the GZK threshold. Both remain unexplained.

What Could Produce Them?

The energy scale alone places severe constraints on possible sources. To accelerate a particle to 10²⁰ eV requires either:

Enormous size: A particle gains energy as it spirals in a magnetic field. The maximum energy depends on the product of magnetic field strength and size. A source needs either powerful fields (like a magnetar) or huge dimensions (like a galaxy cluster), or both.

The Hillas criterion: Physicist Michael Hillas showed that a source must satisfy E_max ∝ Z × B × R, where Z is particle charge, B is magnetic field strength, and R is source size. Very few known objects fall in the allowed region of this parameter space for 10²⁰ eV protons.

Possible accelerators include:

• Active galactic nuclei jets (but none identified in arrival directions)
• Gamma-ray bursts (but they're transient and distant)
• Young, fast-rotating pulsars or magnetars
• Galaxy cluster shock waves
• Something we haven't discovered yet

The Heavy Nucleus Question

One partial explanation: if these particles are heavy nuclei (like iron) rather than protons, the situation changes.

A heavy nucleus is deflected more by magnetic fields, potentially explaining why arrival directions don't point to obvious sources — the real source could be tens of degrees away. Heavy nuclei also have different GZK horizons (they undergo photodisintegration rather than pion production).

Unfortunately, determining composition for individual events is essentially impossible. Air shower characteristics provide statistical composition information, but can't identify what any single cosmic ray was made of.

Why They Matter

The Oh-My-God particle and Amaterasu are more than curiosities. They represent the absolute extreme of the cosmic ray spectrum — the outer boundary of what the universe produces.

Their existence proves that somewhere, somehow, nature accelerates particles to energies that push the limits of known physics. Understanding how — and where — this happens would illuminate some of the universe's most extreme environments.

They also highlight how much we don't know. After 113 years of cosmic ray research, and billions of dollars spent on massive detectors, we still can't explain a single particle.

But perhaps that's the point. Science advances by confronting mysteries, not solving them. The Oh-My-God particle launched three decades of investigation. Amaterasu has launched a new phase. Somewhere in the data — or in theoretical insights yet to come — the answer waits.