A single cosmic particle once hit Earth with the force of a fast pitch

On the night of 15 October 1991, a detector array called Fly’s Eye, set up at the Dugway Proving Ground in the Utah desert to watch for faint flashes of light in the upper atmosphere, registered something its operators had never seen before. A single subatomic particle had slammed into the atmosphere carrying roughly 320 exa-electronvolts of energy, a number so far outside expectations that physicists studying the readout nicknamed it the Oh-My-God particle on the spot. More than three decades later, it remains the most energetic cosmic ray ever recorded striking Earth.

A number too large to picture in normal terms

Physicists trying to explain what 320 exa-electronvolts actually means have settled on one comparison more than any other: that single particle, smaller than an atom, carried roughly the same kinetic energy as a thrown baseball. Consider that the energy of one 140-gram ball moving at highway speed had been compressed into a particle far too small to see, and the scale of the event becomes clearer. The reading was roughly 40 million times more energetic than the highest-energy protons ever produced inside a human-built particle accelerator, according to figures compiled from published analyses of the event, putting it well beyond anything a laboratory on Earth has managed to replicate before or since.

Traveling close enough to light speed to defy easy math

If the particle was an ordinary proton, as most cosmic rays are, its speed worked out to within a hair’s breadth of the speed of light itself, close enough that a photon traveling alongside it would need roughly 245,000 years, from Earth’s point of view, to pull ahead by a single centimeter. Because of relativistic time dilation at that velocity, a proton traveling from a source 1.5 billion light-years away would experience the entire journey as lasting only about 1.71 days from its own reference frame, even though billions of years would pass for any outside observer. When the particle eventually struck a nitrogen nucleus in the atmosphere, the collision released roughly 2,900 teraelectronvolts of energy in the center-of-mass frame, about 200 times more than the highest collision energy ever reached inside the Large Hadron Collider.

A cosmic ray that broke the rules on where it came from

The discovery unsettled astrophysicists because theory said a particle carrying that much energy should not have been able to travel far at all. Photons left over from the Big Bang fill the universe as the cosmic microwave background, and ultra-high-energy protons are expected to lose energy rapidly through collisions with those photons over cosmological distances, a limit known as the Greisen-Zatsepin-Kuzmin cutoff. Yet no source powerful enough to produce the Oh-My-God particle has ever been identified along the direction it appeared to travel from, and it is not even confirmed to have been a proton at all; more recent neutrino studies suggest a meaningful share of the very highest-energy cosmic rays are instead heavier ions, such as iron, which would travel somewhat differently and complicate efforts to trace their origin.

Catching a flash instead of the particle itself

No instrument catches one of these particles directly. When an ultra-high-energy cosmic ray strikes the upper atmosphere, it triggers a cascading shower of billions of secondary particles that spreads outward as it falls toward the ground, and detectors like Fly’s Eye work by watching for the faint streak of ultraviolet fluorescence that shower produces in the night sky, similar in principle to how a meteor briefly lights up as it burns. Ground-based arrays such as the Pierre Auger Observatory in Argentina pair that fluorescence detection with networks of surface tanks spread across hundreds of square kilometers of desert, cross-checking each candidate event against both methods before it is logged, a combination that has produced a large public catalog of the highest-energy cosmic rays recorded to date.

Hundreds of lesser cousins, and one near-equal rival

Since 1991, detector arrays around the world have logged hundreds of similar ultra-high-energy events above roughly 57 exa-electronvolts, confirming that the original detection was not a fluke or an instrument error. The closest rival arrived on 27 May 2021, when the Telescope Array experiment in Utah recorded a particle exceeding 240 exa-electronvolts, later named the Amaterasu particle after a sun goddess in Japanese mythology, as reported by researchers at the University of Utah, which operates the Telescope Array. That particle appeared to originate from the direction of the Local Void, a large, unusually empty region of space bordering the Milky Way, and like the original Oh-My-God particle, no plausible astronomical source has been matched to its arrival direction.

A mystery that newer instruments are still chasing

Researchers using the Telescope Array have since reported a broad “warm spot” of excess arrivals clustered within about 20 degrees of the constellation Ursa Major, a pattern that hints at a real, if still unidentified, source or cluster of sources somewhere in that direction. Because events at these energies are exceedingly rare, arriving perhaps once per square kilometer per century, expanding detector arrays across larger areas of desert and, eventually, orbit remains the most direct way to gather enough examples to pin down where particles like the Oh-My-God particle actually come from, and what in the universe is capable of accelerating anything to that speed in the first place.

This article was produced with the assistance of AI and reviewed by an editor.

More from Morning Overview