The Fly’s Eye was a crude cosmic ray detector perched on top a Utah mountain in the early 80s and 90s. It’s long obsolete now, but it’ll always have a place in astronomy history: On October, 15 1991, it detected something called the Oh-My-God particle, a cosmic ray going faster than astronomers thought possible.

Natalie Wolchover in Quanta Magazine has penned an eloquent history of the Oh-My-God particle and how it’s shaped physics in the decades since. But first, you might want to understand exactly how fast this thing was going and why it shocked astronomers so:

A cosmic ray from space, it possessed 320 exa-electron volts (EeV) of energy, millions of times more than particles attain at the Large Hadron Collider, the most powerful accelerator ever built by humans. The particle was going so fast that in a yearlong race with light, it would have lost by mere thousandths of a hair. Its energy equaled that of a bowling ball dropped on a toe. But bowling balls contain as many atoms as there are stars. “Nobody ever thought you could concentrate so much energy into a single particle before,” said David Kieda, an astrophysicist at the University of Utah.

Aside from the sheer magnitude of its energy, the Oh-My-God particle went up against a cosmic speed limit known as the GZK cutoff, which held that cosmic rays going faster than 60 EeV should be slowed down by background radiation. That mean’s ultrahigh energy cosmic ray must have originated from somewhere nearby, but where?

In the quarter century since, astronomers have erected ever larger and more powerful detectors in search of an answer. Head over to Quanta to read more.


Top image: The Fly’s Eye array in Utah. Credit:HiRes

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