highest-energy particles in nature

Among the cosmic rays that strike the Earth, a tiny handful arrive carrying almost unbelievable amounts of energy — far more than anything human beings have ever produced in a machine. The most famous single example, detected in 1991, was nicknamed the Oh-My-God particle. It was a single subatomic particle, probably a lone proton, yet it carried about the same energy as a baseball thrown at fifty miles an hour, all concentrated in one speck far smaller than an atom. Such ultra-high-energy cosmic rays are extraordinarily rare: one strikes a given square kilometre of the Earth only about once a century.

Their energies are staggering on the particle scale: tens of millions of trillion electronvolts, written as a few times 10 to the 20th electronvolts. For comparison, the most powerful accelerator on Earth, the Large Hadron Collider, reaches only about 10 to the 13th. So nature, in these rare events, out-accelerates our best machines by a factor of tens of millions. To catch them, physicists build enormous detector arrays — the Pierre Auger Observatory in Argentina spreads tanks of water across an area larger than a small country — and watch for the vast air showers a single particle creates.

These particles pose a genuine puzzle. Theory says there should be an upper limit, called the GZK cutoff, because a proton above a certain energy ought to lose energy by colliding with the photons of the cosmic microwave background as it crosses millions of light-years of space — yet a few particles seem to scrape past it. Where they come from, and what cosmic engine — supermassive black holes, colliding galaxies, something stranger — could fling matter to such energies, remains one of the open questions where particle physics and astronomy meet.