Nothing on the tree-less plains of western Argentina seems to expend much energy. Cattle stand nearly motionless as they graze on the thin grass, which grows slowly in the dry heat and high altitude. A cylindrical water tank with a small solar panel and a skyward-facing antenna sits unobtrusively in the nearly motionless landscape. But hidden within this scene is plenty of drama. At any given moment, millions of projectiles from deep space are raining down, penetrating every object in their path. Each particle then vanishes without a trace—unless it happens to pass through the water tank, where it causes a minute spark visible to scientists thousands of miles away.
The tank is one of 1,600 spaced out at one-mile intervals over 1,100 square miles of land, an area bigger than Rhode Island. Collectively they make up the Pierre Auger Cosmic Ray Observatory, a $50 million physics experiment to study bits of atomic shrapnel that blast out from some of the most violent places in the universe. These energetic particles, called (somewhat misleadingly) cosmic rays, tell revealing tales about the exploding stars and black holes that have shaped galaxies and seeded the cosmos with the essential elements of life.
Traditional telescopes are blind to many of these cataclysms. Some 600 miles to the north, atop Chile’s high mountains, some of the world’s greatest observatories are surveying the distant universe in breathtaking detail, and yet they have little new to say about the inner core of a quasar, the edge of a stellar shock wave, or clumps of dark matter. Visible light and radio waves do not or cannot escape from such regions. Cosmic rays, which fly straight from the site of the conflagration, can.
