The Advanced Thin Ionization Calorimeter, shown here in Antarctica
before a 2005 launch, detected cosmic rays in the upper atmosphere.

Image Courtesy of T, Gregory Guzik, ATIC

Also see the other articles in this issue's special Beyond Einstein section: Back From the Future and Is the Search for Immutable Laws of Nature a Wild-Goose Chase.

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.

To attain a new perspective on the cosmos, astronomers are teaming up with particle physicists to develop clever ways of detecting these wayward particles. Pierre Auger’s water tanks represent one way to do it; experiments borne by balloon, like the Advanced Thin Ionization Calorimeter, are another. “To think of these devices as ‘telescopes’ is revolutionary,” says Dan Hooper, a high-energy astrophysicist at Fermi National Accelerator Laboratory (Fermilab), outside Chicago. “Telescopes are not just something you look through and point at something. You have to be pretty creative.”

The first thing to know about observing the universe with cosmic-ray eyes is that Earth’s atmosphere destroys these particles before they reach the ground. A cosmic ray—usually a proton, but sometimes other particles—will slam into air molecules 50 miles or so above the surface, rapidly shedding energy and giving rise to a shower of billions of electrons, positrons, and muons that rain down onto the terrain below.

That is where Pierre Auger’s network of water tanks comes in. The scientists chose them as detectors precisely because light moves more slowly in water than it does in air. A particle will come screaming through the atmosphere at close to light speed; as soon as it passes into the water, it finds itself in violation of nature’s speed limit. Whenever electrically charged particles go faster through an insulating material (like water) than the speed of light would allow, they disrupt nearby electrons, causing a flash of light (known as Cherenkov radiation).

Scientists know that a particle shower has occurred when multiple tanks detect flashes at the same time. By combining data on the precise timing of the flashes from all the tanks, Auger physicists can reconstruct the collision that took place high in the atmosphere and determine the energy and direction of the original cosmic ray. That’s why they call it a telescope.

In the six years that the Pierre Auger Observatory has been in operation in Argentina, it has detected 1.6 million particle showers. Recently scientists traced the origin of a few such showers to violent supernovas in the galaxy M82, located 12 million light-years away in the constellation Ursa Major. These star explosions are among the most powerful events ever observed—each one emits so much light that it can outshine an entire normal galaxy. And yet the cosmic rays that supernovas emit are ho-hum—just medium power by astrophysical standards. Nearly all the showers that scientists measure are like this: interesting, but unspectacular.