The sparkles on the roof

This illustration gives an artist’s impression of an air shower over a particle detector at the Pierre Auger Observatory, seen against a starry sky. Image: A. Chantelauze, S. Staffi, L. Bret, Pierre Auger Observatory

One night in 1939, Professor Pierre Auger’s daughter asked him, “Papa, what are you doing?” In French, of course. “I’m studying the sparkles on the roof,” he said, with a twinkle in his eye. He had discovered that very energetic subatomic particles coming from outer space, cosmic rays, smash into atoms in the upper atmosphere making huge showers of particles that reach the ground. Some of those particles hit you every few minutes but you don’t feel a thing. However, if you laid a sheet of fluorescent plastic on the roof, and it was a very dark night, and you had eyes as big as saucers, you could see those sparkles.

Last month I was in Argentina at the Pierre Auger Observatory, celebrating its 20th anniversary. The observatory named in his honor measures the energies of the most energetic cosmic rays and from where in the sky they come. They are so rare they have a rate of about one per square kilometer per century! Jim Cronin, a professor at the University of Chicago, had the grand idea of building a detector three times the size of Chicago, 3,000 square kilometers (1,200 square miles)! Then he and Alan Watson from England made it happen.

When those showers reach the ground, there can be billions of particles spread over many square kilometers. The physicists placed 1,600 tanks of water, the size of family hot tubs but closed, 1 kilometer apart. Each had photomultipliers inside to detect flashes of light when electrically charged particles zip through. When several tanks detect flashes from a single shower, one can calculate the cosmic ray’s direction from their slightly different times.

On clear dark moonless nights, “eyes” made of hundreds of photomultipliers look at the sky above the tanks. They can measure streaks of light made by the shower particles zipping down the atmosphere at nearly the speed of light, looking like a meteor but taking a fraction of a millisecond. A few times a year a flash is so bright that the cosmic ray must have had the momentum of a Serena Williams tennis serve! It must have come from some extremely violent event in the cosmos, far beyond our galaxy. But what are those events, and how do they give particles energies millions of times higher than our best accelerators? We have not yet identified any weird and wonderful objects that could be their source. It’s a puzzle; that’s what makes it exciting.