Even on a sunny day, our planet is continuously pelted with cosmic rain. Millions of subatomic particles flying through space with extremely high energies smash into Earth’s atmosphere every second.
These particles are called cosmic rays, and they are so powerful that they travel at near–light speeds.
While nothing can outrun light in a vacuum, when it moves through air, water, or other media, its speed slows. In the Earth’s atmosphere cosmic rays exceed the local speed of light. This generates an “electromagnetic sonic boom” that releases bluish light called Cerenkov radiation, said Scott P. Wakely, an assistant professor of physics at the U of C, in an e-mail interview.
“The highest-energy cosmic rays we see are, in fact, the most energetic particles we know of in nature—they were almost certainly produced in unusual or exotic environments,” Wakely said.
But scientists are still unsure exactly where cosmic rays come from. Some come from the sun. The highest-energy rays, more energetic than anything mankind has created, may originate from the centers of faraway active galaxies, according to new data from the Pierre Auger Cosmic Ray Observatory in Argentina.
However, physicists believe the majority of cosmic rays that strike earth come from unknown sources within the Milky Way, perhaps exploding stars. Detailed measurements of Cerenkov radiation may help provide an answer.
“We use the Cerenkov radiation to simultaneously and precisely tell us the energy and mass of the cosmic ray with high precision,” Wakely said. “These are two of the things you need to know to try to unravel the mystery of their origins.”
To that end, the National Science Foundation has granted $625,000 to Wakely and Simon Swordy, a professor of physics at the University, in order to build a new Cerenkov radiation detector, unlike anything else on Earth.
Each cosmic ray makes two flashes of Cerenkov light: one when it hits Earth’s atmosphere, and a second from the shower of particles when the original ray shatters on impact with the atmosphere.
Ground-based arrays can detect the second burst. But the shower of particles and the density of the atmosphere often obscure the first Cerenkov emission.
Helium balloon–based detectors, which float above many air molecules, can detect the first burst but cannot make readings over a large area. Ground-based systems solve this problem but provide poor resolution.
Recently, the ground-based High Energy Stereoscopic System in Namibia observed the first Cerenkov burst, but Wakely called the equipment “non-optimum.” “It’s like using a screwdriver as a chisel—while it will work in a pinch, for precision work, you really want to use a tool designed for the job,” Wakely said.
Although most cosmic rays are protons and the nuclei of helium atoms, a very small portion are the nuclei of heavier atoms. The Cerenkov radiation from these heavier nuclei is easier to observe: An iron nucleus releases about 700 times more light than a proton.
This light is what Wakely and Swordy want to observe with their new ground-based detector. They hope to have a prototype in two to three years.
Wakely says the device will contribute much to mankind’s understanding of physics. “Apart from a few minor exceptions, cosmic rays are our only sample of matter from the wider world outside our solar system. As such, they can teach us much about the composition and distribution of matter in the galaxy and universe,” Wakely said.
