How to turn a continent into a telescope
Ever since Galileo turned "telescopio" into a household word, tiny packets of light have helped inquisitive people unlock the mysteries of the universe. From radio frequencies to visible light to gamma rays, these photons have led to one stunning discovery after another.
Nearly 400 years later, Peter Gorham is hoping to use far more ephemeral cosmic particles to delve into the most violent processes of the universe - a task in which photons are of little help. And to do it, the University of Hawaii physicist and his colleagues have embarked on a unique project to turn an entire continent - the Antarctic ice sheet - into the world's largest telescope.
Call it astronomy beyond light. Instead of relying on common photons, researchers are turning to elusive neutrinos to unlock secrets about the universe. These particles have virtually no mass and carry no electrical charge, so they rarely interact with other matter.
Such qualities make neutrinos extremely useful in detecting objects beyond the energy limits of today's astronomy. Besides helping researchers understand some of the most violent processes since the Big Bang, neutrinos could shed light on the mysteries of very high-energy cosmic rays and test theories about the formation of the universe.
"We've got this entire regime of astrophysics that we cannot explore by traditional astronomy. How are we going to get there?" asks Dr. Gorham. "Neutrinos are the obvious choice."
But the same qualities that make neutrinos so valuable also make them terribly difficult to detect. That's why astronomers are trying to fashion such huge "telescopes." At least four other high-energy neutrino projects are in various stages of planning, where neutrino detectors are lowered into Antarctic ice or submerged in the Mediterranean Sea.
Indeed, "the field is exploding," notes John Bahcall, an astrophysicist with the Institute for Advanced Study in Princeton, N.J.
Although neutrino astronomy has been practiced for decades, the main target of observation was the sun. But in 1987 astronomers detected neutrinos in a massive stellar explosion in the Large Magellanic Cloud. That single event not only breathed new energy into supernova research, it also raised hopes that, with the right detectors, other neutrino sources outside the Milky Way could be detected and studied.
The choice to focus on neutrinos is dictated by the peculiarities of high-energy physics. On a human scale, the energy regime Gorham and his colleagues want to probe represents a small percentage of a Curt Schilling fastball. It smacks a mitt and generates a little heat. The dynamics change when that level of energy is imparted to a subatomic particle.
If such a turbocharged particle collides with the nucleus of a particle, "you'll blow the nucleus to smithereens," Gorham says. For photon-based astronomers, this presents a problem.
When a turbocharged photon hits another photon, both vanish and form electrons and positrons. Such collisions - and disappearances - are inevitable because the universe is awash in very low-energy photons. These particles make up the afterglow of the Big Bang, known as the cosmic microwave background. In practical terms, that means astronomers can't use very high-energy photons to study the processes that Gorham and others are investigating.
Neutrinos, however, so rarely interact with any kind of matter that they zip right by the low-energy photons. And their electrically neutral nature prevents magnetic fields from deflecting or accelerating them. So neutrinos can preserve their original energy content - critical to determining their source.
Of course, because they rarely interact with matter, neutrinos are very hard to spot. Compared with Gorham's project, the current crop of detectors is relatively small - a few cubic kilometers or less - so they find far fewer particles. With an Antarctic-sized detector, Gorham and his team are looking for bursts of radio energy that neutrino interactions give off as they smack into atoms deep in the ice.
This new approach raised eyebrows at first. The notion that neutrino interactions could trigger brief bursts of radio signals was first proposed by Russian physicist Gurgen Askaryan in the early 1960s. "But very few people believed the Askaryan effect was real," Gorham says. The idea languished until 2000, when a team led by Gorham and David Salzberg, a physicist at the University of California at Los Angeles, confirmed that the effect was real - and surprisingly strong.
The finding led to his project, called Antarctic Impulse Transient Array. ANITA could capture in 30 days the amount of data that would take its counterparts a decade to gather.
Funded through NASA and drawing on research and engineering assets from eight universities and NASA's Jet Propulsion Laboratory, the array consists of a set of 40 to 50 antennae clustered beneath a high-altitude balloon. The first full-scale launch of this payload, the size of a small school bus, is planned for December 2006. The balloon's path is expected to take it around the continent once every 15 days and over the thickest ice. At any one time, the balloon can track some 1.5 million cubic kilometers of ice.
Why go to all this trouble? Because neutrinos may offer clues to several astronomical mysteries, including the source of very high-energy cosmic rays.
Only 10 to 20 have been recorded over the past 30 years, says Paul Mantsch, an astrophysicist at the Fermi National Accelerator Laboratory in Batavia, Ill. One reason, scientists say, is that these particles are believed to run into the same photons from the big bang that limit traditional astronomy. Collisions are thought to break the cosmic rays into a shower of other particles - including neutrinos. Thus, neutrinos appear to be the only way of tracking these cosmic rays to truly distant sources.
Researchers have proposed a number of theories as to what generates these cosmic rays. But physicists have a hard time explaining how these mechanisms could accelerate cosmic rays to the high- energy levels observed, notes Steven Barwick, a physicist at the University of California at Irvine and the lead investigator for another neutrino observatory in Antarctica. Each theory carries its own prediction of the range of energy signatures that neutrinos from these processes should carry.
For particle physicists, neutrinos are valuable because they travel at energies no earthbound particle accelerator imaginable could generate. So they could represent a natural source of crash dummies, whose collision debris could help test theories about the origins and evolution of matter in the universe.
One puzzle involves gravity: Why is it so weak relative to the other three fundamental forces of nature? ANITA may only hint at the answers to these questions.
"We're only expecting a few dozen detections in one flight," Gorham says. "That's not a lot, but only a few years ago, people threw up their hands at the thought of trying to detect these neutrinos at all."
Yet researchers note that the detection of a handful of neutrino events in 1987 stimulated at least a decade's worth of study of supernovas. It's not unreasonable, they say, to see dozens of events inject fresh energy into astrophysics.
"It's a wonderful time to be a neutrino physicist," says Dr. Bahcall.