The Enigma of High Energy Cosmic Rays
In 1913, there was talk among scientists that an unusual type of radiation had been discovered high in the atmosphere. Austrian physicist Viktor Hess decided to take his own measurements in a hot-air balloon. He rose five kilometers skyward with three electroscopes, devices that measure radiation emitted by radioactive elements such as radium. The rumors were true. Hess's electroscopes picked up a pervasive signal. The higher he flew, the stronger the energy was. In fact, it was far stronger than known radioactive substances could release. Given the signal direction, Hess suggested that this mysterious energy did not come from Earth. It was so energetic, he said, that it must have been able to travel a very long distance indeed.
In the decades since Hess flew his balloon, physicists have learned much about this energy, now called "cosmic rays." But even today, scientists don't know where the highest-energy cosmic rays come from in space. The Pierre Auger Project, a huge experiment now underway in Argentina, hopes to finally answer this most basic question.
An Array of Rays
Cosmic rays are not a form of radioactivity. Nor are they a form of electromagnetic radiation, the most typical radiation we encounter, which includes sunlight, X-rays, and gamma rays. Instead, cosmic rays are made of atomic particles moving toward Earth at near light speed. The particles themselves are very ordinary—the kind that all matter is made of. They are not atoms of matter exactly. Instead, they are the particles that make up atoms, such as protons.
The energy of cosmic ray particles is staggering. Energy is measured in electron volts, or eV. A medical X-ray machine emits about 150,000 eV. Cosmic rays measure anywhere from 1 billion eV to 100 billiontimes that much. Even the Large Hadron Collider, the world's largest and fastest particle accelerator, can only hurl particles fast enough to reach energies at the lower end of that spectrum. Some mechanism in the Universe is accelerating particles of space stuff at an incredible velocity. But what?
After decades of research, physicists have some idea of where lower-energy cosmic rays come from. These particles hit Earth abundantly. This suggests that they are relatively easy to accelerate and probably originate from a short distance away—within in our own galaxy. The likeliest sources are violent explosions called supernovas, which occur when stars use up their fuel. The expanding cloud of gas that remains after a supernova lasts for thousands of years. Particles from the original star or the region around it are likely accelerated by this expanding cloud, and some hit Earth.
Yet no ordinary exploding star could propel particles to the energies seen in the higher-energy cosmic rays. Unlike lower-energy cosmic ray particles, the higher-energy particles are rare. "You would have to wait 10,000 years to have one land in a ball field," says Paolo Privitera, a physicist from the University of Chicago working on the Pierre Auger project. "It's very difficult to find an explanation of their existence—a physical process that either generates or accelerates particles at this enormous energy."
To find an explanation, an international team of physicists has built a giant catcher's mitt, of sorts on the vast, flat plains of Argentina. Named after Pierre Auger, another pioneer in the field, it's the largest scientific instrument ever built. Science teams have spent $50 million and 17 years assembling the project in the hope that its data will finally pinpoint where ultra high-energy cosmic rays originate.
The Pierre Auger Observatory is not one instrument, but an array of 1,600 detectors evenly dotting 3,000 square kilometers. That's an area the size of Rhode Island. The detectors, each about the size of a compact car, are hard plastic cylinders containing water and instruments. For such a cutting-edge experiment, the impression is surprisingly low-tech.
This array detects about 30 high-energy particles per year. Or rather, it detects evidence of the particles—a much easier task. When a cosmic ray particle of any energy level strikes air molecules high in the atmosphere, the collision triggers a chain reaction of collisions with other air molecules. This shower of secondary particles falls toward Earth, distributing the energy of the original particle. The more energy carried by the original particle, the wider the shower. For the highest-energy cosmic rays, the showers consist of 10 billion particles spread over 20 kilometers, hitting about 65 detectors at once.
When the shower of particles drills through the pitch-black water in each detector, they create light. Detectors in the tanks register the light and send the information to the observatory headquarters through a cellular network. Meanwhile, a secondary detection method of 24 telescopes has sensed the shower's approach in the night sky. As the shower falls to the ground, any nitrogen atoms in the atmosphere that are caught in the collision emit bluish fluorescent light. The telescopes detect this light, producing a rapid movie of the shower's descent.
"Getting this profile—the shape of the shower as it hits the ground—tells us both the direction and the energy of the shower," says Angela Olinto, a University of Chicago astrophysicist on the project. For example, "if the shower is vertical, then many tanks are going to be hit at the same time." Tiny differences in the shower's time of arrival at different detectors help the scientists triangulate the original particle's source in the night sky.
"One of the things we would like to do is have a picture of the Universe at the highest energies to understand the most powerful things that are going on in the Universe," says Olinto. "These particles are so energetic that we may be able to probe interactions at scales that we cannot probe in accelerators today."
Hungry for Data
By late 2007, after collecting a year's worth of data, the Pierre Auger Observatory had mapped 27 ultra high-energy particles on the Southern Hemisphere sky. The scientists noticed an interesting pattern. Twenty of the detections—70 percent—came from the direction of active galactic nuclei (AGNs): enormous black holes that lie in the middle of some galaxies. Such a pattern was highly unlikely to be due to chance.
"It was the first time in history that one has ever had even any inkling where these cosmic rays come from," says the University of Chicago's James Cronin, a Nobel Prize-winning physicist who, with his colleague Allan Watson, first proposed the Pierre Auger Observatory in 1992. "We were extremely excited about these first results." They were published in the journal Science on November 9, 2007, and were hailed by researchers and the press as one of the top scientific discoveries of the year.
Since the announcement, however, Auger scientists have detected double the number of ultra high-energy particles. At a May 2009 meeting at the American Physical Society, the team reported that in this larger sample, the connection between cosmic rays and AGNs is much weaker. Only about 40 percent point to AGNs. The only thing to do now is get more data, much more.
The team plans to start building a second observatory—Pierre Auger North—in 2011, so the number of detections will eventually increase significantly. The sister observatory will stretch across the plains of southeastern Colorado and will be seven times larger than the Argentina facility. The Northern Hemisphere observatory will also scan the half of the sky that cannot be seen from the Southern Hemisphere.
While the result-reversal was a disappointment, Auger scientists are accustomed to the enigma of cosmic rays—and are excited by it. As Pierre Auger wrote in 1941: "If we knew all the answers, we would be deprived of the joy of exploring the unknown, and science would become as dull as a dead language." On Auger's watch, there's no chance of that.
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