Imagine a mysterious particle smaller than an atom that strikes with all the force of a high-speed baseball. Imagine a particle like this hitting a football field once every 30 years. Now imagine you are a scientist and have to figure out what these particles are, where they came from and how they got so much energy.
This is the task facing LSU physicist James Matthews. But Matthews is not alone in his task; he is part of a team of highly respected cosmic-ray investigators at LSU, and a member of an international coalition that has recently been funded to investigate the mysteries of the highest-energy cosmic-rays known -- those that are a trillion times more energetic than the run-of-the-mill model that bombard earth at the rate of one per square meter every second.
"Cosmic rays are believed to be one of the main contributors to slight genetic damage, so they may well play a part in evolution," Matthews said. "We've only been studying these things for about 50 years, so we don't really know whether the earth has been bombarded with the same number of them throughout history or whether the number changes."
But the cosmic rays that cause genetic mutations are not the ones that really interest Matthews. The ones he studies are rare and fast -- so rare only 10 of them have been recorded in the past 30 years, and so fast they must be created by something bigger than a galaxy.
"It is almost impossible to imagine how that tiny little particle got so much energy," Matthews said. He explained the difficulty this way:
"The energy a particle has is measured in electron volts. If you want to get a particle moving at one electron volt, you hook it up to a 1-volt battery. If you want it to move at 1,000 electron volts, which is three orders of magnitude higher, you accelerate it with a classroom generator. To get 1 million electron volts, you have to use a generator the size of those in the Grand Coulee dam. To go up to a trillion electron volts, you need something like Fermilab in Chicago, and to go up another three orders of magnitude (a thousand trillion electron volts) you need a supernova. You can get higher energies from a snarling black hole, or the heart of an active galaxy, but the particles we're looking at are six orders of magnitude (one billion trillion) more energetic than even these can generate, so it's hard to conceive of what might be making them," he said.
Compounding the mystery of these high-energy particles is the fact that they can't travel very far. "They have to come from our galactic neighborhood -- about 200 million light years away. That seems like a long way, but in cosmic terms, it's not."
Because even "empty" space is filled with wandering bits of protons, atoms, photons, dust and background radiation, the laws of probability say that these high-speed cosmic rays are going to run into something by the time they've traveled 200 million light years. "And there's just no obvious source of these things that close to us," Matthews said.
But now, Matthews is participating in an international collaboration that is on the verge of an experiment that may reveal answers to these perplexing mysteries within the next few years. Called the Auger (pronounced Ojay) Observatory after Pierre Auger who discovered the ultra-high energy cosmic rays, it is an array of detectors larger than the state of Delaware that will detect the effects of one of these cosmic-ray hits in the atmosphere.
"When one of these high-energy particles hits the atmosphere, it creates what we call an 'air shower' -- a rain of secondary particles, mostly electrons, three miles in diameter and a few hundred yards thick. So the plan is to set out detectors one mile apart on a 1600-square-mile grid. That way, when two or three detectors register activity at the same instant, we can be pretty sure it's an air shower," Matthews said.
There will actually be two of these grids, Matthews said -- one about 100 miles south of the Great Salt Lake in a desert area of Utah, and a second in Western Argentina, on similar desert terrain.
Each detector will be a covered tank of water about five feet high and 10 feet in diameter. It will have photon detectors on the inside, a small computer, a global positioning satellite locator and a radio transmitter on it, all powered by a solar panel. When particles from an air shower hit the water, tiny flashes of light will be picked up by the detectors and transmitted to the computer, which will send the information by radio signal, along with the exact position of the tank, to a central station where the researchers will be waiting.
"We expect to get about 50 hits from these high-energy cosmic rays per year per site," Matthews said.
With those 100 hits, scientists will be able to aim their radio and optical telescopes toward the source of the cosmic rays and perhaps come a little closer to understanding the nature of this strange cosmos we live in.
The entire project will cost about $100 million, with Argentina and the United States putting up $15 million apiece and Brazil, France, the United Kingdom and other countries -- 20 in all -- contributing the balance. The U.S. Department of Energy, Fermilab, UNESCO and the National Science Foundation are among organizations supporting the project.
Matthews, who came to LSU in 1997, was hired especially to enlist a team of cosmic-ray researchers to take part in the Auger Project. On the project he is in charge of computer simulation of the working of the detectors. "This is so we don't build something that doesn't work right," he said.
Construction of the Argentina site will begin in 1999, and construction of the Utah site will begin in 2000 or 2001 if all goes well at the Argentine site. The entire experiment is projected to run for 20 years.
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