CHEMISTRY
Joel Miller
Thanh N. Truong
Peter J. Stang
MATHEMATICS
Graeme W. Milton
Jim Carlson
PHYSICS
Charles Jui
Charles Jui
Craig Taylor
Valy Vardeny
Valy Vardeny
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(Cosmic) Rays of Mystery
By Lee Siegel
The Salt Lake Tribune
DUGWAY PROVING GROUND -- During dark, moonless nights when coyotes howl, a team of physicists rolls open garage-like doors on 33 metal sheds perched atop
two lonely peaks.
Most sheds contain two 7 1/2-foot-wide mirrors. The mirrors scan the vast desert sky, looking for faint fluorescent blue streaks that flash too quickly to be seen by the naked
eye. The flashes are focused onto photomultiplier tubes, which convert the light into electronic signals recorded by computers.
Out here in Utah's west desert, atop Camels Back Ridge and Little Granite Mountain, Charles Jui (pronounced "ray") and other University of Utah physicists are looking for
subtle signs of ultrahigh-energy cosmic rays -- the most energetic particles in the universe.
Out there, somewhere in the universe, an unknown, incredibly powerful process sends these particles screaming through space and into Earth's atmosphere. Although smaller
than an atom, if one ultrahigh-energy particle hit you in the face it would feel like a fast-pitched baseball. But they never get that far.
Instead, the cosmic ray particles hit nitrogen gas five to 10 miles high in the atmosphere, generating an "air shower" -- a cascade of billions of other particles. Those particles produce faint blue and ultraviolet flashes when they hit air molecules.
Below, at the newly upgraded, $14 million High-Resolution Fly's Eye cosmic ray observatory, the physicists study the flashes and wonder what mysterious force propels the powerful cosmic rays.
It is "the largest cosmic ray experiment in the world today," said Jui, an associate professor of physics. "The whole study is tied to our understanding of ourselves and our place in the universe. It really is about us as inhabitants of the universe."
Jui outlined research at the Fly's Eye during a tour last week and during Wednesday's Science at Breakfast lecture sponsored by the university's College of Science.
Cosmic rays were discovered in 1912. Most cosmic rays are bare protons -- a hydrogen atom without an electron. They also can be alpha radiation particles or nuclei of elements like oxygen, carbon, nitrogen and iron.
The rays' energy is measured in terms of electron volts. The most energetic cosmic ray ever measured was detected by the old Fly's Eye in 1991. Its energy--that of a speeding baseball or "a brick dropped from your chest onto your toe" --measured 300 billion billion electron volts, Jui said.
That is 100 million times more energetic than particles that will zip through the most powerful atom smashers physicists now hope to build, he added.
Somewhat less powerful cosmic rays -- those with energies of about 1 million billion to 10 million billion electron volts -- may be hurled into space by exploding stars, Jui said. Weaker cosmic rays are spewed into space by the sun and other stars.
But the source of the most energetic cosmic rays remains elusive. Possibilities include noisy "radio galaxies," shock wavesĘ from colliding galaxies, or extremely bright, active centers of certain galaxies.
Those "active galactic nuclei" may harbor supermassive black holes made of billions of collapsed stars with gravity so powerful not even light can escape. But monster black holes may hurl the most energetic cosmic rays across space, Jui said.
More bizarre possible sources may be cosmic strings, magnetic monopoles and other "exotic primordial particles left over from the formation of the universe," he added.
But those are all just theories.
"There is no evidence cosmic rays come from any of them," Jui said. To make matters worse, cosmic ray particles come from all directions. That is perplexing because physicists believe the ultrahigh-energy particles probably originate within our local supercluster of galaxies, which "is a small part of the
universe," Jui said. So their source should be apparent. But it is not.
If the most energetic rays originated outside our supercluster, Albert Einstein's special theory of relativity predicts they could not reach Earth. Their energy should be absorbed by collisions with "afterglow" radiation from the Big Bang that formed the universe roughly 13 billion years ago.
"That's the mystery," Jui said, raising the unpopular possibility that special relativity might be wrong. "Not only do we not know how they [ultrahigh-energy cosmic rays] are made, but we shouldn't be seeing them. The fact we are seeing them suggests there are powerful forces in the universe we don't yet understand."
Over the past 2 1/2 years, since observations from the upgraded Little Granite Mountain site began, they have detected seven cosmic rays of with ultrahigh energies above 10 billion billion electron volts. At least one was in the highest-energy category, exceeding 100 billion billion electron volts, and the others are now being analyzed, Jui said.
Since the Camels Back Ridge site started running last August, another half-dozen rays at least one-tenth as powerful were observed from both peaks, he added.
"Our job is to figure out where these particles are made, how they're made and what they are," Jui said. "To answer these questions, we need to observe many of them."
That is why the old Fly's Eye was upgraded.
The method used by the old and upgraded observatories-- mirrors focusing light onto photomultiplier tubes-- was developed in the 1950s to estimate the yield from atmospheric nuclear weapons tests. For nukes and cosmic ray flashes, "the brightness tells you how much energy is released," Jui said.
Cornell University scientists built a Fly's Eye in upstate New York in the late 1960s, "but it never saw anything" because the atmosphere there was too humid. Under the leadership of Jack Keuffel, University of Utah physicists tested a prototype Fly's Eye detector in 1976 in New Mexico.
The Utah Fly's Eye was built at two Dugway Proving Ground sites -- Little Granite Mountain and on the desert floor -- during 1980-1981 and improved in 1986 for a total of $1 million, Jui said. It got its name because it looked at the sky with many mirrors, just as a fly sees with multifaceted eyes.
A 4-foot-wide mirror and 12 to 14 photomultiplier tubes were placed in a barrel-shaped device. One site had 67 such cosmic ray detectors; the other site had 34.
The old detectors were removed from Little Granite to make room for the High-Resolution Fly's Eye, which was built from 1994 to 1999, first on Little Granite, then eight miles away on Camels Back Ridge. Scientists nicknamed the two-site observatory "HiRes." It began full-scale observations in November.
"I like it out here-- just the solitude," said Benjamin Stokes, a U. physics doctoral student. "It gives you lots of time for reflection. . . . Most of physics is focused on little details. The things we're looking at are big, with vast implications."
Jui added: "This is a very romantic place to pursue these studies. You are out here and your closest companions are the coyotes, a kit fox and maybe an antelope or two. . . . We call ourselves cowboy physicists. It's a much less rigid, less corporate and less industrial style of physics" than organizing hundreds of scientists to run large, expensive
atom smashers to study subatomic particles.
Little Granite Mountain has 22 mirrors in 12 sheds. Camels Back Ridge has 42 mirrors in 21 sheds. Each mirror reflects air-shower flashes onto 256 photomultiplier tubes.
With all the sheds, the Camels Back site "looks like a Wal-Mart warehouse," Jui joked.
The large number of mirrors and photomultiplier tubes make HiRes 25 times better than the old Fly's Eye at ignoring light other than cosmic ray air-shower flashes, Jui said.
HiRes can detect the flashes up to 25 miles away, while the old Fly's Eye could see them only eight miles away. That means the upgraded observatory watches 10 times more sky than the old Fly's Eye and thus can measure more of the most energetic cosmic rays.
The two observatory sites allow "stereo" observations of cosmic rays, providing better data on the rays' energy levels, chemical makeup and direction of travel.
Originally published March 23, 2000, in The Salt Lake Tribune.
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