xilihutu
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作者：xilihutu 在 驴鸣镇 发贴, 来自 http://www.hjclub.org
Biggest Atom Smasher to Fire Up Wednesday
Mason Inman
for National Geographic News
September 9, 2008
The long-awaited start-up of the world's largest science experiment will begin tomorrow underneath villages and cow pastures at the French-Swiss border.
The Large Hadron Collider will smash protons at nearly the speed of light inside a circular, 17-mile (27-kilometer) long tunnel.
"It was first proposed more than 20 years ago," said Django Manglunki, an accelerator physicist at the European Organization for Nuclear Research (CERN). "We've been preparing that beam for more than ten years."
"It's difficult to realize that the machine, at last, is starting now," he added. (See photos of the collider.)
By creating hundreds of thousands of head-on collisions each second, physicists hope to understand the fiery conditions of the universe a trillionth of a second after the big bang.
The findings could also help resolve some of the biggest mysteries in physics, such as the existence of one long-hypothesized particle called the Higgs boson—or the "God particle"—thought to be responsible for giving all other particles their mass.
(Read about the God particle in National Geographic magazine.)
Another enigma that could be at least partially explained is dark matter, the invisible material thought to be the most common in the universe.
But first, researchers have to get the machine's beams of protons running.
Very Big Staircase
In several months CERN's physicists plan to use two beams, each with 2,808 bunches of protons, each of which contains a hundred billion protons—positively charged particles found in the nuclei of atoms.
Out of each collision, a spray of energy and other assorted particles will form. Scientists will study which particles show up, how often, and exactly how they fly out of the collisions. (Learn more about atom smashers.)
But on Wednesday, CERN scientists will first try to thread a single bunch of two billion protons through the collider.
"We will have a very low-intensity beam, so [in case of a problem] we can lose the beam without damaging the machine," Manglunki said.
Once the team gets the beam circulating all the way around the tunnel—which may happen in a couple of hours—the scientists will send in several bunches at a time.
Getting the first beam circulating, Manglunki said, is "one step in a very big staircase"—the long process of conceiving, designing, building, and finally running the experiment.
(See: "Broken Magnet Highlights Largest Collider's Engineering Challenges" [April 13, 2007].)
Although the physicists have done various tests on the machine already, "ultimately it's the beam that can tell you if everything is working," he added.
Later, they will attempt to get another beam of protons circulating through the tunnel in the opposite direction—a prelude to colliding the two beams.
Dark Matter Particle
In addition to spotting the Higgs boson, another early reward could be evidence of supersymmetry. The supersymmetry theory says that all the particles known today have much more massive—and as yet undetected—partners.
"There are strong reasons to believe that these new particles include the particle that makes up the cosmic dark matter that accounts for 80 percent of the matter in universe," said Michael Peskin, a particle physicist at the Stanford Linear Accelerator Center in Menlo Park, California.
(Related: "Dark Matter Proof Found, Scientists Say" [August 22, 2006].)
The collisions could also create a zoo of new particles, experts say.
"If any of these theories are right, the LHC should be turning up the evidence for these particles by next summer," Peskin said.
But there could also be some big surprises.
"It might turn out to be like the 1950s, when we were discovering many new particles and had no clue about how they fit into a coherent picture," Peskin said.
"I hope it will turn out like that," he added. "This is what makes science fun."
No Cause for Alarm
Some people are worried that the experiments could also create unwelcome discoveries, such as particles and other exotic phenomena that could swallow up Earth or destroy the universe as we know it.
For instance, one possibility is that the collisions will pack matter together so tightly that it may collapse to form miniature black holes.
But reviews by both CERN physicists and independent researchers argue that, even if such black holes do form, there's no reason for alarm.
"Collisions just like those the [atom-smasher] will make have been produced by cosmic rays bombarding the Earth throughout its existence," said a statement from the American Physical Society.
The most energetic cosmic rays are particles that pack much more energy than those in the Large Hadron Collider—so much so that physicists still aren't sure how the most powerful cosmic rays get created. (See: "Black Holes Belch Universe's Most Energetic Particles" [November 8, 2007].)
Steve Giddings, a physicist at the University of California, Santa Barbara, is actually hoping miniature black holes do show up, along with other evidence supporting string theory—an unproven theory that describes subatomic particles as though they are tiny vibrating strings.
"It would be extremely exciting to see string properties directly. And that is possible if there are extra dimensions of space that are configured just the right way," Giddings said.
"[Seeing] all of this would be the ultimate jackpot scenario."
作者：xilihutu 在 驴鸣镇 发贴, 来自 http://www.hjclub.org
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是你对还是我right？ |
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有谁能 update 去年欧洲CERN 粒子碰撞的消息？
