2006年VOA标准英语-Ion-Smashing Yields New Knowledge, But Som（在线收听）

By Carolyn Weaver
Washington, DC
09 February 2006
 
watch Brookhaven report

Experimental results at Brookhaven National Laboratory’s supercollider that replicate in miniature the “Big Bang,” when the universe exploded into being, were named the top physics story of 2005 by the American Institute of Physics. But some observers say that the possible risks of the experiments, which smash together gold ions traveling near the speed of light, remain an open question.

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Brookhaven National Laboratory's Relativistic Heavy Ion Collider (courtesy Brookhaven)  
  

Seen from above, the Relativistic Heavy Ion Collider, or RHIC, at New York’s Brookhaven National Laboratory, looks like a racetrack. And it is a kind of race track: two “beam pipes” in a tunnel nearly four kilometers around, in which gold nuclei are accelerated to close to the speed of light, and are crashed into each other at intersecting points along the way. Out of the kinetic energy of those collisions, new matter is created for a brief instant: a shower of quarks and gluons, the smallest particles known – and at seven trillion degrees, hotter than anything now in the universe. 

 
Brookhaven physicist Peter Steinberg
  
“It’s basically a living embodiment of E=mc squared,” says Brookhaven physicist Peter Steinberg. “Einstein’s theory told us a hundred years ago that you can trade off energy for mass, and vice versa. We’re essentially converting the kinetic energy, the energy from the motion of these nuclei, converting it into lots of particles.”

The four detectors that bestride the collision points are massive machines, with “time projection chambers” that record the collisions and their after-moments. The latest results made big news last year when Brookhaven physicists reported that the quark-gluon plasma was not a gas as expected, but rather a very dense liquid.

 
Artitst's drawing of gold ions colliding (courtesy Jeffery Mitchell, Brookhaven National Laboratory) 
  
“Most people believe the universe itself, a couple of microseconds after the Big Bang, was essentially one big state of quark-gluon plasma,” Mr. Steinberg explains. “And so learning about the properties and the nature of this material at RHIC, we are probably gaining some insight into what the universe looked like right after the Big Bang.”

Scientists from more than a dozen nations collaborate at RHIC. The work is unclassified. But is it safe to recreate conditions that haven’t existed since the beginning of time? Before the supercollider opened in 2000, several reputable physicists raised the possibility that the experiments could create a chain reaction of “strangelets,” quarks missing an electron that would turn other nearby atoms into strange matter, too. 

Author Richard A. Posner, a critic of RHIC  
  
Federal appeals court judge and author Richard A. Posner outlines the scenario in his latest book, Catastrophe: Risk and Response.  “Within a fraction of a second, this strange matter would expand, so the entire earth was a strangelet,” he said in an interview. “And because of its great density, the earth in its strange form would be only 100 meters in diameter. And then it would blow up.” 

In response to these concerns, Brookhaven asked a panel of four leading physicists to assess the risks. Their report concluded that the chance of a strangelet accident was no more than one in fifty million – and might not even be physically possible. They also dismissed the risk of other cosmic accidents. But some observers still object that RHIC experiments amount to “betting the planet” -- however unlikely the chance of losing.  Richard A. Posner, who advocates the use of cost-benefit analyses in making policy decisions, says, “It’s a very abstract scientific interest that drives this research. It doesn’t sound like a particularly good investment, in which event why take even a very small risk?” 

A view of the STAR detector, one of four such experiments at RHIC   
  
“If we really saw it in those terms, if we sat at a table and asked ourselves, are we going to bet the planet to learn something that could potentially be very useful to mankind in the fullness of time, we’d probably go, ‘no,’” Peter Steinberg replies. “But the reality is, nobody I know doing this has even any gut-level feeling, ‘oh gosh, maybe we’re doing something dangerous.’” A critical point, says Mr. Steinberg, is that RHIC’s experimental findings in five and a half years of operation have shown no sign of any dangers, and have reduced any grounds for concern over risk. 

“The more we learn, the experts are less worried,” he says, “and when that happens, you should worry less, too. If the experts get more worried, then you should start to get more worried, but the experts feel better about it year by year, because we’re learning more. And it’s only the people who aren’t the experts who are actively keeping these discussions going.”

Yet Mr. Posner and a few other critics, including University of Cambridge physicist and mathematician Adrian Kent, say that a process for deciding on such experiments should be established that includes risk-analysis scholars and the public – particularly since next year the European Organization for Nuclear Research, or CERN, in Geneva, will open a collider 30 times more powerful than RHIC, that’s expected to offer the deepest views into the nature of matter yet.  

Some images and animations courtesy of Brookhaven National Laboratory and CERN.