标题: Doomsayers' LHC Fear Untenable, Scientists Say [打印本页]
作者: 悟空 时间: 2008-6-30 16:40 标题: Doomsayers' LHC Fear Untenable, Scientists Say
Scientists: Nothing to fear from atom-smasher
By DOUGLAS BIRCH
The Associated Press
Saturday, June 28, 2008; 3:08 PM
MEYRIN, Switzerland -- The most powerful atom-smasher ever built could make some bizarre discoveries, such as invisible matter or extra dimensions in space, after it is switched on in August.
But some critics fear the Large Hadron Collider could exceed physicists' wildest conjectures: Will it spawn a black hole that could swallow Earth? Or spit out particles that could turn the planet into a hot dead clump?
Ridiculous, say scientists at the European Organization for Nuclear Research, known by its French initials CERN _ some of whom have been working for a generation on the $5.8 billion collider, or LHC.
"Obviously, the world will not end when the LHC switches on," said project leader Lyn Evans.
David Francis, a physicist on the collider's huge ATLAS particle detector, smiled when asked whether he worried about black holes and hypothetical killer particles known as strangelets.
"If I thought that this was going to happen, I would be well away from here," he said.
The collider basically consists of a ring of supercooled magnets 17 miles in circumference attached to huge barrel-shaped detectors. The ring, which straddles the French and Swiss border, is buried 330 feet underground.
The machine, which has been called the largest scientific experiment in history, isn't expected to begin test runs until August, and ramping up to full power could take months. But once it is working, it is expected to produce some startling findings.
Scientists plan to hunt for signs of the invisible "dark matter" and "dark energy" that make up more than 96 percent of the universe, and hope to glimpse the elusive Higgs boson, a so-far undiscovered particle thought to give matter its mass.
The collider could find evidence of extra dimensions, a boon for superstring theory, which holds that quarks, the particles that make up atoms, are infinitesimal vibrating strings.
The theory could resolve many of physics' unanswered questions, but requires about 10 dimensions _ far more than the three spatial dimensions our senses experience.
The safety of the collider, which will generate energies seven times higher than its most powerful rival, at Fermilab near Chicago, has been debated for years. The physicist Martin Rees has estimated the chance of an accelerator producing a global catastrophe at one in 50 million _ long odds, to be sure, but about the same as winning some lotteries.
By contrast, a CERN team this month issued a report concluding that there is "no conceivable danger" of a cataclysmic event. The report essentially confirmed the findings of a 2003 CERN safety report, and a panel of five prominent scientists not affiliated with CERN, including one Nobel laureate, endorsed its conclusions.
Critics of the LHC filed a lawsuit in a Hawaiian court in March seeking to block its startup, alleging that there was "a significant risk that ... operation of the Collider may have unintended consequences which could ultimately result in the destruction of our planet."
One of the plaintiffs, Walter L. Wagner, a physicist and lawyer, said Wednesday CERN's safety report, released June 20, "has several major flaws," and his views on the risks of using the particle accelerator had not changed.
On Tuesday, U.S. Justice Department lawyers representing the Department of Energy and the National Science Foundation filed a motion to dismiss the case.
The two agencies have contributed $531 million to building the collider, and the NSF has agreed to pay $87 million of its annual operating costs. Hundreds of American scientists will participate in the research.
The lawyers called the plaintiffs' allegations "extraordinarily speculative," and said "there is no basis for any conceivable threat" from black holes or other objects the LHC might produce. A hearing on the motion is expected in late July or August.
In rebutting doomsday scenarios, CERN scientists point out that cosmic rays have been bombarding the earth, and triggering collisions similar to those planned for the collider, since the solar system formed 4.5 billion years ago.
And so far, Earth has survived.
"The LHC is only going to reproduce what nature does every second, what it has been doing for billions of years," said John Ellis, a British theoretical physicist at CERN.
Critics like Wagner have said the collisions caused by accelerators could be more hazardous than those of cosmic rays.
Both may produce micro black holes, subatomic versions of cosmic black holes _ collapsed stars whose gravity fields are so powerful that they can suck in planets and other stars.
But micro black holes produced by cosmic ray collisions would likely be traveling so fast they would pass harmlessly through the earth.
Micro black holes produced by a collider, the skeptics theorize, would move more slowly and might be trapped inside the earth's gravitational field _ and eventually threaten the planet.
Ellis said doomsayers assume that the collider will create micro black holes in the first place, which he called unlikely. And even if they appeared, he said, they would instantly evaporate, as predicted by the British physicist Stephen Hawking.
As for strangelets, CERN scientists point out that they have never been proven to exist. They said that even if these particles formed inside the Collider they would quickly break down.
When the LHC is finally at full power, two beams of protons will race around the huge ring 11,000 times a second in opposite directions. They will travel in two tubes about the width of fire hoses, speeding through a vacuum that is colder and emptier than outer space.
Their trajectory will be curved by supercooled magnets _ to guide the beams around the rings and prevent the packets of protons from cutting through the surrounding magnets like a blowtorch.
The paths of these beams will cross, and a few of the protons in them will collide, at a series of cylindrical detectors along the ring. The two largest detectors are essentially huge digital cameras, each weighing thousands of tons, capable of taking millions of snapshots a second.
Each year the detectors will generate 15 petabytes of data, the equivalent of a stack of CDs 12 miles tall. The data will require a high speed global network of computers for analysis.
Wagner and others filed a lawsuit to halt operation of the Relativistic Heavy Ion Collider, or RHIC, at the Brookhaven National Laboratory in New York state in 1999. The courts dismissed the suit.
The leafy campus of CERN, a short drive from the shores of Lake Geneva, hardly seems like ground zero for doomsday. And locals don't seem overly concerned. Thousands attended an open house here this spring.
"There is a huge army of scientists who know what they are talking about and are sleeping quite soundly as far as concerns the LHC," said project leader Evans.
Copyright@2008 The Associated Press
作者: 悟空 时间: 2008-9-9 11:18
Originally posted by 悟空 at 2008-6-30 09:40 PM:
Scientists: Nothing to fear from atom-smasher
By DOUGLAS BIRCH
The Associated Press
Saturday, June 28, 2008; 3:08 PM
MEYRIN, Switzerland -- The most powerful atom-smasher ever built..
A follow-up to my original post in June: LHC will be switched on tomorrow. Scientists reiteriate safty of the first high-energy collision:
CERN Press Release:
Geneva, 5 September 2008: A report published today in the peer reviewed Journal of Physics G: Nuclear and Particle Physics1 provides comprehensive evidence that safety fears about the Large Hadron Collider (LHC) are unfounded. The LHC is CERN’s2 new flagship research facility. As the world’s highest energy particle accelerator, it is poised to provide new insights into the mysteries of our universe.
“The LHC will enable us to study in detail what nature is doing all around us,” said CERN Director General Robert Aymar. “The LHC is safe, and any suggestion that it might present a risk is pure fiction.”
Safety has been an integral part of the LHC project since its inception in 1994, and the project has been subject to numerous audits covering all aspects of safety and environmental impact. A comprehensive report by independent scientists addressing safety issues related to the production of new particles at the LHC was presented to CERN’s governing body, the CERN Council, in 2003. It concluded that the LHC is safe. This report was updated and its conclusions strengthened in a new report incorporating recent experimental and observational data that was presented to Council at its most recent meeting in June 2008. This new report confirms and strengthens the conclusion of the 2003 report that there is no basis for any concern about the safety of the LHC.The CERN Council is composed of representatives of the governments of the 20 European Member States of CERN.
The report was prepared by a group of scientists at CERN, the University of California, Santa Barbara, and the Institute for Nuclear Research of the Russian Academy of Sciences. The papers comprising the report have been accepted for publication in leading peer-reviewed scientific journals. The report was reviewed carefully by the Scientific Policy Committee (SPC), a body composed of 20 independent external scientists that advises the CERN Council on scientific matters. Five of these independent scientists, including one Nobel Laureate, examined in detail the 2008 report and endorsed the authors’ approach of basing their arguments on irrefutable observational evidence to conclude that new particles produced at the LHC will pose no danger. The full SPC agreed unanimously with their findings.
“The LHC safety review has shown that the LHC is perfectly safe,” said Jos Engelen, CERN’s Chief Scientific Officer, “it points out that Nature has already conducted the equivalent of about a hundred thousand LHC experimental programmes on Earth – and the planet still exists.”
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For technically savvy readers, a full review can be found at :
http://www.iop.org/EJ/abstract/0954-3899/35/11/115004/
Review of the safety of LHC collisions
published in J. Phys. G: Nucl. Part. Phys. 35(2008) 115004
John Ellis1, Gian Giudice1, Michelangelo Mangano1, Igor Tkachev2 and Urs Wiedemann1 (LHC Safety Assessment Group)
1 Theory Division, Physics Department, CERN, CH 1211 Geneva 23, Switzerland
2 Institute for Nuclear Research of Russian Academy of Sciences, Moscow 117312, Russia
ABSTRACT: The safety of collisions at the Large Hadron Collider (LHC) was studied in 2003 by the LHC Safety Study Group, who concluded that they presented no danger. Here we review their 2003 analysis in light of additional experimental results and theoretical understanding, which enable us to confirm, update and extend the conclusions of the LHC Safety Study Group. The LHC reproduces in the laboratory, under controlled conditions, collisions at centre-of-mass energies, less than those reached in the atmosphere by some of the cosmic rays that have been bombarding the Earth for billions of years. We recall the rates for the collisions of cosmic rays with the Earth, Sun, neutron stars, white dwarfs and other astronomical bodies at energies higher than the LHC. The stability of astronomical bodies indicates that such collisions cannot be dangerous. Specifically, we study the possible production at the LHC of hypothetical objects such as vacuum bubbles, magnetic monopoles, microscopic black holes and strangelets, and find no associated risks. Any microscopic black holes produced at the LHC are expected to decay by Hawking radiation before they reach the detector walls. If some microscopic black holes were stable, those produced by cosmic rays would be stopped inside the Earth or other astronomical bodies. The stability of astronomical bodies strongly constrains the possible rate of accretion by any such microscopic black holes, so that they present no conceivable danger. In the case of strangelets, the good agreement of measurements of particle production at RHIC with simple thermodynamic models severely constrains the production of strangelets in heavy-ion collisions at the LHC, which also present no danger.
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