这是纽约时报的原文。
By DENNIS OVERBYEPublished: September 23, 2011
The world was watching as scientists announced that neutrinos from the CERN laboratory had raced to an Italian site faster than it would take a light beam.
Thanks to the Internet, the whole physics world was watching on Friday when Dario Autiero, of the Institut de Physique Nucléaire de Lyon in France, in front of a palpably skeptical roomful of physicists, put a whole new category of speed demons on the table, namely the shadowy subatomic particles known as neutrinos. He was describing a recent experiment in which neutrinos were clocked going faster than the speed of light, the cosmic speed limit set by Albert Einstein in his theory of relativity back in 1905.
According to Dr. Autiero’s team, neutrinos emanating from a particle accelerator at CERN, outside Geneva, had raced to a cavern underneath Gran Sasso in Italy — a distance of 454 miles — about 60 nanoseconds faster than it would take a light beam. That amounts to a speed greater than light by about 25 parts in a million.
“We cannot explain the observed effect in terms of systematic uncertainties,” Dr. Autiero told the physicists at CERN, the European organization for nuclear research. “Therefore, the measurement indicates a neutrino velocity higher than the speed of light.”
Dr. Autiero said his group had spent six months trying to explain away the result, but could not do it. Given the stakes for physics, he said, it would not be proper to attempt any sort of theoretical interpretation of the results. “We present to you this discrepancy or anomaly today,” he said.
The purported effect sounds slight, but to be even slightly on the wrong side of the speed of light is forbidden in the world that Einstein described. Faster-than-light travel can also lead to the possibility of time travel, something that most physicists do not believe is possible.
Relativity has been tested over and over again for a century, and as Carl Sagan, the late Cornell astronomer, liked to say: extraordinary claims require extraordinary evidence. “This is quite a shake-up,” said Alvaro de Rujula, a theorist at CERN. “The correct attitude is to ask oneself what went wrong.”
And the assembled CERN physicists were only too happy to oblige, diving in, after Samuel C. C. Ting, an M.I.T. Nobelist in the audience, offered his congratulations for work “very carefully done.” They asked detailed questions about, among other things, how the scientists had measured the distance from CERN to Gran Sasso to what is claimed to be an accuracy of 20 centimeters, extending GPS measurements underground. Had they, for example taken into account the location of the Moon and tidal bulges in the Earth’s crust?
The recent history of physics and astronomy is strewn with reports of suspicious data bumps that might be new particles or new planets and — if true — could change the way we think about the world, but then disappear with more data or critical scrutiny. Most physicists think the same will happen with this finding. The prevailing attitude was perhaps illustrated best by an XKCD cartoon, in which a character explains his intention to get rich betting against the new discovery.
Neutrinos are still a cosmic mystery. They are among the weirdest denizens of the weird quantum subatomic world. Not only are they virtually invisible and able to sail through walls and planets like wind through a screen door, but they are shape-shifters. They come in three varieties and can morph from one form to another as they travel along, an effect Dr. Autiero and his colleagues were trying to observe.
Their experiment, known clunkily as Oscillation Project with Emulsion-Tracking Apparatus, or Opera, is a collaboration of 160 physicists from 11 countries, primarily Japan and Italy. It is based at the Gran Sasso laboratory, a center for underground physics experiments that need sheltering from cosmic rays.
The action begins in a tank of hydrogen gas inside a building at CERN. Atoms in puffs of gas from the tank get stripped of their electrons, becoming naked protons, and then get sent on a Coney Island-style speed ride through a series of particle accelerators, eventually winding up in the main ring of the Large Hadron Collider — the mother of all particle accelerators.
For the Opera experiment, some of the protons are siphoned off at an intermediate energy and slammed in pulses 10 microseconds long into a graphite target, where they produce a pulse of lesser particles called mesons. The mesons in turn decay into neutrinos, which then disappear into the Earth in the direction of Gran Sasso. There, the arriving neutrinos run into an assemblage of lead bricks and photographic emulsion.
In theory, during the trip, which takes a few milliseconds, some of the neutrinos should shape-shift from a variety known as muon neutrinos to tau neutrinos. The goal of the Opera experiments was to study this transformation: In three years, the researchers have recorded some 16,000 neutrinos in their detector, but only one tau neutrino.
Measuring the speed of the neutrinos was only a side ambition, explained Antonio Ereditato of the University of Bern, the head of the Opera collaboration. “Now it is becoming a main issue,” he said, adding, “we would like to see some tau neutrinos,” to appreciative laughter from the audience.
In the old days, when scientists sent around copies of journal articles and wrote letters to one another, the process of scrutiny of a controversial measurement could have happened quietly, but the Web has changed all that. Dr. Autiero’s talk at CERN and the appearance of a paper by the Opera group on the Internet Thursday night came at the end of a drumbeat of rumors and blog postings. One blog called it “Rumour of the Century.”
Some physicists, inside and outside of CERN, were critical of this process, saying the laboratory was giving too much weight to a premature result by a group that was not even part of CERN.
Nima Arkani-Hamed, a particle theorist at the Institute for Advanced Study in Princeton, said in an e-mail, “There was no need for a press release or indeed even for a scientific paper, till much more work was done. They claim that they wanted the community to scrutinize their result — well, they could have accomplished that by going around and giving talks about it.”
Rolf-Dieter Heuer, director general of CERN, said in an e-mail from Spain, “I agreed to the seminar at CERN because it is the duty of a lab like CERN to give the collaboration the possibility to ask the community for scrutiny of their findings.”
The scrutiny is surely coming.
An earlier measurement of neutrino speeds was performed by a collaboration known as Minos, for Main Injector Neutrino Oscillation Search, in 2007. Jenny Thomas of University College London, said the Minos experiment would be able to do a more precise measurement in four to six months.
“They’ve done their best,” Professor Thomas said of the Opera group. “The light’s going to shine on us now while we repeat our experiment.”
http://www.nytimes.com/2011/09/24/science/24speed.html?ref=science
|
置顶卡
变色卡
千斤顶
显身卡
关注！
