超光速中微子

译者 dlq

Who travelled faster than light

什么的速度比光更快呢？

NEUTRINOS possess a seemingly endless capacity to discombobulate. First the elusive particles, which theorists believe to be as abundant in the universe as photons, but which almost never interact with anything, turned out to have mass. That discovery, made at Japan's Super-Kamiokande detector in 1998, flew in the face of the Standard Model, a 40-year-old rulebook of particle physics which predicted they ought to be massless (and which has since been tweaked to accommodate the result). Now researchers at CERN, the world's main particle-physics laboratory, report that their neutrinos appear to confound what is, if anything, an even bigger theoretical colossus: Albert Einstein's special theory of relativity. They did it by apparently travelling faster than the speed of light.

中微子是物理世界的调皮鬼。首先，理论物理学家认为难以琢磨的它具有质量，在宇宙中和光子一样无处不在，但却几乎不与任何物质发生相互作用。按照标准模型它是没有静止质量的，这一理论曾统治了物理学界四十余年，但是1998年日本超级神冈探测器观测结果却证实它具有微小质量。而日前欧洲核子研究中心（CERN，世界上最大的粒子物理实验室)的研究员表示中微子又很可能造成了更大的物理学界混乱：爱因斯坦的狭义相对论可能不正确。因为他们发现了一种超光速的中微子。

Physicists from OPERA, one of the experiments at CERN, send beams of neutrinos from the organisation's headquarters on the outskirts of Geneva, through the Earth's crust to an underground laboratory 730km away underneath Gran Sasso, a mountain in the Apennines. They use fancy kit like high-precision GPS and atomic clocks to measure the distance the neutrinos travel to within 20cm and their time of flight to within ten nanoseconds (billionths of a second). The neutrinos in question appear to be reaching the detector 60 nanoseconds faster than light would take to cover the same distance. That translates to a speed 0.002% higher than the 299,792,458 metres per second at which light zaps through a vacuum.

来自CERN下属实验室OPERA的物理学家接受到了日内瓦市郊总部发出的中微子束，它穿过地壳到达距日内瓦730km的意大利亚平宁山格兰萨索峰。他们利用诸如高精度的全球定位系统（GPS)、原子钟等特殊仪器测量中微子运动的距离（精确到20cm)和时间(精确到10ns，1ns=10-10s).问题是相同的距离，中微子比光子提前60ns到达，即比真空中光速299,792,458m/s的速度还要快0.002%。

The result, published in arXiv, an online database, is based on data from 15,000 neutrinos detected at Gran Sasso over three years. If it holds up it would be the first chink in what has until now been the impenetrable armour of special relativity, a theory which has been tested—and confirmed—time and again since its publication in 1905. The theory states that as an object speeds up, time slows down until it stops altogether on hitting the speed of light. Anything going faster than light would, in other words, be moving backwards in time.

这一公布在网上数据库arXiv上的结果来源于过去三年格兰萨索接收到的15,000个中微子数据。如果这一结果得到验证，这将是迄今为止狭义相对论面临的最严峻考验，要知道1905年公布以来狭义相对论得到了时间的检验和证实。狭义相对论表明一个物体的速度增加，时间将减慢直到它无限接近光速。换句话说，任何比光更快的物质将回到过去。

A violation of special relativity that affects only neutrinos would be very weird indeed. To confuse matters further, observations of neutrinos emitted by a supernova observed in 1987 established that the particles travel at just below the speed of light through the vacuum of space to a precision four orders of magnitude better than the OPERA claim. That means that the OPERA neutrinos would have to be interacting with matter in some bizarre way that violates special relativity.

这一违背狭义相对论的现象只能说明中微子的确太诡异了。更让人迷惑的是，根据超新星爆1987a的观测结果观测者认为中微子速度要比真空中的光速低，这比OPERA声称的要精确4个数量级。这意味着OPERA的中微子可能以某种违反狭义相对论的奇异途径和物质发生了相互作用。

The odds, it must be admitted, are that a mistake has been made somewhere in the long chain of timing measurements required to compare the moment when neutrinos are created at CERN by smashing a beam of protons into a target, and their detection in Gran Sasso, though OPERA's researchers have done their best to account for all possible instrumental quirks. What makes the result slightly less than incredible is that an experiment in America, called MINOS, detected a similar anomaly in 2007. MINOS's researchers dismissed that result as a mismeasurement. Now, though, the experiment has ten times more data than it did four years ago, as well as ideas about how to make the necessary calculations more accurate. (A proposed upgrade called MINOS+, which could start collecting data in 2013, might be able to determine the flight time to within one nanosecond.)

不得不承认这些怪事的确发生了，长期观测中某个环节出了错误，也许是CERN用质子束轰击靶材或者在格兰萨索被探测到的过程中；虽然OPERA的研究人员已经尽其所能将仪器误差减到最小了。这个不可能的结果还不至于太不可信是因为2007年美国的MINOS团队观测了类似结果。MINOS的研究者对他们的测量结果仍有所怀疑。而现在的实验数据比4年前要多十倍，计算方法也更科学精确。（有一个升级版的MINOS+,将从2013年开始收集数据，也许可以将中微子飞行时间精确到ns。）

Physicists working on another neutrino experiment in Japan, known as T2K, are holding a meeting next week and the OPERA result will be high on the agenda. The effect may be too small to spot in the data recorded before T2K was damaged by the earthquake in March. Moreover, T2K's detector is located just 295km from the neutrino source, so the effect would be just 25 nanoseconds, if it were real. T2K hopes to start taking data again in 2012.

日本的另一组研究中微子的物理学家T2K将在下周举行一场研究会，OPERA将是重要议题。由于T2K在三月份的地震中受损，他们观测到的反应并不明显。而且T2K的探测器里中微子发射源只有295km,所以只观测到25ns的提前时间，前提是它准确的话。T2K希望2012年重新开始。

If the Japanese and American experiments do see the same strange result, it would be the greatest revolution in physics since, well, special relativity burst onto the scene. And it would be fair to say of a neutrino what a wag once quipped about a lady named Bright: that it went away, in a relative way, and came back on the previous night.

如果日本和美国的实验得出了相同的奇怪结论，也许这将是狭义相对论诞生以来最伟大的物理界革命。那么将调皮的中微子以女性名字命名为Bright（意为“光明”）也就合情合理了：她以相对的路径离开，并在前一天晚上回来。

译者福利：相关链接：

 http://www.guokr.com/post/64707/
bbc 英文原文 http://www.bbc.co.uk/news/science-environment-15017484 
Nature原文 http://www.nature.com/news/2011/110922/full/news.2011.554.html 
论文出处 http://arxiv.org/ftp/arxiv/papers/1109/1109.4897.pdf