An early Christmas present? 一份提前送出的圣诞礼物？

The search for dark matter
寻找暗物质

An early Christmas present?
一份提前送出的圣诞礼物？

Dec 17th 2009
From The Economist print edition

Wild rumours are circulating of the discovery of one of physics’s great unknowns: dark matter

消息传开了！发现了物理学中最重要的一个未知物——暗物质

AS The Economist went to press this week, physicists were aflutter about an expected announcement from one of the world’s most important experiments searching for dark matter—the as-yet-undetected material that, if models of the universe are correct, is about six times as abundant as the familiar, visible stuff. Physicists working on the Cryogenic Dark Matter Search (CDMS), a large collaboration whose experimental apparatus is located in Minnesota, will be making presentations on December 17th at Fermilab and the SLAC National Accelerator Laboratory in America, and on December 18th at CERN, the European particle-physics laboratory near Geneva. The speculation is that they will announce the detection of the hitherto unknown particles that make up dark matter. The researchers plan to post their results to the arXiv, an online repository of physics papers, on December 17th, with submissions to a peer-reviewed journal following shortly thereafter.

本周《经济学人》发刊之际，物理学界正在为一个预期的发布会兴奋雀跃着。届时，将宣布世界上一个最重要的实验的结果：寻找暗物质——一种尚未发现的物质。如果宇宙真如宇宙模型的描述般，宇宙中暗物质的含量将是人们熟悉的可见物质的6倍之多。低温暗物质搜寻组（一个大型的合作组，其实验仪器位于明尼苏达）的物理学家们将于12月17日在美国的费米国家实验室和斯坦福直线加速器中心同期举行报告会；18日，报告会将移师日内瓦附近的欧洲粒子物理实验室。据推测，他们将会宣布发现了一种未知粒子，这种未知粒子构成了暗物质。研究者计划在12月17日将研究结果放到arXiv网站（一个网上物理学论文库），并随后将论文提交到一个采用专家评审制择稿的专业学术刊物。

Around a quarter of the universe is thought to be made up of dark matter, which, as the name suggests, neither gives off nor reflects light. (The balance, once the small amount of visible matter is subtracted, is made of even more mysterious stuff known as “dark energy”.) However, dark matter does make itself known through its gravity. This, indeed, is why astronomers believe it must be there. Some galaxies rotate so fast that they should be throwing off their outermost stars. Only the gravitational pull of these galaxies’ unseen halos of dark matter holds those stars in. Observations of the bending of light around clusters of galaxies, as well as the way that galactic structures formed in the early universe, also suggest that there is much more to reality than meets the eye.

通常认为，宇宙的大约1/4是由暗物质组成，就像这个名字表示的那样，暗物质既不发光，也不反光。（宇宙中数量很少的可见物质如果被消减，会由一种叫作“暗能量”的更为神秘的东西来达成平衡。）但是，人们可以通过暗物质的引力来知道其存在。这就是天文学家确信暗物质存在的原因。一些极速自转的星系本该把其最外层的星体抛出，是存在于星系之中的暗物质晕的引力留住了这些本该被遗弃的星体。除此之外，星系团周围光线的屈折现象和宇宙早期星系结构的形成方式都表明：我们看不见的现实比我们看得见的要多得多。

Although the astrophysical case for dark matter is compelling, physicists would like to be able to study it in the laboratory to tease out its true nature. One way to do this would be to produce it in a particle accelerator. When it is running at full strength, the Large Hadron Collider at CERN will certainly try to do this. Another way is to build a detector on Earth that can capture particles of dark matter as they wander in from space. This is what CDMS and other experiments like it have been trying to do for several decades.

尽管天体物理学中关于暗物质存在的证据极为明显，物理学家还是想在实验室中研究暗物质，以理出其真正的本质。一种实现办法是通过粒子加速器的作用来显现暗物质的存在。欧洲粒子物理实验室的大型强子对撞机就试图在其满荷运动时来达到这一目的。另一种办法是在地球建造一台探测器，可以捕获从太空误入地球的暗物质粒子。低温暗物质搜寻组和其它一些类似的实验组在过去的几十年间一直采用这种办法。

The CDMS detector, which is located 700 metres underground in an old iron-ore mine in northern Minnesota, to shield it from quotidian sources of radiation such as cosmic rays that might confuse the detectors, makes use of an array of germanium and silicon crystals that have been cooled to within a whisker of absolute zero. As the Earth makes its way through the Milky Way, it passes through the galaxy’s halo of dark-matter particles. Every so often, one of these particles would be expected to enter one of the crystals and bump into the nucleus of an atom of germanium or silicon. This minuscule nudge will set the crystal vibrating. By listening for these telltale vibrations in the quiet of a cold crystal, the researchers running the experiment should be able to detect the presence of the passing dark matter. At least, that is the theory. However, theory also predicts that such collisions will be exceedingly rare, and observing them requires very large, very sensitive detectors.

低温暗物质搜寻组使用的探测器在一个废弃的矿井之内，位于地面以下700米，这是为了让其与诸如宇宙射线之类的普通辐射源相隔离，这些辐射源可能会干扰探测器的正常运转。搜寻组利用了大量的锗硅晶体，并且将其冷却到接近绝对零度。地球在银河系运行的过程中，会经过银河系中的暗物质微粒形成的晕。估计偶尔会有某个暗物质微粒进入（实验所用的）晶体之中，然后撞到某个锗原子或硅原子的原子核上。这个细微的触碰将会引起晶体的震颤。而静寂、冰冷的晶体一有响动，也就透露了暗物质的行踪。研究者通过仔细聆听，应该能找到途经而过的暗物质。这种办法至少在理论上是圆通的。但是，理论也告诉我们，这种碰撞几率将极其微小，而且观察所用的探测器要非常巨大，非常灵敏。

Besides locating CDMS deep underground, other precautions have also been taken. Most intriguingly, the shielding around the apparatus is made from lead recovered from an ancient sunken ship. This, because of its age, has already lost most of its radioactivity.

除了把探测器深置于地下，还有其它的预防措施。最有趣的是，仪器保护罩用的材料铅是来自一艘古代沉船。这些铅因为年代久远，已失去了它绝大部分的放射性。

The experiment last announced results almost a year ago. Through careful calibration and diligent work, the researchers running it were able to eliminate most of the residual background sources of radiation that could mimic dark matter. When they examined their data then, nothing was seen in the region where a dark-matter signal would have been expected to appear. The new results, however, are based on twice as much data, so might yield a different outcome.

实验组上次宣布研究结果大约是在一年之前。研究者通过仔细的调校和努力的工作，已经有能力剔除背景残留辐射源中可能造成误认的大部分。但在检查实验数据的过程中，他们在预计暗物质信号将会显现的区域什么也没看见。但是，这次的新结果基于的数据相当于上次的两倍，所以可能是一个不同的结果。

If the rumours are true, a solution to one of the great problems of physics may now be within reach. If not, CDMS will at least be able to place the most stringent constraints to date on theories that attempt to explain dark matter—proving some of them as baseless as the tales that eager and hopeful physicists tell.

如果传言为真，物理学上最大难题之一的解决之道可能就离我们不远了。如果传言为传言，低温暗物质搜寻组至少能够为那些迄今为止试图解释暗物质存在的理论带上最厉害的紧箍咒——证明某些理论是站不住脚的，就像某些心急且信心十足的物理学家搭建的空中楼阁一样没有根基。