In search of the neutrino, ghost particle of the universeThe main spectrometer of Katrin on its way to Karlsruhe in 2006. The project is set to get under way in June 2018 Photograph: Michael LatzOn the outskirts of Karlsruhe, in south-west Germany, engineers have buried a giant, stainless steel device, bigger than a blue whale, inside the town’s institute of technology. The machine looks for all the world like a grounded zeppelin or a buried blimp. 德国西南部的卡尔斯鲁厄郊区，工程师在该镇技术研究所中埋了一个比蓝鲸还大的巨型不锈钢设备，该设备像搁浅的齐柏林飞艇，后一个被埋藏起来的飞艇。 In fact, the apparatus is one of the world’s biggest vacuum chambers. Air pressure inside it is lower than that on the surface of the moon and it has been installed to help solve a single, intricate problem: finding the mass of the universe’s most insignificant entity, the neutrino. 事实上，该仪器是世上最大的真空室之一，里面的气压比月球表面还低，它已被用来解决一个单一且复杂的问题：获取宇宙中最微不足道的实体中微子的质量。 Every second, billions of neutrinos pass through our bodies. The sun sends trillions streaming across space every minute. Uncountable numbers have been left over from the Big Bang birth of the cosmos 13.8 billion years ago. 每秒钟，数十亿个中微子穿过我们的身体，而太阳每分钟要向太空中发射数以万亿计的中微子。138亿年前宇宙大爆炸遗留了不计其数的中微子数据。 In fact, there are more neutrinos in the universe than any other type of particle of matter, though hardly anything can stop these cosmological lightweights in their paths. And this inability to interact with other matter has made them a source of considerable frustration for scientists who believe neutrinos could bring new understandings to major cosmological problems, including the nature of dark matter and the fate of our expanding universe. Unfortunately, the unbearable lightness of their being makes them very difficult to study. Hence the decision to build the Karlsruhe Tritium Neutrino Experiment, or Katrin. It is designed to measure the behaviour of neutrinos and electrons that are emitted by the hydrogen isotope, tritium, in order to uncover slight variations in their paths as they fly through the experiment’s vacuum chamber. These variations should reveal precise details about the neutrino’s physical properties, in particular its mass. 然后决定建立卡尔斯鲁厄氚中微子实验室或简称为卡特琳，该实验室旨在测量氢同位素氚发出的中微子和电子的行为，以发现它们穿过实验真空室时的轻微路径变化。这些变化可以揭示中微子物理属性的精确细节，特别是其质量。 “We have pushed technology to the limit in building Katrin,” says the project’s leader, Guido Drexlin. “Apart from creating a near perfect vacuum inside its huge chamber, we also have to keep the temperature of the tritium, which is the machine’s source of neutrinos, inside the device to a constant 30C above absolute zero. We have also had to take incredible care about the magnetic fields inside the machines. Essentially, we have had to demagnetise the whole building.” 该项目负责人吉多·德雷克斯林说：“为了建设卡特琳，我们将技术发挥到极限。除了在巨大的室内空间制造一个接近完美的真空，还必须将装置内氚的温度恒定在绝对零度以上30℃，氚是装置内中微子的来源。我们还得密切关注设备内的磁场，最重要的是，我们得为整个建筑消磁。” It has taken more than a decade of planning and construction to put Katrin together. Its price tag, just over €60m, has been met by the German taxpayer via the country’s state-funded Helmholtz Association, with a further €6m chipped in by US, Russian, Czech and Spanish scientists who will have a minor involvement with the project. 卡特琳历经数十年规划与建设，耗资超过六千万欧元，由德国纳税人通过国有赫姆霍兹联合会出资，美国、俄罗斯、捷克以及西班牙科学家共同出资六百万欧元，他们或多或少将参与项目建设。 Final trials are now being completed and full operations are set to begin in June, though it will take a further five years of gathering data before scientists can expect to have enough information to make an accurate assessment of the neutrino’s mass. 最终测试已近完成，预计六月开始全面运行，虽然还要再花五年时间收集数据，科学家才能有足够信息对中微子质量进行精确评估。 “Even then, we may have to go to a second phase of operations to get our answer,” says Drexlin. “We are moving into unknown territory here.” 德雷克斯林说道:“即使到了那时，我们也许还得进行第二阶段的测试以获取答案，我们将进入一个未知的领域。” The neutrino was first postulated in 1930, by the Nobel physics laureate Wolfgang Pauli, to explain the behaviour of other subatomic particles during radioactive decay. It took a further 26 years of search before neutrinos were first pinpointed in detectors and they remain maddeningly elusive. 1930年，诺贝尔物理学奖得主沃尔夫冈·泡利假定中微子存在以解释放射性衰变过程中其它亚原子粒子的行为。探测器首次捕捉到中微子之前，他又进行了26年的研究，然而中微子依然难以捉摸。 An illustration of their insubstantial nature is provided by Canada’s Sudbury Neutrino Observatory, where a 1,000-tonne tank of heavy water is used to stop some of the 10 million million neutrinos that pass through it each second. Of these, only about 30 are actually detected in an average day. 加拿大萨德伯里中微子天文台提供了一个中微子微不足道的例证，他们用1000吨罐装重水截留每秒穿过它的一千万中微子中的一部分。在这些里面，平均一天也只有大约三十个真正被探测到。 Three different forms of the particle are now known to exist: the electron neutrino, the muon neutrino and the tau neutrino and until relatively recently it was thought that none of them had any mass at all. They were the ultimate in ephemeral ghostliness, a bizarre situation that was celebrated by John Updike in his poem, Cosmic Gall. 目前已知的有三种不同形式的中微子：电子中微子、μ中微子和τ中微子，并且直到最近还被认为是没有质量的。它们是短暂幽灵般的终极目标，是约翰·厄普代克的诗《宇宙的苦味》中表达的一个奇怪情形。 Neutrinos, they are very small. 中微子，它们很小。 也不会交互作用。 地球就是一个愚蠢的球 对它们而言，可以轻而易举的穿梭， 就像走在通风大厅里的扫地阿姨 However, in the late 20th and early 21st century, scientists started to uncover evidence that suggested Updike was not entirely correct in his claims about the neutrino and that it did have some mass after all. This work culminated in experiments, carried out separately by Takaaki Kajita, from Japan, and Arthur McDonald, from Canada, which showed that neutrinos switch form as they travel across space. For example, some of the electron neutrinos emitted by the sun are transformed into muon and tau neutrinos as they hurtle towards the Earth. The process is known as neutrino oscillation. 然而，二十世纪末、二十一世纪初，科学家开始发现一些证据，证据显示厄普代克关于中微子的陈述并非完全正确，毕竟它们确实是有质量的。这项工作在实验中达到高潮，实验由日本的梶田隆章和加拿大的阿瑟·麦克唐纳分别进行，实验表明中微子穿越太空时会变换形式。例如，太阳发出的一些电子中微子会在冲向地球时转换成μ中微子和τ中微子，而这个过程就是中微子振荡。 “The discovery was crucial,” Drexlin insists. “There is a straightforward constraint in cosmological theory that states that only objects with mass can oscillate between different forms in this way. Massless particles could not change in this way. So the inference is clear: neutrinos must have mass.” 德雷克斯林说道：“这个发现至关重要，宇宙学理论有一个最直接的限定，那就是只有具有质量的物体才能像这样以不同的形式振荡。无质量的粒子是无法这样变化的，所以推断显而易见：中微子一定有质量。” Drexlin recalls attending the physics conference where the results of these first experiments were unveiled. “It was like a rock concert. People were cheering and stamping their feet – for a good reason. We knew the universe would never be the same again.” For revealing the neutrino’s massive secret, Kajita and McDonald were awarded the Nobel prize in physics in 2015. 德雷克斯林回忆参加物理大会，第一批实验结果公布时的情景，“就像是一场摇滚音乐会，人们欢呼跺脚，为这美好的理由。我们知道宇宙不会是一成不变的。”梶田隆章和阿瑟因揭示中微子质量秘密被授予2015年诺贝尔物理学奖。 But if neutrinos have mass, exactly how much do they possess? It is not a trivial question, for as Mark Thomson of Cambridge’s Cavendish Laboratory points out, the precise result of such a measurement could have critical consequences. “Neutrinos have mass but they remain staggeringly insubstantial. They are still a billion times smaller than any other type of known subatomic particle. On the other hand, there so many of them that their combined masses could give them cosmological significance. We badly need to know what that mass is in order to figure out how they might affect the future of the universe.” 如果中微子有质量，那么它具体有多大呢？这并不是一个微不足道的问题，正如剑桥卡文迪什实验室的马克·汤姆森指出，这种测量的准确结果会产生重要影响。“中微子有质量但是仍具有虚无缥缈，它们依然比已知其他类型的亚原子粒子要小十亿倍。另一方面，有太多的中微子，一旦它们的质量合起来将产生宇宙级影响。我们急需了解它的质量以搞清楚它们会如何影响宇宙的未来。” For example, if neutrinos prove to be on the heavy side of current estimates, then their combined gravitation pull would effect the expansion of the cosmos and slow it down. However, if their mass is on the light side, neutrinos, despite their cosmological ubiquity, will be unable to act as any kind of meaningful brake to the universe’s expansion. 例如，如果中微子被证实属于目前预估中较重的情况，那么它们的组合引力牵引会影响宇宙的膨胀并减慢它的速度。然而，如果它们的质量属于轻的情况，尽管存在于宇宙各处，中微子也不能以任何形式减速宇宙膨胀。 Nor is this the only reason that scientists are fascinated by the fact that neutrinos have mass. “What is so intriguing is that their mass is just so much less than that of any other particle – by a factor of a billion – which suggests they must get their mass from some other mechanism,” adds Thomson. “All other particles get their mass by attaching themselves to Higgs bosons but the neutrino must do it by a very different route. So there is some other basic force that seems to be involved and uncovering that would be a real prize.” 这也不是科学家执着于中微子有质量的唯一原因。汤姆森补充道：“有趣的是，它们的质量比其它粒子要小十亿倍，这意味着它们必须从其它途径获得质量。所有其它粒子通过附着于希格斯玻色子获得质量，而中微子一定是通过不同的方式获得的。似乎还有其它一些基本力量参与其中，揭示这一力量将会是最好的奖励。” It is for these reasons that scientists have struggled, over the decades, to find the exact mass of the neutrino. First efforts, made after the second world war, placed an upper limit on its mass at around 500 electron volts (ev). This figure is about 1/500th of the mass of the electron, itself a relatively tiny particle. (Using a unit of energy to describe the mass of an object may seem strange but all subatomic particles are measured in electron volts, which can also be used as a unit of mass because energy and mass are convertible concepts according to Einstein’s E=mc² equation.) 正是由于这些原因，科学家们几十年来一直努力探寻着中微子的确切质量。第二次世界大战后作出第一次尝试，科学家们将中微子质量的上限设为约500电子伏特。这个数字大概是电子质量的五百分之一，它本身就是一个相对微小的粒子。（用一个能量单位来描述物体的质量可能看起来很奇怪，但是所有的亚原子粒子都是用电子伏特来衡量的，这也可以用作质量单位，因为根据爱因斯坦的能量公式E=mc²，能量和质量是可以相互转化的两个概念。） Inside Katrin, superconducting magnets will generate a field 70,000 times more powerful than Earth’s. Photograph: Forschungszentrum Karlsruhe Since then, measurements, carried out in Mainz, Germany and Troitsk in Russia, have pushed this figure further and further downwards with the result that the upper limit for the neutrino mass is now put at around a mere 2 ev, about two billionth the mass of the lightest atom. It will be the task of Katrin finally to nail down a precise figure. 此后，在德国美因茨和俄罗斯特洛伊茨克进行的测量将这一数字进一步拉低，结果现在中微子质量的上限仅为约2电子伏特，这大概是最轻的原子质量的二十亿分之一。最终确定一个精确的数字将是卡特琳的任务了。 This work will be carried out using a small supply of tritium, an isotope of hydrogen that has two neutrons and a proton in its nucleus. Tritium is made in nuclear reactors and is extraordinarily expensive. “A gram costs about €10,000 so you do want to be careful with stuff, particularly as it is also highly radioactive,” says Drexlin. 这项工作将使用少量的氚，一种氢原子的同位素，它的原子核里有两个中子和一个质子。氚是在核反应堆中产生的，非常昂贵。德雷克斯林说道：“一克大概10000欧元，因此你的确想小心对待这个东西，特别是它还具有高放射性。” It is this last feature – tritium’s radioactivity – that makes it crucial to Katrin. Tritium decays into an isotope known as helium-3 by emitting an electron and a neutrino. By precisely measuring the energy (and therefore the mass) of the electron as it flies away from its tritium source it should be possible to deduce the energy (and mass) of the neutrino that is emitted with it. 正是这最后一个特征——氚的放射性——使得它对卡特琳至关重要。氚通过发射电子和中微子衰变成一种称为氦-3的同位素。通过精确测量电子在离开氚能源时的能量（以及质量），我们应该可以推导出随它释放出的中微子的能量（以及质量）。 Superconducting magnets will generate a field 70,000 times more powerful than Earth’s and channel the electrons into Katrin’s great vacuum chamber towards a powerful electric field. Only those electrons that have the most energy will be able to get past that field and be counted. These will be the electrons that have taken almost all of the energy from their decay from a tritium atom while the neutrino will get none. About one in every 5 trillion electrons created by the tritium will have this feature. 超导磁体将产生比地球强大70000倍的电场，并将电子引入卡特琳的大真空室、通向强大的电场。只有能量最大的电子可以穿过电场并被计数。这些将是从氚原子衰变中吸收了几乎所有能量的电子，可而中微子将不会得到能量。氚产生的每五万亿个电子中大约有一个具备这种特性。 “These electrons will take up all the kinetic energy of that aspect of the decay of the tritium nucleus,” says Drexlin. “The neutrinos that are emitted will get none. All that will be left in the equation will be the mass of the neutrino that was emitted with the electron. By taking very careful measurements, it should then be possible to calculate what is that exact mass.” 德雷克斯林说:“这些电子将获得氚原子衰变过程的所有动能，而发射出来的中微子一点儿也得不到。等式剩下的就是随电子发射的中微子的质量。通过仔细地测量，我们应该可以算出确切的质量。” “It will take at least five years before we can hope to get a realistic figure and even then we might still not get a result. We have various ideas about what to do then so in the end we are pretty sure we will find out what is the neutrino’s mass. It is going to be an intriguing voyage, nevertheless.” “至少要花五年时间我们才能得到真实的数字，即使那时候我们也许依然得不到结果。那个时候该怎么做，我们有很多想法，因此我们坚信最终我们会发现中微子的质量。当然，这会是一场奇妙之旅。” The 200-tonne giant spectrometer is transported through Leopoldshafen in Germany Photograph: Alamy Stock Photo Modern odyssey: how Katrin took the long way home 当代奥德赛：卡特琳是如何历经长途跋涉到家的 The voyage that brought Katrin’s main component – its giant, 23-metre-long, 10-metre-wide vacuum chamber – to Karlsruhe remains one of modern engineering’s strangest odysseys. 将卡特琳的主部件——23米长、10米宽的巨型真空室——运往卡尔斯鲁厄的征程依然是现代工程史上最奇幻的奥德赛事件之一。 Built in nearby Deggendorf, 150km north-east of Munich, the 200-tonne, zeppelin-like chamber was too large to be taken on the 400km westward journey directly to Karlsruhe, either by air or road. So engineers were forced to take to the water, which in turn obliged them to head east down the Danube before sailing into the Black Sea and then the Mediterranean, across the Bay of Biscay and the Channel to Rotterdam. Finally, it was taken down the Rhine to bring it close to Karlsruhe. The 8,800-km trip took two months. “It was dubbed Europe’s biggest detour and you can see why,” says Drexlin. 200吨齐柏林飞艇式的密室在德根多夫附近，位于慕尼黑东北部150千米，它太大了无法通过航空或公路向西走400千米直接运往卡尔斯鲁厄。于是工程师被迫采取水路运输，这又迫使他们沿着多瑙河东行，然后驶入黑海和地中海，穿越比斯开湾、海峡到达鹿特丹。最终，它驶下莱茵河、靠近卡尔斯鲁厄。8800千米的路程耗时两个月。德雷克斯林说道：“它被戏称为欧洲最大的弯路，原因你懂的。” It was also an eventful excursion. In September 2006, not long after the ship carrying the vacuum chamber had set off, it was found its cargo was actually too light to keep the boat low enough in the water and so allow it to pass under the bridge that crosses the Danube at Jochenstein. “We had to buy 1,000 tonnes of rock and gravel to weigh it down so we could get under the bridge,” Drexlin recalls. 这也是一场麻烦不断的航程。2006年9月，运载真空室的船只出发不久就发现承载物实在太轻了以至于无法在水里保持足够低以通过架设在多瑙河约亨施泰因航段上的桥。德雷克斯林回忆道：“我们不得不购置1000吨岩石、砾石来拉低船只吃水以从桥下通过。” On 27 October, the chamber reached the Black Sea. A few days later, as it crossed the Sea of Marmara en route to the Mediterranean, the ship was struck by a storm. “The protective covers that covered the chamber on deck were blown away and its stainless steel coat was exposed, raising the risk of salt water corrosion,” adds Drexlin. “However, we decided to proceed.” 10月27日，密室到达黑海。几天之后，船只跨越马尔马拉海进入地中海时遭遇了风暴。德雷克斯林补充说：“甲板上密室的防护罩被吹走了，不锈钢外壳裸露在外，这增加了被盐水腐蚀的风险。” Eventually the chamber reached Rotterdam and was then taken up the Rhine, which was by now at its lowest level for decades. “We only scraped over the river bottom by centimetres this time,” says Drexlin. 终于密室到达鹿特丹，接着占据莱茵河，莱茵河的水位目前为止是十年来最低了。德雷克斯林说道：“我们这一次驶过（莱茵河）只距河底几厘米。” Eventually, at Leopoldshafen, outside Karlsruhe, the chamber was lifted on to dry land, using one of the world’s most powerful cranes, and placed on to a giant trailer. “I was asked how many people might come to watch the chamber being brought to its final resting place,” says Drexlin. “I said I thought about 300 would turn up. On the day, 30,000 came. The town ran out of food within an hour and we had to ferry in 10,000 sausages to feed everyone.” 在卡尔斯鲁厄郊外的利奥伯德港，工程师用世上最强大的起重机将密室放到旱地上，然后放在一个巨型拖车上。德雷克斯林说道：“我被问到有多少人会来观看密室运往栖身之地，我说我想大概会有300人出现。结果，那一天来了30000人。一小时内镇上食物被吃光，我们不得不运10000根香肠给大家吃。” This final stage of the chamber’s journey also proved to be the most nerve-racking. “At some points, there was a clearance of only 3cm between the chamber and the town’s buildings,” adds Drexlin. The sight of the great machine scraping its way through the town is extraordinary: a spectacle eerily reminiscent of a Hollywood alien invasion film. 事实证明，密室运输的最终阶段最让人头痛。德雷克斯林补充说：“有些时候真空室与镇上的建筑只距3厘米。”巨型仪器穿过小镇的景象非同寻常，是一个让人想起好莱坞外星人入侵影片的奇观。 Eventually, the chamber reached its resting place at the Karlsruhe Institute of Technology. “It was filthy by now,” says Drexlin. “So the first thing we did when we got it on site was to clean it. We are Germans, after all.” RM 最终，密室在卡尔斯鲁厄技术研究所找到栖身之处。德雷克斯林说道：“那个时候它已经很脏了，所以我们把它运到预定地点后的第一件事就是打扫它。毕竟我们是德国人。” |
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