[转帖] [6.28]The ghosts and the machine幽灵和机器

showcraft

http://www.ecocn.org/thread-198289-1-1.html
Particle physics
粒子物理学

The ghosts and the machine幽灵和机器

Studying the diaphanous neutrino will be America’s contribution to a new generation of physics
对几乎一无所知中微子的研究将是美国对新时代物理的贡献

Jun 28th 2014 | Batavia, Illinois | From the print edition
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1 DEEP beneath the plains of Illinois, in a man-made cavern filled with racks of scientific equipment, someone has spray-painted a white circle onto the bare rock wall. Stand in front of it and you are standing in the path of the most powerful beam of neutrinos in the world, which is emerging from a nearby particle accelerator at Fermilab, America’s main particle-physics laboratory. With any other kind of accelerator, standing in the beam would have spectacular and fatal consequences. But your correspondent was not vapourised—nor, several weeks later, has he developed either cancer or superpowers.

在伊利诺伊州的平原地下深处，有一个人造洞穴，里面充满了科研设备的机架，有人在裸露的岩壁表面喷上了白色的圆圈。站在这个洞穴前，你已经站到了世界上最强中微子光束的路径上，这个光束是附近费米实验室--美国的主要粒子物理实验室--的粒子加速器发射出的。和其它任何一种粒子加速器一样,站在这种光束的路径上会引起非常壮观的，而且致命的后果。但是，本刊记者在这里却并没有被气化，也没有在几周后患上癌症或者变成超人。

2 And that is the point: neutrinos are ghostly things. Billions a second stream through every cubic centimetre of space. But because they feel only the two weakest of the four fundamental physical forces—gravity and the aptly named weak nuclear force, rather than electromagnetism and the strong nuclear force—they hardly interact with the rest of creation.

关键是：中微子是很可怕的物质。几十亿光束会在一秒钟内穿过空间的每个角落。但是，因为它们只能感应到四种基本物理力中最微弱的两种力—重力和弱核力（这个命名很恰当），而不是电磁力和强核力—中微子光束很难与其它物质互相感应。

3 They are, nonetheless, a hot topic, for studying them could reveal the physics that must exist beyond the Standard Model. This is the theoretical framework of particle physics, which was completed in 2012 when its missing element, the Higgs boson, was run to ground by the Large Hadron Collider (LHC) at CERN, Europe’s chief particle-physics laboratory. Looking beyond the Standard Model is physics’s next big thing. And a report on the future of American physics, published last month, outlined a plan to make America the world’s leader in neutrino research.

尽管如此，它们仍旧是热点话题，因为研究它们可能会揭示物理肯定是超越标准模型存在的。这是粒子物理学的理论框架—该理论产生于2012年，当时粒子中失踪元素--希格斯玻色子，通过欧洲核子研究中心--欧洲首席粒子物理实验室—的大型强子对撞机（LHC）的作用，已坠向地面。 寻找超越标准模型是物理界的下一件大事。上个月出版的，一篇有关美国物理未来的报告提出了一个计划，可以使美国成为世界中微子研究的领导者。

4 Neutrinos are interesting because they do not obey the rules. The Standard Model said they should be massless. According to Einstein, this means they necessarily travel at the speed of light—and, as a consequence, do not experience the flow of time. But in 1998 an experiment in Japan showed that neutrinos spontaneously transform between the three varieties of them that exist (known as electron, muon and tau neutrinos) as they zip through space—a process known as oscillation.

因为中微子不符合物理标准规则，所以显得特别吸引人。按照标准模型的解释，这些中微子应该是没有质量的。根据爱因斯坦的研究，没有质量意味着它们应该是以光束传播的—因而，它们不会随着时间的流动而流动。但是，1998年在日本进行的一项实验表明，当中微子三个变种（被称为电子，μ子和τ中微子）快速通过空间时，中微子在它们之间自发性地转变是存在的—这个过程被称为振荡过程。

Variations on a theme 主题变奏曲
5 Oscillation means neutrinos experience time. This, in turn, means they must travel slower than light and so have mass after all. That is a lever to break the Standard Model open. Steve Weinberg, a doyen of particle physics who was one of the Model’s architects, has described neutrino mass as the most important discovery in particle physics for a quarter of a century.

振荡表明中微子是随着时间变化的。反过来，也就是说，它们的传播速度肯定比光速慢，因此它们应该是有质量的。这个结果打破了物理界的标准模型。粒子物理学的资深研究者--史蒂夫温伯格，他也是标准模型的创建者之一，称中微子质量的发现是四分之一个世纪以来，粒子物理学界最重要的发现。

6 But the details of oscillation remain incomplete, which is where Fermilab’s neutrino beam comes in. By the end of July work should have finished on building NOVA, an experiment designed to pin those details down. The beam that passes through the white circle will carry on for 810km (500 miles) through the Earth to a detector in northern Minnesota. When it arrives, some of the muon neutrinos in it will have transformed themselves into electron neutrinos. NOVA will measure precisely how often this occurs.

但是振荡过程的细节仍旧有待研究，这也是费米试验室中微子光束存在的原因。到7月底，建造NOVA—一项设计用于记录这些未知细节的试验--工作应该能完成。通过白色光圈的光束会穿越地球上空810公里（500英里），到达明尼苏达州北部的一个探测器。当它被探测器接收时，其中的一些μ介子中微子会已转型为电子中微子。NOVA将会精确测量出这种现象的发生频率。

7 Besides their three varieties, neutrinos have three different masses. Scientists know that two of these are closely matched, while the third is either much bigger or much smaller. NOVA should be able to discover which. That, in turn, should shed light on how neutrinos acquire mass in the first place.

除了这三种变种以外，中微子还有另外三种不同的质量。科学家们知道，这三种不同的质量当中有两种非常接近，而第三种的质量不是比前两种大得多，就是小得多。NOVA应该能够发现其中奥妙。反过来，这对首先揭示中微子如何获得质量应该大有帮助。

8 All this may sound rather technical. But neutrino physics could have some very big implications indeed. Specifically, it could explain why there is any matter in the universe at all. Like other kinds of matter, neutrinos have antimatter opposites. Matter and antimatter, as is well known, annihilate each other on contact. That is a problem because existing theory says the Big Bang should have produced equal quantities of both, and this means the modern universe ought to be little more than a thin soup of post-annihilation radiation—which, evidently, it is not.

这一切听起来非常深奥。但是，中微子物理可能实际上会有一些非常大的影响。具体来说，它可以解释宇宙中存在的一切是怎么回事。同样，它也像其它的物质，中微子存在反物质的对立面。众所周知，物质和反物质在相互接触时会湮灭对方。这产生了一个问题，因为现有的理论认为宇宙大爆炸应该产生了相同数量的物质和反物质，这意味着现代宇宙应该比湮没后辐射的稀薄宇宙小些—但是显然，并不是这样。

9 The best explanation for the preponderance of matter is that it and antimatter are not quite equal and opposite. A few inequalities are known, involving particles called B-mesons and K-mesons, but they are insufficient to explain the amount of matter around. Many physicists suspect that neutrinos oscillate in a different way from their antineutrino counterparts, and that this could be a big enough effect to fill in the gap and explain why the universe is as it appears. By switching its beam between neutrinos and antineutrinos, NOVA may be able to glimpse such differences. And an even more sensitive successor experiment, the Long-Baseline Neutrino Facility (LBNF), planned to be finished in 2022, would be able to explore them more comprehensively.

对物质数量上优势的最好解释是，物质和反物质的数量并不是完全相同的，而且它们也不是完全对立的。比如几个已知的物质，包括B-介子和K介子颗粒，它们与各处反物质的数量就不相等，但是，，仅凭这些个别现象又无法完全解释其它物质数量不等的原因。许多物理学家猜测，中微子振动与它们的反中微子配对物的振动方式完全不同，所以这也可能会填补物质与反物质之间的间隙，也能够解释为什么宇宙是现在的样子了。如果在中微子和反中微子之间切换光束，那么NOVA可能就会发现这种差异。一个更加灵敏的后续试验可能会在探索中微子和反中微子方面更加全面，试验用的设备--长基线中微子设备（LBNF）计划于2022年完成。

10 The LBNF will detect neutrinos by watching for flashes of light caused when one of them deigns to interact with an atom in a giant tank of liquid argon. But neutrinos are not the only things that cause such flashes. The Standard Model says that protons, which help make up the nuclei of atoms, are stable, but many post-Standard Model theories disagree. They predict protons should decay into lighter particles. The fact that no proton has been seen to do so suggests such events are exceedingly rare. But the LBNF’s 34,000-tonne tank of argon will contain an awful lot of carefully watched protons. If these particles really can combust spontaneously, LBNF stands a fair chance of spotting it.

在充满液氩的大罐中，当其中一个中微子不得不与一个原子相互作用时，就会产生光，LBNF将会通过探测光的闪烁来检测中微子。中微子并不是造成这种闪烁的唯一物质。标准模型认为，原子中原子核的组成部分—质子是稳定不变的，但是许多后标准模型理论却不支持这种观点。它们预测质子应该会衰变为更轻的粒子。但是，由于质子的这种转变还没有被物理学家们捕捉到，所以这就表明这种转变的发生几率极小。不过，LBNF34,000吨的氩罐中会有非常多可以仔细观察的质子。如果这些颗粒真的能自燃，那么LBNF极有可能可以观察到这种变化。

11 It could even act as a telescope. Neutrinos are generated in prodigious quantities in stellar explosions known as supernovas, and their ghostliness lets them escape the superdense centre of such an explosion unhindered. By registering the neutrino pulse escaping a supernova, the LBNF, alongside other, existing neutrino detectors, could let astronomers probe what takes place in such extreme environments.

LBNF甚至可以作为一种望远镜。中微子是在被称为超新星的恒星爆炸时，产生的无数碎片中形成的，它们的幽灵使它们远离了这种无法阻挡的爆炸的超密度中心地带。通过记录逃离超新星的中微子脉冲，LBNF与其它的中微子探测器可以让天文学家们明白，在这样一种极端的环境中究竟发生了什么。

12 Assuming, that is, the LBNF gets built. As physicists probe deeper and deeper into the subatomic world they must make use of bigger and bigger machines. Particle physics is, increasingly, an international endeavour, as governments try to spread the cost of the kit. America will be the biggest contributor to the LBNF’s $1.5 billion budget, but crucial cash must be raised from other countries, including Brazil, Britain and India. Still, next to the $9 billion the LHC cost, that might seem a bargain.

也就是说，假设LBNF建造成功了。当物理学家们想要探测到更深层次的亚原子世界时，他们必须使用更大的机器。当各国政府在相关设备的投入上越来越多时，也表明现在的粒子物理已经越来越成为国际物理学努力的方向了。美国将成为LBNF项目15亿美元预算的最大投资者，但是，决定性的资金投入者却是巴西，英国和印度这些国家。然而，与90亿美元的LHC投资相比，这好像还可以再商量商量。