You might think that physicists would be satisfied by now. They have been testing Einstein’s theory of general relativity, which explains what gravity is, ever since he first described it 100 years ago this year. And not once has it been found wanting. But they are still investigating its predictions to the nth decimal place, and this centenary year should see some particularly stringent tests. Perhaps one will uncover the first tiny flaw in this awesome mathematical edifice.
你可能以为，物理学家现在已经满意了。他们一直在对爱因斯坦的广义相对论进行检验。爱因斯坦在整整100年前第一次提出了广义相对论，它解释了引力是什么。科学家们一直没有发现它存在任何不足之处，但却仍在调查根据它做出的预测，精确到第n位小数。在该理论100周年之际，科学家会做一些特别严格的验证。也许会有人发现这座非凡数学大厦的第一个微小缺陷。

Stranger still is that, though general relativity is celebrated and revered among physicists like no other theory in science, they would doubtless react with joy if it is proved to fail. That’s science: You produce a smart idea and then test it to its breaking point.
更为奇怪的是，虽然在物理学家中，广义相对论获得的赞颂和尊崇超过了所有其他科学理论，但如果验证证明它站不住脚，他们无疑会感到欣喜。这就是科学：你提出了一个聪明的想法，然后检验它至极限。
But this determination to expose flaws isn’t really about skepticism, far less wanton nihilism. Most physicists are already convinced that general relativity is not the final word on gravity. That’s because the theory, which is applied mostly at the scale of stars and galaxies, doesn’t mesh with quantum theory, the other cornerstone of modern physics, which describes the ultra-small world of atoms and subatomic particles. It’s suspected that underlying both theories is a theory of quantum gravity, from which general relativity and conventional quantum theory emerge as excellent approximations just as Isaac Newton’s theory of gravity, posed in the late 17th century, works fine except in some extreme situations.
但是揭示该理论缺陷的这种决心，其实无关乎怀疑主义，和肆意的虚无主义更是远远扯不上关系。大多数物理学家已经确信，广义相对论并不是引力的最终定论。这是因为该理论主要应用在恒星和星系的规模，和量子理论没有交集。量子理论是现代物理学的另一块基石，针对的是原子和亚原子粒子级别的微观世界。科学家们觉得，这两个基本理论的依托是一个量子引力理论，广义相对论和常规量子理论是它的绝佳近似值，这就像艾萨克·牛顿在17世纪后期提出的万有引力理论，除某些极端情况外，应用起来通常都没问题。
The hope is, then, that if we can find some dark corner of the universe where general relativity fails, perhaps because the gravitational fields it describes are so enormously strong, we might glimpse what extra ingredient is needed — one that might point the way to a theory of quantum gravity.
科学家的希望是，如果能找到广义相对论站不住脚的一些黑暗角落——这有可能是因为它描述的引力场如此强大——那么我们或许会发现它欠缺了哪些成分，而这可能会指明通向量子引力理论的道路。
General relativity was not just the last of Einstein’s truly magnificent ideas, but arguably the greatest of them. His “annus mirabilis” is usually cited as 1905, when, among other things, he kick-started quantum theory and came up with special relativity, describing the distortion of time and space caused by traveling close to the speed of light. General relativity offered a broader picture, embracing motion that changes speed, such as objects accelerating as they fall in a gravitational field. Einstein explained that gravity can be thought of as curvature induced in the very fabric of time and space by the presence of a mass. This, too, distorts time: Clocks run slower in a strong gravitational field than they do in empty space. That’s one prediction that has now been thoroughly confirmed by the use of extremely accurate clocks on space satellites, and in fact GPS systems have to adjust their clocks to allow for it.
广义相对论不仅仅是爱因斯坦最后一个宏伟想法，而且可以说是他最伟大的构想。他的“奇迹年”通常被认为是1905年，这一年他开始构想量子理论，并提出了狭义相对论，描述了接近光速的运动导致的时空扭曲。广义相对论则描绘了更加广阔的画面，探讨了变速运动，比如物体在进入引力场时出现的加速。根据爱因斯坦解释，引力可以看成是由于质量的存在，时间和空间结构中出现的弯曲。这也扭曲了时间：与没有引力场的空间相比，时钟在一个强大的引力场中走得慢一些。利用在空间卫星上极其精确的时钟，科学家们彻底证实了这个预测的正确性。事实上，GPS系统必须考虑到这种影响，来调整自己的时钟。
Einstein presented his theory of general relativity to the Prussian Academy of Sciences in 1915, though it wasn’t officially published until the following year. The theory also predicted that light rays will be bent by strong gravitational fields. In 1919 the British astronomer Arthur Eddington confirmed that idea by making careful observations of the positions of stars whose light passes close to the sun during a total solar eclipse. The discovery assured Einstein as an international celebrity. When he met Charlie Chaplin in 1931, Chaplin is said to have told Einstein that the crowds cheered them both because everyone understood him and no one understood Einstein.
爱因斯坦1915年向普鲁士科学院(Prussian Academy of Sciences)提交了广义相对论的论文，不过正式发表是在第二年。该理论还预测，强大的引力场会导致光的弯曲。在1919年，英国天文学家亚瑟·爱丁顿(Arthur Eddington)通过仔细观察一次日全食中一些恒星的位置，证实了这一预测，这些恒星的光线会通过临近太阳的区域。爱因斯坦自此成为国际名人。当他在1931年与查理·卓别林(Charlie Chaplin)见面时，据说卓别林对他说，公众为他们两人喝彩，是因为每个人都理解自己的电影，但没有一个人理解爱因斯坦的理论。
General relativity predicts that some burned-out stars will collapse under their own gravity. They might become incredibly dense objects called neutron stars only a few miles across, from which a teaspoon of matter would weigh 10 billion tons. Or they might collapse without limit into a “singularity” — a black hole from whose immense gravitational field not even light can escape, since the surrounding space is so bent that light just turns back on itself.
广义相对论预言，一些燃料耗尽的恒星将因自身引力而崩塌。它们被称为中子星，其密度可能会变得非常之大，直径只有几英里，但一小勺就有100亿吨。或者可能会无限地崩塌下去，变成“奇点”，也就是一个黑洞，其巨大引力场甚至连光都无法逃逸，因为周围的空间太过弯曲，光会直接转弯回到原处。
Many neutron stars have been seen by astronomers: Some, called pulsars, rotate and send out beams of intense radio waves from their magnetic poles, beams that flash on and off with precise regularity. Black holes can only be seen indirectly from the X-rays and other radiation emitted by the hot gas that surrounds and is sucked into them. But astrophysicists are certain that they exist.
自那之后，天文学家发现了很多中子星：有些被称为脉冲星，它们旋转运动，从磁极发射出强烈的电波，发射和停止存在着精准的规律性。黑洞只能通过X射线和热气体散发的其他辐射被间接看到，黑洞被这些热气体包围着，并将它们吸入。但是天体物理学家坚信黑洞是存在的。
While Newton’s theory of gravity is mostly good enough to describe the motions of the solar system, it is around very dense objects like pulsars and black holes that general relativity becomes indispensable. That’s also where it might be possible to test the limits of the theory with astronomical investigations. Last year, astronomers at the National Radio Astronomy Observatory in Charlottesville, Virginia, discovered the first pulsar orbited by two other shrunken stars, called white dwarfs. This situation, with two bodies moving in the gravitational field of a third, should allow one of the central pillars of general relativity, called the strong equivalence principle, to be put to the test by making very detailed measurements of the effects of the white dwarfs on the pulsar’s metronome flashes as they circulate. The team hopes to carry out that study this year.
虽然牛顿的引力理论基本上足以描述太阳系的运动，但对于密度极大的物体，比如脉冲星和黑洞，广义相对论就不可或缺了。这也是用天文研究检验这个理论的局限的地方。去年在弗吉尼亚州夏洛茨维尔，国家射电天文台(National Radio Astronomy Observatory)的天文学家发现了一颗脉冲星，绕着它运动的另外两颗缩小的恒星被称为白矮星，而这一现象是前所未见的。在这种情况下，有两个星体在第三个的引力场中运动，如果在白矮星绕脉冲星运动的时候，非常细致地测量它们对脉冲星电波发射规律的影响，应该可以检验广义相对论的核心支柱之一“强等效原理”。该团队希望今年开展这项研究。
But the highest-profile test of general relativity is the search for gravitational waves. The theory predicts that some astrophysical processes involving very massive bodies, such as supernovae (exploding stars) or pulsars orbited by another star (binary pulsars), should excite ripples in space-time that radiate outwards as waves. The first binary pulsar was discovered in 1974, and we now know the two bodies are getting slowly closer at just the rate expected if they are losing energy by radiating gravitational waves.
但最引人注目的广义相对论检验是对引力波的寻找。该理论预测，一些非常庞大的星体，比如超新星（爆炸的恒星）或者被另一颗恒星围绕盘旋的脉冲星（脉冲双星），和它们有关的天体物理过程应该在时空中激发涟漪，像波一样向外辐射。第一个脉冲双星是在1974年发现的，科学家假设两个星体辐射了引力波，因而损耗了能量，计算出了它们靠拢的速率，我们现在已经知道，它们确实在以这个速率慢慢靠拢。
The real goal, though, is to see such waves directly from the tiny distortions of space that they induce as they ripple past our planet. Gravitational-wave detectors use lasers bouncing off mirrors in two-kilometer-long arms at right angles, like an L, to measure such minuscule contractions or stretches. Two of the several gravitational-wave detectors currently built — the American LIGO, with two observatories in Louisiana and Washington, and the European VIRGO in Italy — have just been upgraded to boost their sensitivity, and both will start searching in 2015. The European Space Agency is also launching a pilot mission for a space-based detector, called LISA Pathfinder, this September.
不过，真正的目标是，当这些波经过我们的星球时，直接从它们导致的微小空间扭曲中看到它们。引力波探测器让激光在长两公里、摆成L形的干涉臂上来回反射，从而对这种微小收缩或扩张进行测量。目前世界上许多台引力波探测器，其中两台——美国的LIGO，在路易斯安那州和华盛顿有两个观察站；以及欧洲的VIRGO，位于意大利——刚刚对灵敏性进行了升级，它们都将在2015年开始寻找引力波。去年9月，欧洲航天局还用太空中的LISA Pathfinder探测器开展了一个试点任务。
If we’re lucky, then, 2015 could be the year we confirm both the virtues and the limits of general relativity. But neither will do much to alter the esteem with which it is regarded. The Austrian-Swiss physicist Wolfgang Pauli called it “probably the most beautiful of all existing theories.” Many physicists (including Einstein himself) believed it not so much because of the experimental tests but because of what they perceived as its elegance and simplicity. Anyone working on quantum gravity knows that it is a very hard act to follow.
幸运的话，2015年就会是我们确认广义相对论优势和局限性的一年。但这不会对它受到的推崇产生太大影响。奥地利-瑞士物理学家沃尔夫冈·泡利(Wolfgang Pauli)称广义相对论“可能是现有理论中最美的”。很多物理学家（包括爱因斯坦本人）相信它，并不是因为它经过了实验的检验，而是因为他们认为它简洁优雅。每个在量子引力领域工作的人都知道，简洁优雅是多么难以达到。