引力波新发现令天文学家雀跃不已

这可能揭开巨型黑洞甚至宇宙起源的面纱

It could reveal giant black holes—or the beginnings of the universe

二〇一七年的诺贝尔物理学奖表彰了科学家对101年前的一个预测的证实。爱因斯坦的狭义和广义相对论彻底改变了科学家对恒星和星系尺度上的物理规律的理解，他在1916年预言，在某些情况下，宇宙结构本身会摆动和弯曲。

THE 2017 Nobel prize for physics was given for the confirmation of a prediction made 101 years earlier. In 1916 Albert Einstein, whose theories of special and general relativity revolutionised scientists’ understanding of physics at the scale of stars and galaxies, predicted that, in certain circumstances, the fabric of the universe itself should wobble and flex.

起因就是引力波，它和引力的关系相当于无线电波或可见光之于电磁辐射。2015年，科学家首次直接观测到引力波。美国的激光干涉引力波天文台（以下简称LIGO，在美国西北的华盛顿州和东南的路易斯安那州设有观测站）检测到一对质量各约为太阳的30倍的黑洞相互碰撞产生的波。这在时空中形成了频率约150赫兹（即每秒周期数）的涟漪，波长约2000公里。

The culprits are gravitational waves, which are to gravity as radio waves or visible light are to electromagnetism. In 2015 gravitational waves were directly observed for the first time. LIGO, an American observatory based in Washington state in that country’s north-west, and Louisiana in the south-east, detected waves produced by a pair of colliding black holes, each about 30 times the mass of the sun. That produced ripples in spacetime with a frequency of about 150Hz, or cycles per second, and a wavelength of around 2,000km.

这一发现标志着引力波天文学时代开启，该学科通过引力研究宇宙，就像传统天文学利用来自可见光、无线电波和伽马射线的电磁辐射一样。6月29日，由美国、澳大利亚、中国和欧洲的研究人员主持的四个科研合作团队声称这门新兴研究取得了重大突破。他们宣布初步探测到新的超低频引力波，可能为了解宇宙中最深不可测的一些奥秘带来启示。

This detection marked the beginning of the era of gravitational-wave astronomy, which uses gravity to examine the universe in the same way that conventional astronomy uses electromagnetic radiation, from visible light to radio waves and gamma rays. On June 29th four collaborations led by researchers in America, Australia, China and Europe claimed to have pushed forward the state of that emerging art. They announced the tentative detection of new, ultra-low frequency gravitational waves which could offer insights into some of the hardest-to-study bits of the universe.

引力波探测器主要是干涉仪。这些仪器的工作原理是把一束光一分为二，分别射入一对互相垂直的测量长臂中，在末端被反射回源头，重新合并。如果在这一过程中没有受到干扰，返回的光束在重新合并时将相互抵消。如果没有相消，就表明两者在光路中受到了某种干扰，有时不过是轻微的地震震颤，但偶尔会是行经的引力波。

Most gravitational-wave detectors are interferometers. These work by splitting a beam of light in two, and sending each half down one of a pair of long, perpendicular arms. At the end of the arms the light pulses are reflected back towards the source, where they are recombined. If that journey is uninterrupted, the returning beams will cancel each other out when they are put back together. If they do not, then that suggests some disturbance—sometimes a mere seismic tremor, but occasionally a passing gravitational wave—has disturbed them on their journey.

狩猎大目标

Big-game hunters

“狩猎”引力波需要大型仪器。LIGO的测量臂有四公里长；位于欧洲的室女座干涉仪（Virgo）的测量臂长三公里。想探测的引力波越低频，所需的探测仪就越大。例如，要探测到频率在一赫兹左右的引力波，探测仪得要比地球还大。所以欧洲航天局正在建造名为激光干涉仪空间天线（LISA）的航天器，计划于2030年代末升空，它将用一系列空间激光器和镜子组合而成一条长达250万公里的测量“臂”。

Hunting for gravitational waves requires big instruments. LIGO’s arms are 4km long; those at Virgo, a European instrument, span 3km. And the lower the frequency of the waves you want to detect, the bigger you have to go. Waves with a frequency around 1Hz, for instance, require detectors bigger than Earth itself. That is why the European Space Agency is building a spacecraft called LISA, due to fly in the late 2030s. It will use a system of space-going lasers and mirrors to create “arms” that are 2.5m kilometres long.

但最新的突破涉及的是频率接近纳米赫兹的引力波，比上述频率还要低几十亿倍。为探测这些引力波，天文学家必须依靠大自然创造的光束，也就是由脉冲星发射的光脉冲。脉冲星是已坍塌并高速旋转的恒星，会像节拍器般规律地发射脉冲。如果有引力波经过，扭曲了脉冲星和地球之间的时空，部分光脉冲就会早于或晚于预期到达。监测脉冲星群就相当于创造出测量臂跨越星际的干涉仪。

But the latest result concerns waves with frequencies in the nanohertz range, billions of times lower still. To detect those, astronomers must rely on light pulses created by Mother Nature—specifically, by pulsars. These are collapsed, spinning stars that emit flashes of light with metronomic regularity. If a passing gravitational wave distorts a region of spacetime between the pulsar and Earth, then some pulses would arrive earlier or later than expected. Monitoring groups of pulsars can create, in effect, interferometers with arms of interstellar size.

现在已探测到了超低频扭曲。这殊为不易，需要极大的耐心等待来自各种天文台的观测结果历经多年涓滴成河。此次发布的研究包含的部分数据收集于25年前。

And ultra-low frequency distortions have now been spotted. Doing so was not easy. A great deal of patience was required, as results from the various observatories trickled in over the years. Some of the data included in this week’s research were collected over 25 years ago.

参与合作的四个科研团队都不认为自己已有足够的证据一锤定音。物理学家用“西格玛”（sigma）这一统计术语来衡量一个结果的显著性。黄金标准为五西格玛，即研究结果实为偶然产物的概率约为350万分之一。单独来看，这四个团队探测结果的西格玛值分别在2到4.6之间。但把它们的数据综合到一起，可能会在一年内跨过五西格玛的大关。“这真的只是时间的问题，我毫不怀疑这一点。”英国卡迪夫大学的天体物理学家维维安·雷蒙德（Vivien Raymond）说。他本人没有参与这项工作。

None of the collaborations believe they have quite enough evidence for a conclusive discovery just yet. Physicists measure the significance of a result using a statistical term called sigma. A score of 5, the gold standard, indicates a roughly 1-in-3.5m chance that what seems like a result is instead the product of chance. Individually, the four detections have sigma values between 2 and 4.6. But combining their data could take them over the 5-sigma mark within a year. “I have no doubt it’s really just a matter of time,” says Vivien Raymond, an astrophysicist at Cardiff University, who was not involved in the work.

米兰比可卡大学（University of Milano Bicocca）的阿尔贝托·塞扎纳（Alberto Sesana）表示，波源最可能是一对超大质量黑洞，每一个的质量都相当于太阳的数百万倍。它们最常出现在星系的中心，被认为是在这些星系碰撞和合并时配对的。据预测，在几十亿年里，这种配对频繁发生，在宇宙中形成一种引力波的“嗡嗡”背景声。然而，塞扎纳说，“这将是第一个观察证据，证明超大质量黑洞对在自然界确实存在。”

The most likely source of the waves, says Alberto Sesana at the University of Milano Bicocca, are pairs of supermassive black holes, each with a mass millions of times that of the sun. They are most commonly found at the centres of galaxies, and are thought to pair off when those galaxies collide and merge. Over billions of years, such pairings are predicted to be frequent, producing a background gravitational “hum” across the sky. Still, says Dr Sesana, “this would be the first observational proof that supermassive black hole binaries do indeed occur in nature.”

还有另一种可能，几率小得多，但也令人兴奋得多。也许可以想象一下，探测到的新信号可能是人类有史以来首次窥见宇宙最早的历史，当时名为宇宙暴胀（宇宙在短时间内迅速膨胀）的现象可能令时空产生了嗡鸣。

There is another possibility, much less likely but far more exciting. It is just about conceivable that the new signal could be the first-ever glimpse of the universe’s earliest history, when a phenomenon known as inflation—in which the size of the universe briefly increased rapidly—would have set spacetime ringing.

假如探测到的真是这种现象，很难想象还有什么东西能以更激动人心的方式展现引力天文学的力量。在诞生最初的38万年里，宇宙的温度和密度极高，被认为是电磁辐射无法穿透的。这意味着一般的天文望远镜（都靠探测各种波长的光）都无法探测到在此之前发生的任何事情的痕迹。引力望远镜则不受此限制。关注这个领域吧。或者说，关注这个时空吧。

If that is indeed what has been detected, it is hard to think of a more dramatic demonstration of the power of gravitational astronomy. Because it was so hot and dense, the universe is thought to have been opaque to electromagnetic radiation for the first 380,000 years of its existence. That means that no standard telescope (all of which depend on detecting light of various wavelengths) can detect traces of anything that happened before that. That is not a limit to which gravitational telescopes are subject. Watch this space. Or spacetime. ■