Frank Wilczek
弗兰克·维尔切克是世界顶尖科学家协会（WLA）指导委员会委员、麻省理工学院物理学教授、量子色动力学的奠基人之一。因发现了量子色动力学的渐近自由现象，他在2004年获得了诺贝尔物理学奖。
中文版
一系列强大的新天文工具正在揭示银河系中重元素是如何起源的。
最近，科学家着迷于对引力波的观测。这项新兴的天文学研究已经初露锋芒，似乎正在以一种令人惊讶和美妙的方式填补我们在理解元素起源上的巨大空白。新的发现解决了一个长期存在的谜团：那些中子占比最多的重元素，比如金、银和铀等等，是如何形成的？
我们知道，原子核是由质子和中子构成。中子本身并不稳定，它会迅速地衰变，其半衰期不到15分钟。但在原子核内部，中子却是可以稳定存在的 ：与质子和其他中子的相互作用降低了中子的质量并抑制了它们的衰变。
从宇宙大爆炸中产生的所有物质都是由氢（原子核中有一个质子）、氦（两个质子和一个或两个中子）和一些微量杂质组成的。它们的中子数都没有超过质子数。恒星则对其进行了进一步的“核烹饪”：它们把这三种成分组合起来，形成了更大的单元，从而产生了别的元素。
但是普通的恒星很难制造出原子核里中子比质子多得多的元素。例如，银有47个质子和60或62个中子，金有79个质子和118个中子，最稳定的铀则有92个质子和146个中子。如此过剩的中子是如何产生的？又是在哪儿产生的？
我们或许可以在中子星中找到这个问题的答案。中子星是超新星爆炸后的残留物。当质量为太阳好几倍的恒星耗尽了自身的核燃料而死亡时，它们的内部就会发生塌缩，最终成为一个只有地球大小、但质量却超过太阳的物体。实际上，这就是一个靠引力作用结合在一起的巨大原子核。在这种环境中，中子明显比质子轻，因此质子是不稳定的，它会衰变成中子或被喷射出去，直到剩下的绝大多数都是中子，成为名副其实的中子星。
现在我们已经有办法形成巨量的中子了，下一个挑战在于，如何把这些中子偷偷地从中子星带出去。下面是它可能发生的方式。有时候，双子星的两个成员都会演化成中子星。这两颗中子星会绕着彼此旋转，但是轨道会逐渐缩小，辐射出引力波。当它们相互靠近时，会被巨大的潮汐力撕得粉碎。在激烈的碰撞中，那些炽热致密的碎片会被抛出去。这些碎片中包含着丰富的中子，在寒冷的宇宙环境中可以逐渐形成稳定的富含中子的原子核。理论上讲，这种激烈的过程可以产生大量含有众多中子的原子，也就是说，这个过程中能够产生我们今天大部分的金子、银子以及所有的铀。
但在不久之前，这种猜想只不过是一种虚幻飘渺的假想。直到2017年，LIGO（位于美国路易斯安那州和华盛顿州的激光干涉引力波观测站）观测到了两颗中子星合并时爆发的强烈引力辐射，才让科学家有机会进一步验证这个猜想。
此后，一套涉及范围更广的引力波探测器网络上线了，天文学家可以使用不同的“望远镜”来协同观测与引力波相关的现象 ：从射电望远镜到伽马射线卫星，甚至中微子探测器。这些功能不同的设备可以聚焦于一个重点，共同去揭示引力波事件所包含的信息。由此，伴随着“多信使天文学”的兴起，中子星炼金术的奇观将成为一场多媒体盛会。
目前我们能够确认的是，两颗中子星的合并在我们的星系中是极不寻常的事件。在粉碎后，它们的碎片也将散落在星系的各处。或许有的地方留下的碎片可能比留在我们太阳系中的多得多，但也有很多地方留下的碎片要比太阳系中的少得多。这将是一个有趣的研究。
另外，假如地球上有更多或更少的金、银和铀，人类的历史会改写吗？想想这个问题也是蛮有趣的。
英文版
Powerful new astronomical tools are revealing how thegalaxy’s heaviest elements were formed.
Recent work observing gravitational waves seems to fill a big gap in our understanding of the origin of the elements in a surprising and beautiful way. This is the first taste of a promising new kind of astronomy.
The new discoveries address a long-standing mystery: How were the heaviest elements, with the biggest share of neutrons—such as gold, silver and uranium—formed? Atomic nuclei are made from protons and neutrons. Neutrons on their own are unstable and decay quickly, with a half-life of less than 15 minutes. But inside nuclei, neutrons can be stabilized; interactions with protons and other neutrons lower their mass and limit their decay.
Coming out of the Big Bang, all matter consisted of hydrogen (with one proton in its nucleus) and helium (with two protons and either one or two neutrons), plus some trace impurities. There was no material in which neutrons outnumbered protons. Stars take nuclear cooking further: Basically, they combine those three ingredients into larger units, producing other elements.
But normal stars have a tough time producing elements whose nuclei contain far more neutrons than protons. Silver, for example, has 47 protons and 60 or 62 neutrons, gold has 79 protons and 118 neutrons, and uranium in its most stable form has 92 protons and 146 neutrons. How and where can such an overabundance of neutrons arise?
Neutron stars, the burnt-out remnants of supernova explosions, are a likely suspect. When stars whose mass is a few times that of our sun run out of nuclear fuel and “die,” their cores collapse. The end result is an approximately Earth-sized body whose mass exceeds that of the Sun. It is, in effect, a single gigantic nucleus, held together by gravity. In this environment, neutrons are significantly lighter than protons, so it is protons that are unstable. The protons decay into neutrons or are ejected, until there is an overwhelming preponderance of neutrons in the remnant, aptly called a neutron star.
The next challenge is to smuggle some neutrons out. Here’s how it mighth appen. Sometimes both members of a binary star system evolve into neutron stars. The neutron stars then revolve around one another. The orbit decays, however, releasing gravitational radiation. As the neutron stars come close together, gigantic tidal forces rip them apart, and the shattered remnants collide. In these violent encounters fragments of hot, dense material that is still neutron-rich are thrown off and then settle into stable, neutron-rich nuclei. Theoretically, this tempestuous process could generate most of our silver and gold and all of our uranium.
Until recently, that scenario was extremely hypothetical. But in 2017, LIGO (the Laser Interferometric Gravitational-Wave Observatory, based in Louisiana and Washington State) caught two neutron stars in the act, by observing the tell-tale burst of intense gravitational radiation at their merger.
As a more extensive network of gravitational wave detectors comes online, astronomers using many kinds of “telescopes”—from radio dishes togamma-ray satellites, and even neutrino detectors—will be able to focus on the events they reveal. With this “multi-messenger astronomy,” the spectacle of neutron star alchemy will become a multimedia extravaganza.
Since mergers of two neutron stars are unusual events in our galaxy, different areas might contain more or less of their debris than our solar system. This will be interesting to investigate.
In the meantime, it’s entertaining to consider how human history would have been different if Earth contained a lot more—or a lot less—silver, gold and uranium.
编辑：黄琦
作者 | Frank Wilczek
翻译 | 胡风、梁丁当