Surveying black holes and neutron stars with gravitational waves

黑洞和中子星是我们宇宙中最奇怪的物体，它们是在质量最大的恒星生命末期的超新星爆炸中形成的。中子星将太阳的质量压缩成一个直径只有几十公里的球体；密度大到原子无法存在，所以整个恒星是由引力作用下的中子组成的。黑洞被认为质量更大，密度更大，以至于光都无法从它们的视界外逃逸。这些迷人的物体是物理学家梦寐以求的实验室：在这里，我们所有的理论都被推向了极限和超越。了解它们是如何形成的，也有助于了解它们所来自的恒星的演化过程。当两个这样的天体在一个双星系统中被发现时，它们的小尺寸允许它们在几个小时或更短的时间内绕对方运行。根据爱因斯坦的广义相对论，这些系统在如此小的空间里有如此多的物质，以至于它们扭曲了周围的时空，产生引力波，从轨道中提取能量，并改变它们所经过的一切物体的尺寸。幸运的是，当它们到达地球时，这些引力波非常微弱，以至于长度的变化相当于太阳和土星之间距离的变化一根头发的宽度，太小了，我们注意不到。最终，大量的能量被提取出来，以至于两个物体在最终碰撞之前每秒绕对方旋转数百次，释放出巨大的能量，如果涉及中子星，就会产生宇宙中最明亮的爆炸，但如果只有黑洞，就完全看不见了。也就是说，不可见，除了引力波天文台之外，引力波天文台是用来测量宇宙中这些事件造成的时空扭曲的。这些天文台的第二代，被称为先进的LIGO和先进的处女座，将在2015年上线，并在不久之后开始探测双星碰撞。通过精确测量引力波经过时产生的扭曲，就有可能测量产生引力波的双星系统的性质，即使在没有电磁信号可见的情况下。引力波实际上形成了一个全新的光谱，我们可以用它来观察宇宙，为发现开辟了新的途径。寻找引力波是一个非常令人兴奋的科学领域。它在设计的每个部分都推动了技术可能的极限。知道我们很快就能观测到黑洞和中子星，激发了我们的好奇心，推动了这项全球努力的向前发展。我们即将开启一种新的工具，它将使我们能够观察时空本身，并最终使我们更好地理解宇宙的本质和演化。

Black holes and neutron stars are some of the strangest objects in our universe, formed in the supernova explosions at the end of the lives of the most massive stars. Neutron stars contain the mass of the Sun compressed into a sphere only a few tens of kilometres across; so dense that atoms can no longer exist, so the entire star consists of neutrons packed together by gravity. Black holes are thought to be more massive still, and so dense that not even light can escape from beyond their event horizons. These fascinating objects are a physicist's dream laboratory: a place where all our theories are pushed to their limits and beyond. Understanding how they are formed can also shed light on the evolution of the stars that they came from.When two of these objects are found in a binary system, their small size allow them to orbit each other in a few hours or less.According to Einstein's general theory of relativity these systems have so much matter in such a small space that they distort the space-time around them, whipping up gravitational waves that extract energy from the orbit and change the dimensions of everything they pass through. Luckily, by the time they reach us at Earth, these gravitational waves are so weak that the changes in length are equivalent to changing the distance between the Sun and Saturn by one hair's width, far too small to notice. Eventually so much energy is extracted that the objects orbit each other hundreds of times a second before they finally collide, releasing a huge amount of energy that can produce the brightest explosions in the universe if a neutron star is involved, but are completely invisible if only black holes are. Invisible that is, except to gravitational wave observatories, which are designed to measure the distortion of space-time produced by these events throughout the universe. The second generation of these observatories, called Advanced LIGO and Advanced Virgo, will come online in 2015 and start to make detections of binary collisions soon afterwards. By precisely measuring the distortions produced as a gravitational wave passes by, it will be possible to measure the nature of the binary system that produced it, even when no electromagnetic signal can be seen. Gravitational waves form in effect an entirely new spectrum with which we can observe the universe, opening up new avenues for discovery.The hunt for these gravitational waves is an incredibly exciting field of science. It pushes the limits of what is technologically possible in every part of its design. Knowing that we will soon be observing black holes and neutron stars fuels the curiosity that drives this global effort forward. We are about to open up a new tool that will allow us to observe space-time itself, and will eventually lead to a better understanding of the nature and evolution of the universe.