Chapter 6 How Do You Weigh a Black Hole?

你怎样给黑洞称重

The Sun, the planets that orbit around it, together with dwarf planets (of which Pluto is the most famous example), asteroids, and comets collectively comprise the Solar System. The Solar System itself orbits within the disc of our Galaxy around its centre of mass at the Galactic Centre. The speed at which our Solar System travels around its circular path through the Galactic disc is about 7 km/s, and to complete an entire circuit around the Galactic Centre will take a couple of hundred million years. In addition to this orbital motion, the whole Solar System moves perpendicular to the Galactic plane. The kind of motion it exhibits is well known to physicists as simple harmonic motion with the restoring force, which pulls our Solar System back towards the equilibrium position of the plane of the Galaxy, coming from the gravitational pull of the stars and gas that comprise the Galactic disc. At the moment, we are about 45 light-years above this equilibrium point. In about 21 million years from now the Solar System will be at its extreme point 320 light-years above the Galactic plane. 43 million years after that, the Solar System will be back in the mid-plane of the Galaxy.

太阳以及围绕它运行的行星、矮行星(其中冥王星是最著名的例子)、小行星和彗星共同组成了太阳系。太阳系本身在银盘上绕着在 的质心转 动。我们的太阳系在银盘中的圆轨道上绕转的速度大约是7千米/秒，绕着银心转完一整圈需要几亿年。除了这种轨道运动之外，整个太阳系还垂直于银道面运动。它所表现出的这种运动是物理学家所熟知的简谐运动，而把我们的太阳系拉回到位于银道面上的平衡位置的回复力，则来自构成银盘的恒星和气体的引力。目前我们在这个平衡点上方约45光年的地方。从现在起大约 2100万年后，太阳系将到达银盘上方320光年处的极值点;在此之后再过4300 万年，太阳系将重新回到银河系的中心平面。

When the Solar System lies in the centre of the Galactic plane then, the Earth will suffer maximum exposure to the cosmic rays that are whizzing around in the plane of the Galaxy, trapped along lines of magnetic field, and travelling around them on some kind of a cross between a helter-skelter and a tramline. There have been speculations that the Sun's motion through the Galactic plane could have been responsible for the mass extinction of dinosaurs. But this kind of speculation is hard to verify or refute because the timescales for this orbital motion are of course rather tricky for human observers, who don't tend to live longer than one century. This is a common problem in observational astronomy when we want to follow some process that changes on timescales much longer than the few centuries over which we've been making astronomical observations of any reasonable accuracy and thoroughness. 

当太阳系位于银道面的中心时，地球将最大限度地暴露在宇宙射线中。这些宇宙射线会在银道面上呼啸着转圈，它们被磁力线所俘获，以介于完全杂乱无章和完全有序之间的某种方式运动—— 一边沿着磁力线前进，一边绕着磁力线旋转。有人猜测可能是由于太阳穿过银道面的运动导致了恐龙灭绝。但是这种推测很难被证实或反驳，因为这种轨道运动的时标对于寿命通常不会超过一个世纪的人类观测者来说显然是难以观察的。人们采用足够精确和彻底的手段进行天文学观测也只有几个世纪。因此，在观测天文学中，当我们想要关注某种以比这还长得多的时标变化的过程时，这是一个很常见的问题。 

There are, however, orbital motions within the Galaxy that are significantly easier to measure, at least in the sense that the relevant timescales are commensurate with the attention spans of humans and their telescopes. Of particular interest in the context of black holes are the orbital motions of the stars in the innermost regions of theMilkyWay, that appears in a part of the sky known as SagittariusA*. Looking into this region, most easily seen from the southern hemisphere, one is looking towards the very centre of our own Galaxy, 27,000 light-years away from us. This is a particularly densely populated region of space, which leads us to two problems when we want to study the Galactic Centre. The first is that there is a relatively high space density of stars and the second is that there is lots of dust. 

然而，至少在相关的时标与人类及其望远镜所关注的时间尺度差不多的情况下，银河系中的轨道运动非常容易测量。既然我们在讨论黑洞，那么最令人感兴趣的显然是银河系最内部区域中恒星的轨道运动，这一区域位于天 空中被称为人马座A*的那一部分。当我们观察这个在南半球最容易看到的区域时，也是在看向距我们27 000光年远的银河系的正中心。这是一个天体特别稠密的空间区域，而当我们想研究银心时，会导致两个问题。首先是恒星的空间密度较高，其次是尘埃很多。 

The first problem means you need to use a measurement technique that enables high resolution imaging, i.e. fine details can be separated from one another in the way that a telephoto lens gives finer detail on a given camera than a wide-angle lens does.Just using a larger telescope is invariably insufficient for this, but there are various techniques developed for untangling the turbulence in Earth's atmosphere through which we inevitably view all celestial objects, unless we put the telescope on a satellite above the atmosphere. Of particular importance is a technique known as adaptive optics. This technique corrects for atmospheric variations by observing the blurring of a bright star (called a guide star) and deforming the primary mirror of the telescope to cancel out this varying blurring. When a bright star isn't available in the part of sky that is of interest, a powerful collimated laser beam can be shone up to excite atoms in the atmosphere and the atmospheric corrections derived from that. 

第一个问题意味着你需要使用一种能够实现高分辨率成像的测量技术， 也就是精细的细节可以被区分开，就像在给定的相机上长焦镜头提供的细节 比广角镜头提供的细节更精细一样。仅仅使用更大的望远镜肯定不足以解决这个问题，因为除非我们把望远镜放在大气层外的卫星上，否则我们将不可避免地通过具有湍流的大气来观察所有的天体。不过，人们已经开发出了各种各样的技术来消除地球大气中湍流的影响。最为重要的是一种被称为自适应光学的技术。这种技术的工作原理是观察明亮恒星(被称为导星)模糊的图 像，通过使望远镜的主镜变形以抵消这种变化着的使图像模糊的效应，从而校正大气变化的影响。当所感兴趣的天空中没有明亮的恒星时，则可以向上发出高功率的准直激光束，以激发大气中的原子，并由此进行大气校正。