Chapter 7 Eating More and Growing Bigger

吃得更多，长得更大

How fast do they eat?

它们吃得有多快

The popular notion of a black hole `sucking in everything' from its surroundings is only correct near the event horizon, and even then, only if the angular momentum of the infalling matter isn't too great. Far away from the black hole, the external gravitational field is identical to that of any other spherical body having the same mass. Therefore, a particle can orbit around a black hole in accordance with Newtonian dynamics, just as it would around any other star.What could unravel this pattern of going round and round in circles (or indeed ellipses) and pave the way for more exotic behaviour? The answer is that there is invariably more than one particle orbiting the black hole. The richness of the astrophysical phenomena we observe arises because there is a lot of matter orbiting around a black hole and this matter can interact with itself.What is more, gravity isn't the only law of physics that must be obeyed: so too must the law of conservation of angular momentum. Applying these laws to the bulk quantities of matter that may be attracted towards the black hole gives rise to remarkable observable phenomena, good examples of which are found in the case of exotic objects known as quasars. Quasars are objects at the centres of galaxies having a supermassive black hole at their very heart which, because of its effect on nearby matter,can cause it to outshine the collective light from all the stars in one of those galaxies, across all parts of the electromagnetic spectrum.We shall meet quasars, and other examples of `active galaxies', in Chapter 8, together with scaled-down counterparts of these called microquasars whose black holes are orders of magnitude less massive than those inside quasars. For now let's get back to thinking about the matter around a black hole. 

黑洞会“吞噬一切其周围物质”的流行观点，只在事件视界附近成立， 并且坠入物质的角动量还不能太大。在远离黑洞的地方，外部的引力场与质量相同的任何其他球形物体的引力场相同。因此，一个粒子可以按照牛顿动力学，像绕着其他恒星一样，绕着黑洞公转。是什么打破了粒子绕着圆圈(实际上是椭圆)不停转下去的模式，而按照更奇特的轨迹运行呢?答案是，总是有不止一个粒子在绕着黑洞转。我们观察到的天体物理学现象之所以丰富多彩，是因为有许多物质在黑洞周围绕转，这些物质之间可以发生相互作用。 此外，引力并不是唯一的必须遵守的物理定律:角动量守恒定律也必须成立。将这些定律应用于可能被黑洞吸引的大量物质，会引发显著的可观测现象，被称为类星体的奇异天体就是一个很好的例子。 就是核心有一个超大质量黑洞的星系中心的天体，它对自身附近物质有影响，这种影响，使它在整个电磁波谱发出的光甚至比某些星系中所有恒星还要亮。我们将在第8章中讨论类星体和其他类型的“活动星系”，还有缩小版的微类星体——它们的黑洞要比类星体内的黑洞质量小几个量级。现在，让我们回过头来继续探究黑洞周围的物质。 

As we have noted, you cannot directly observe an isolated black hole because it simply won't emit light; you can only detect a black hole by its interactions with other material. Any matter falling towards a black hole gains kinetic energy and by turbulence, that is to say swirling against other infalling matter doing a similar thing, becomes hot. This heating ionizes the atoms leading to the emission of electromagnetic radiation. Thus, it is the interaction of the black hole on the nearby matter that leads to radiation being emitted from the vicinity of the black hole, rather than direct radiation from the black hole itself. 

正如我们所看到的，你没有办法直接观测一个孤立的黑洞，因为它根本不会发光，你只能通过黑洞与其他物质的相互作用来探测它。任何落入黑洞 的物质都将获得动能，并且与其他下落物质一起形成旋涡。这个旋涡被称为湍流，通过湍流的物质会被加热，而这种加热会使原子电离，发出电磁辐射。因此，黑洞对附近物质的作用会导致黑洞周围发出辐射，而黑洞本身不会辐射。

Black holes are not aloof, non-interacting entities in space. Their gravitational fields attract all matter, whether nearby gas or stars, towards them. Because gravitational attraction increases strongly with proximity, stars are ripped apart if they are unfortunate enough to have a close encounter with a black hole; an example is pictured in Figure 15. A certain fraction of the attracted matter will be entirely swallowed or accreted by the black hole. Matter doesn't just accelerate into the black hole whooshing through the event horizon. Rather, there is something of an elaborate courtship ritual as the gravitationally- attractedmatter draws near the black hole. Very often it is found that a particular geometry characterizes accreting matter: that of a disc. If the gravitational field were spherically symmetric, the black hole would play no role in determining the plane within which the gas would settle to form an accretion disc-the disc plane would be determined by the nature of the gas flow far from the black hole. If, however, the black hole has spin, accreted gas will eventually settle into the plane perpendicular to its spin axis, regardless of how it flows at large radii. If there is any rotation at all in the attracted matter,then this must be thought of in terms of the conservation of angular momentum that we met in Chapter 3 when we considered the rotation of material that ultimately collapsed to form a spinning black hole. The rotation means that the matter will be following (fairly circular but actually) spiralling-in orbits as it loses energy. Close to the black hole, the Lense-Thirring effect that we met in Chapter 3 means that at small radii the accretion disc may become aligned with the equatorial plane of the spinning black hole. (In this context, this effect is known as the Bardeen-Petterson effect.)

黑洞不是在太空中孤立的、没有相互作用的实体。它们的引力场会将所有物质吸过去，无论是附近的气体还是恒星。由于引力随着距离的缩短而急剧增加，如果恒星不幸与黑洞发生了近距离接触，它们就会被撕裂。图15就是一个例子。被吸过去的物质中有一部分将被黑洞完全吞噬或 。物质不会只是加速冲向黑洞，并飞快地穿过事件视界。相反，在引力吸引物质并使其靠近黑洞时，会有一些精心设计的“求爱仪式”。人们发现，吸积物质会具有特殊的几何形状——通常是盘状。如果引力场是成球状对称的，黑洞将无法决定气体沉积到哪个平面上形成吸积盘——吸积盘的平面位置将由远离黑洞的气体流的性质决定。但是，如果黑洞具有自转，那么无论在半径较大 的地方气体如何流动，物质最终都会沉积到垂直于其自转轴的平面上。如果被吸引的物质根本没有旋转，那么就必须考虑在第3章中当我们讨论最终会坍缩成黑洞的物质的转动时所提到的角动量守恒。旋转意味着物质在失去能量时将沿着非常圆但实际上是螺旋状的轨道向内运动。在黑洞附近，我们在第3 章中提到的伦泽-蒂林效应则意味着，在半径较小的地方，吸积盘可能会与旋转黑洞的赤道面一致(这个论点中，此效应称为巴丁-彼得森效应)。 

If gas is a significant component of the collapsing matter then gas atoms can collide with other gas particles on their own orbits and these collisions result in electrons in those atoms being excited to higher energy states. When these electrons fall back to lower energy states they release photons whose energies are precisely the difference between the higher energy level of the electron and the lower energy level it has fallen to. The release of photons means that radiative energy leaves the collapsing gas cloud and so this loses energy.While energy is released in these processes, bulk angular momentum is not. Because angular momentum remains in the system, the coalescing matter continues to rotate in whatever plane conserves the direction of the original net angular momentum.Thus, the attracted matter will invariably form an accretion disc:a rather long-lived holding pattern formaterial orbiting the black hole. Depending on just how close to the black hole the orbiting material can get, the matter can get so hot that the radiation emitted from the accretion disc actually comprises X- ray photons, corresponding to high temperatures of ten million degrees (it doesn’t matter too much whether the Kelvin or Celsius temperature scale is being used when the temperatures are quite this hot!). 

如果气体是坍缩物质的重要组分，那么气体原子就可以与位于其所在轨道上的其他气体原子发生碰撞，而这些碰撞会导致那些原子中的电子被激发到更高的能级。当这些电子回落更低的能级时，它们所释放的光子能量恰好是电子所在的较高能级与较低能级间的能量之差。释放出光子就产生了辐射能，这意味着坍缩中的气体云损失了能量。尽管能量被释放了出来，但整体的角动量保持不变。因为角动量依然留存在系统中，所以坍缩中的物质仍然会在某个平面上保持初始净角动量的方向旋转。因此，被吸引的物质将总会形成一个吸积盘—— 一种可以维持很长时间的物质绕着黑洞运转的结构。由于绕转的物质可以离黑洞非常近，物质实际的热度可能达到令吸积盘所发出的辐射包含X射线光子，温度几乎等于1000万度(温度这么高的时候，使用开氏温标还是摄氏温标并没有太大关系)。 

A simple analysis of some familiar equations fromNewtonian physics shows that the gravitational energy release for a given amount of infalling mass depends on the ratio of its mass multiplied by that of the black hole it is spiralling towards, and how close to the black hole the infalling mass gets. For a given mass of attractor such as a black hole, the closer the infalling mass approaches it, the greater the gravitational potential energy released as can be seen in the cartoon in Figure 16. The energy that is available to be radiated out is the difference between the energy the infalling mass has far away before it is accelerated(calculated using Einstein's famous formula E = mc2, where E is energy, mis mass, and c is the speed of light) and the energy it has at the innermost stable circular orbit of the black hole. 

对牛顿物理学中一些熟悉的方程的简单分析表明，给定质量的下落物质所释放出的引力能，取决于其质量与它旋转落入的黑洞质量的乘积，以及下落物质最终距离黑洞的远近。如图16所示，对于给定质量的，类似黑洞这样产生引力的物体，下落的物质离它越近，释放出的引力势能就越大。可辐射出的能量是下落物质在加速之前位于远处时的能量(使用爱因斯坦著名的公式 E=mc2计算，其中E是能量，m是质量，c是光速)与它在最内稳定圆轨道上的能量之差。 

Although fusion holds great hope as a future source of energy for Earth, it can only yield at most 0.7% of the available `E = mc2' energy. In contrast, significantly more of the available rest mass can be released as energy from accreting material, via electromagnetic or other radiation. Quite how close to a black hole the accreting material can get depends, as described in Chapter 4,on how fast the black hole is spinning. If the black hole is spinning fast, the holding pattern of the material can be orbiting much closer in, on much smaller orbits. In fact, accretion of mass onto a spinning black hole is the most efficient way known of using mass to get energy. This process is thought to be the mechanism by which quasars are fuelled. Quasars are the sites of the most powerful sustained energy release in the Universe and are discussed further in Chapter 8. 

尽管聚变是地球未来能源的巨大希望，但它最多只能产生可用能量(由 E=mc2计算出)的0.7%。相比之下，可用的静止质量的更多部分，则可以通过 电磁辐射或其他辐射从吸积物质中以能量的形式释放出来。如第4章中所述， 吸积物质能够到达离黑洞多近的地方，取决于黑洞的旋转速度。如果黑洞旋转很快，则物质就可以维持在更小(或者说距离黑洞更近的)轨道上绕转的模 式。事实上，将物质吸积到旋转的黑洞上，是用质量换取能量的最有效方 法。人们认为正是这个过程为类星体提供了燃料。类星体是宇宙中最强大的持续释放能量的场所，我们将在第8章中进一步讨论这个问题。 

I've already mentioned there is an equivalence between mass and energy and for a Schwarzschild (non-rotating) black hole, an amount of energy equivalent to 6% of its original mass could in principle be liberated, and that Roy Kerr's solutions to the Einstein field equations show that the last stable circular orbit has a much smaller radius from the spinning black hole than would a non-rotating black hole of the same mass. In principle, vastly more rotational energy can be extracted from a Kerr black hole, but only if the infalling matter is orbiting in the same sense as the black hole itself. If matter is orbiting in the opposite direction to the way the black hole is spinning, i.e. it is on a retrograde orbit, then not quite 4% of the rest energy could be released as electromagnetic radiation. If, however, the matter infalling towards a maximally spinning black hole were orbiting in the same sense as the black hole were spinning, then in principle a remarkable 42% of the rest energy could be released as radiation, if the matter could lose sufficient angular momentum that it could orbit the black hole as close as the innermost stable prograde circular orbit. 

我已经提到过质量和能量之间是等效的，并且对于史瓦西(无旋转)黑洞来说，原则上可以释放相当于其初始质量6%的能量。罗伊·克尔找出的爱因斯坦场方程的解表明:旋转黑洞的最内稳定圆轨道的半径比相同质量的无旋转黑洞小得多。原则上可以从克尔黑洞中提取更多的转动能，但前提是下落的物质按照与黑洞本身相同的方向转动。如果物质按照与黑洞自转方向相反的方向转动，也就是说它处于逆行轨道上，那么就只有不到4%的静止能量会以电磁辐射的形式被释放出来。假如物质坠入一个以最大限度自转的黑洞， 并且自转方向与黑洞自转的方向相同，那么原则上如果该物质能够损失足够多的角动量，并且能够在顺行的最内稳定圆轨道上绕转，将有多达42%的静止能量能以辐射形式被释放出来。