Chapter 8 Black Holes and Spin-of
黑洞和副产品
Black holes don’t just suck
黑洞不只是吸收
If our eyes could observe the sky at radio or at X-ray wavelengths, we would see that some galaxies are straddled by vast balloons or lobes of plasma. This plasma contains charged particles that move at speeds close to the speed of light and radiate powerfully across a range of wavelengths. The plasma lobes exhibited by some of these galaxies (examples of `active galaxies') are created by jets, travelling at speeds so fast that they are comparable with the speed of light, that are squirted out from the immediate surroundings of a black hole, outside its event horizon. Roger Penrose showed in general terms how extraction of the spin energy of a black hole from its ergosphere might be possible in principle. Roger Blandford and Roman Znajek have shown explicitly how the energy stored in a spinning black hole could actually be transferred into electric and magnetic fields and thereby provide the power to produce these relativistic jets of plasma. There are also other explanations for the mechanism by which jets are launched from near black holes. However, which of these is correct is the subject of active and exciting current research.
如果我们的眼睛可以在射电或者X射线波段观察天空，就会看到一些星系被巨大的气球或等离子体波瓣包裹。这些等离子体中含有运动速度接近光速的带电粒子，发出一定波长范围内的辐射。其中一些星系(比如“活动星 系”)所表现出的等离子波瓣是由喷流所产生的，它们是从黑洞事件视界周围喷出来的，运动速度快到可以与光速相提并论。罗杰·彭罗斯概括性地指 出:理论上，从黑洞的能层中提取自转能量是可能的。罗杰·布兰福德 (Roger Blandford)和罗曼·扎纳克(Roman Znajek)明确提出了将旋转黑洞中存储的能量转移到电场和磁场中的方法，从而为产生相对论性等离子体喷流提供动力。从黑洞附近发出喷流的机制也有其他解释，这些解释中哪个才是正确的，正是当前活跃且令人兴奋的研究主题。
Whatever the mechanism(s) turn out to be, these jets are highly focused, collimated flows ejected from the vicinity of the black hole, but of course outside the event horizon. The regions in between galaxies are not, in fact, empty space. Instead they are filled with a very diffuse and dilute gas termed the intergalactic medium. When the jets impinge on the intergalacticmedium, shock waves form within which spectacular particle acceleration occurs, and the energized plasma which originated in a jet from near the black hole billows up and flows out of the immediate shock region. As the plasma expands, it imparts enormous quantities of energy to the intergalacticmedium. There are many instances of these plasma jets extending over millions of light-years. Thus black holes have tremendous cosmic influence,many light years beyond their event horizons. In this chapter, I will describe the influence and interactions of black holes on and with their surroundings.
最终无论哪种机制被证实，这些喷流都是从黑洞附近(当然是在事件视界外)喷出的高度聚焦且准直的射流。实际上，星系之间的区域并不是真空。与此相反，其中弥散着非常稀薄的被称为星际介质的气体。当喷流撞击星际介质时会形成激波，其内部会发生壮观的粒子加速，而被黑洞附近喷流所激发的等离子体，其内部也会发生极其剧烈的运动，从而流出当前的激流区域。 随着等离子体膨胀，它会向星际介质传输大量能量。这些等离子喷流中，有许多会延伸至数百万光年外。因此，黑洞对超过事件视界很多光年的宇宙依然有着巨大影响。在本章中，我将描述黑洞对其周围环境的影响以及与周围环境的相互作用。
As discussed in Chapter 6, at the centre of (probably) most galaxies is a black hole, on to which matter accretes, giving rise to emission of electromagnetic radiation. Such galaxies are called active galaxies. In some of these galaxies, the process of accretion is extremely effective and the resulting emission of radiation extremely luminous. Such galaxies are called quasars (a term which derives from their original identification as `quasi- stellar radio sources', vastly distant, highly luminous points of radio emission).We now understand that quasars are the sites of the most powerful sustained energy release known in the Universe. Quasars radiate energy across all of the electromagnetic spectrum, from long wavelength radio waves, through optical(visual) wavelengths, to X-rays and beyond. The radio lobes, mentioned above, can be especially dramatic because they extend across distances of over hundreds of thousands of light-years (see Figure 19). The energy radiated at radio wavelengths arises from those large lobes-reservoirs of ultra-hot magnetized plasma, powered by jets that transport energy over vast distances in space.Highly energetic electrons (highly energetic here meaning travelling extremely close to the speed of light) experience forces across their direction of travel from the ambient magnetic fields that pervade the plasma lobes within which they are travelling. This acceleration causes them to emit photons of radiation (which may be radio, or in rare, highly energetic instances, at shorter wavelengths still, all the way up to X-rays) known as synchrotron radiation.
如第6章所述，在大多数星系的中心(可能)有一个黑洞，物质会被其吸积，从而产生电磁辐射。这样的星系被称为活动星系。它们其中一些的吸积过程非常有效，产生的辐射光度极高。这样的星系被称为类星体(这个词源于它们最初被识别为“类似恒星的射电源”，因为它们是遥远的亮度很高的射电点光源)。我们现在知道，类星体是宇宙中已知的最强大的持续能量释放场所。类星体辐射的能量跨越整个电磁频谱，从长波射电波到光学(可见光)波段，再到X射线并继续向后。上面提到的射电瓣之所以特别引人注目，是因为它们跨越了数十万光年(见图19)。射电波段辐射的能量来自那些巨大的波瓣——也就是存有超热的磁化等离子体的地方，其能量是由空间上长距离传输能量的喷流所提供的。高能电子(此处高能是指其传播速度非常接近光速)在行经等离子体波瓣时会受到来自遍布其中的磁场所施加的垂直于运动方向的力。这种加速使它们发出被称为同步辐射的光子(可能是射电的，也可能是红外的;或者在极短的波长下高能的情况，也可能是X射线)。
To give a sense of the scale of the power produced by quasars, consider the following values. The LEDs by whose light I am working have a power output of ten watts. They are illuminated by electricity frommy local power station which produces a few billion watts (a billion watts is 109 watts or a gigawatt). The Sun outputs about 4 x 1026 watts, more than ahundred million billion times that from this power station. Our Galaxy, theMilkyWay, contains more than a hundred billion stars, and its power output is approaching1037 watts. But the power produced by a quasar can exceed even the Galactic power output by more than a factor of 100. Remember, this power is being emitted not by a galaxy of one hundred billion stars but by the processes going on around a single black hole. Such radiation could do considerable damage to the health of living creatures here on Earth, so it is just as well for us that there are no examples of such powerful quasars too near our Galaxy!
要了解类星体产生的功率的规模，我们来考虑以下几个典型情况。我用 于工作的LED灯，输出功率是10瓦。它们由输出功率高达数十亿瓦的本地发电站提供的电能所点亮(10亿瓦等于109瓦或1吉瓦)。太阳的输出功率约为 4×1026瓦，是这个发电站功率的10亿亿倍。我们所在的星系，也就是银河系包含超过1000亿颗恒星，其输出功率接近1037瓦。但是类星体产生的功率甚至 可以比银河系的输出功率高100倍以上。请记住，这个功率不是由一个星系或 1000亿颗恒星发出的，而是由单个黑洞周围的能量所产生的。这样的辐射可能会对地球上生物的健康造成极大损害，因此可以说我们非常幸运，因为在距离银河系很近的地方没有这样强大的类星体!
Jets in quasars are thought to persist for a billion years or less, an idea that comes from estimates of the speed at which these objects' jets grow and from measurements of the size they have grown out to. A simple relationship between distance and time and speed therefore gives a guide to the likely durations of jet activity in the quasars that are observed across the cosmos.
人们认为类星体中的喷流可以持续10亿年或更短时间，这个判断来自对这些物体喷流的成长速度的估计，以及对它们长大后的尺寸的测量。因此根据距离、时间和速度之间的简单关系，可以为整个宇宙中类星体中可能观测到的喷流活动的持续时间提供参考。
As these radio-emitting lobes expand, their magnetic fields weaken as do the `internal' energies of the individual electrons in the lobes. These two effects serve to diminish the intensity of the radiation with time and with distance from the black hole; how dramatically this intensity falls off depends on how many highly energized electrons there are compared with how many less energetic ones there are. It's a property of synchrotron radiation that the lower the magnetic field strength is, the more energetic the electrons need to be to produce the radiation at the wavelength that your radio telescope is tuned to receive at. This compounds the diminishing of the synchrotron radiation as the plasma lobes expand into outer space. Not only do the electrons lose energy as the plasma expands, but because the magnetic field strength is weakening, only increasingly energetic electrons are relevant to what is observed by your telescope and, very often, there are vastly fewer of these than there are of the lower energy electrons anyway.As far as radio lobes of quasars are concerned, the lights can go out really quite rapidly.
随着这些射电波瓣的扩张，它们的磁场会减弱，波瓣中各个电子的“内部”能量也随之减弱。这两种效应会让辐射强度随着时间的推移和到黑洞距离的增加而减小，强度下降的幅度取决于其中高能电子与低能电子的相对数量。同步辐射的一个特性是，磁场强度越弱，所需的产生射电望远镜可接收长波的辐射的电子能量就越高。所以当等离子体波瓣扩展到外部空间时，同步辐射也随之减弱。不仅电子会随着等离子体的膨胀损失能量，随着磁场强度的减弱，与被望远镜观测到的现象有关的，只有那些和高能量电子相关的现象，而且通常情况下，这些电子的数量要远少于低能电子。就类星体的射电波瓣而言，光可以在很短时间内就熄灭。