Chapter 1 What Is a Black Hole?
黑洞是什么？
A black hole is a region of spacewhere the force of gravity is so strong that nothing, not even light, can travel fast enough to escape from its interior. Although they were first conceived in the fertile imaginations of theoretical physicists, black holes have now been identified in the Universe in their hundreds and accounted for in their millions. Although invisible, these objects interact with, and can thus influence, their surroundings in a way that can be highly detectable. Exactly what the nature of that interaction is depends on proximity relative to the black hole: too close and there is no escape, but further afield some dramatic and spectacular phenomena will play out.
黑洞就是一个引力很强的空间区域。任何东西——甚至连光——都因为不够快，不能从其内部逃离。虽然这一概念最初是在理论物理学家丰富的想象中被构思出来的，但现在我们已经在宇宙中发现了数百个黑洞，并且它们以百万计。尽管这些黑洞是不可见的，但它们以一种很容易被探测到的方式与周围环境相互作用，并对其产生影响。确切地说，这种相互作用的性质取决于相对黑洞的距离:太近的话是不能逃脱的，但更远的地方就会出现一些戏剧性的壮观现象。
The term `black hole' was first mentioned in print in an article by Ann Ewing in 1964, reporting on a symposium held in Texas in1963, although she never mentioned who coined the expression.In 1967, American physicist JohnWheeler needed a shorthand for`gravitationally completely collapsed star' and began to popularize the term, though the concept of a collapsed star was developed by fellow Americans Robert Oppenheimer and Hartland Snyder back in 1939. In fact, the mathematical foundations of the modern picture of black holes began rather earlier in 1915, with German physicist Karl Schwarzschild solving some important equations of Einstein's (known as the field equations in his General Theory of Relativity) for the case of an isolated non- rotatingmass in space.
1964年，安·尤因(Ann Ewing)在一篇报道1963年于得克萨斯州举办的一 个研讨会的文章中首次提到了“黑洞”一词，然而她从未说明是谁发明了这个词。1967年，美国物理学家约翰·惠勒(John Wheeler)需要一个词作 为“引力坍缩彻底的恒星”的简写，于是开始推广这个术语——不过坍缩的恒星这一概念早在1939年就由他的美国同事罗伯特·奥本海默(Robert Oppenheimer)和哈特兰·斯奈德(Hartland Snyder)提出来了。事实上，关于现代黑洞概念的数学基础在1915年就已经诞生了。德国物理学家卡尔·史瓦 西(Karl Schwarzschild)在空间中有孤立无转动的质量的条件下解出了爱因斯坦的重要方程(在他的广义相对论中被称为场方程)。
Two decades later in the UK, a little before Oppenheimer and Snyder's work, Sir Arthur Eddington had worked out some of the relevant mathematics in the context of investigating work by the Indian physicist Subrahmanyan Chandrasekhar on what happens to stars when they die. The physical implications of Eddington's calculations, namely the collapse of massive stars when they have used up all their fuel to form black holes, Eddington himself pronounced to the Royal Astronomical Society in 1935 as being‘absurd'. Despite the apparent absurdity of the notion, black holes are very much part of physical reality throughout our Galaxy and across the Universe. Further advances were made in the United States by David Finkelstein in 1958, who established the existence of a one-way surface surrounding a black hole whose significance for what we shall study in the coming chapters is immense. The existence of this surface doesn't allow light itself to break free from the powerful gravitational attraction within and is the reason why a black hole is black. To begin to understand howthis behaviour might arise we need to first understand a profound feature of the physical world: there is a maximum speed at which any particle or any object can travel.
在此之后过了20年，印度物理学家苏布拉马尼扬·钱德拉塞卡 (Subrahmanyan Chandrasekhar)研究了恒星死亡时会发生什么。以此为基础，英国的亚瑟·爱丁顿爵士(Sir Arthur Eddington)解决了一些相关的数学问题——比奥本海默和斯奈德的工作稍早一点。爱丁顿的计算表明，当大 质量恒星耗尽所有燃料时会坍缩形成黑洞，不过爱丁顿自己在1935年向皇家天文学会宣称其物理含义是“荒谬的”。尽管这个概念看起来荒谬，但黑洞无疑是我们的银河系乃至整个宇宙物理现实的重要组成部分。1958年，美国的大卫·芬克尔斯坦(David Finkelstein)取得了更进一步的进展，他明确了黑洞周围存在一个单向表面。这对于我们将在下一章中讨论的内容具有重要的意义。这个表面的存在不允许光从黑洞内部强大的引力中脱离，而这也是黑洞是黑色的原因。要理解这种现象是如何产生的，我们首先要理解物理世界的一个深刻的特性:任何运动的粒子或物体都存在一个最大速度。
How fast is fast?
快多有是快?
A lawof the jungle is that if youwant to escape a predator you need to run fast. Unless you have exceptional cunning or camouflage, you will only survive if you are swift. The maximum speed with which a mammal can escape an unpleasant situation depends on complex biochemical relationships between mass,muscle strength, and metabolism. The maximum speed with which the most rapidly travelling entity in the Universe can travel is that exhibited by particles that have no mass at all, such as particles of light (known as photons). This maximum speed can be given very precisely as 299,792,458 metres per second, equivalent to 186,282 miles per second, which is almost approaching a million times faster than the speed of sound in air. If I could travel at the speed of light, I would be able to travel from my home in the UK to Australia in one fourteenth of a second, barely time to blink. Light travelling from our nearest star,the Sun, takes just eight minutes to travel to us. From our outermost planet, Neptune, it's a journey time of just a few hours for a photon.We say that the Sun is eight light-minutes away from Earth and that Neptune is a few light-hours away from us. This has the interesting consequence that if the Sun stopped shining or if Neptune suddenly turned purple, no one on Earth could find out about such important information for eight minutes or a few hours respectively.
丛林法则之一是:不想死得快，就得跑得快。除非你异常狡猾或者善于 伪装，否则只有足够敏捷才能存活下来。哺乳动物摆脱劣势的最大速度取决 于其质量、肌肉力量和新陈代谢之间复杂的生化关系。宇宙中运动最快的实 体所能达到的最大速度是由完全没有质量的粒子所呈现的，例如光的粒子(被 称为 )。这个最大速度被精确地定为每秒299 792 458米，相当于每秒186 282英里，几乎比空气中的音速快100万倍。如果能以光速旅行，我将能够在十四分之一秒内从我在英国的家中到达澳大利亚，就是一瞬间的事情。从离我们最近的恒星，也就是太阳出发的光只需要8分钟就可以到达我们这里。而 从太阳系最外层的行星海王星出发，光子到地球的行程时间也只有几个小时。我们说太阳离地球有8光分，而海王星离我们有几光时。这会导致一个 趣的后果，如果太阳停止发光或海王星突然变成紫色，地球上的任何人发现这些重要信息都分别需要花上8分钟或几小时。
Let's now consider how fast light can travel from even more immensely distant points in space back to Earth. The Milky Way,the Galaxy in which our Solar System resides, is a few hundred thousand light-years across. This means that light takes a few hundred thousand years to travel from one side of the Galaxy to the other. The Fornax cluster is the nearest cluster of galaxies to the local group of galaxies (of which the Milky Way is a significant member) and is hundreds of millions of light-years away from us.Thus, an observer on a planet orbiting a star in a galaxy within the Fornax cluster looking back to Earth right now might, if equipped with appropriate instrumentation, see dinosaurs lumbering around on Earth. However, it is only the mind-boggling vastness of the Universe thatmakes the motion of light look sluggish and time- consuming. The role of the speed of light as a mandatory upper limit has an intriguing effect when we start to consider how to launch rockets into space.
现在让我们来考虑光线从太空中更加遥远的地方传回地球的时间有多长。我们的太阳系所在的银河系是一个长达几十万的星系。这意味着光 从银河系的一侧行进到另一侧需要几十万年。离本星系群(银河系是其中的重 要成员)最近的星系团，也就是天炉座星系团，离我们有几亿光年。因此，在围绕天炉座星系团中的某颗恒星运行的行星上如果有一位观察者，手头配备了恰当的仪器回看地球，可能会看到恐龙在地球上徘徊。不过这只是由于宇宙浩瀚得令人难以置信，才使得光的运动看起来迟缓且费时。但当我们开始考虑如何将火箭发射到太空时，宇宙规定光速是上限这一点就会带来一种有趣的效应。
Escape velocity
逃逸速度
If we wish to launch a rocket into space but its launch speed is too slow then the rocket will have insufficient kinetic energy to break free from the Earth's gravitational field. However, if the rocket has just enough speed to escape the gravitational pull of the Earth, we say it has reached its escape velocity. The escape velocity of a rocket from a massive object such as a planet is larger the more massive the planet is and larger the closer the rocket is to the centre of mass of the planet. The escape velocity vesc is written as vesc= where Mis the mass of the planet and R is the separation of the rocket from the planet's centre of mass and G is a constant ofNature known asNewton's gravitational constant.Gravity always acts so that it pulls the rocket towards the centre of the planet or star in question, towards a point known as the centre of mass. However, the value of the escape velocity is completely independent of the mass of the rocket. Thus, the escape velocity of a rocket at Cape Canavaral, some 6,400 kmaway from the centre of mass of Planet Earth, takes the same value, just over 11 km/s or approximately 34 times the speed of sound (which may be written as Mach 34), irrespective of whether its internal payload is a few feathers or several grand pianos. Now, suppose we could shrink the entire mass of Planet Earth so that it occupies a much smaller volume. Let's say that its radius becomes one quarter of its current value. If the rocket was launched at a distance of 6,400 km away from the centre of mass, its escape velocity would remain the same. However, if it relocated to the new surface of the shrunken Earth 1,600 km from its centre, then the escape velocity would be double the original value.
如果我们希望将火箭送到太空但发射速度太慢，那么火箭将没有足够的来挣脱地球的引力场。反之，如果火箭的速度恰好足以逃离地球引力的拉扯，我们就说它已达到了 。火箭从诸如行星之类的大质量物体上 逃离时，行星质量越大，火箭距行星的越近，逃逸速度也就越大。逃逸速度Vesc可以写成 ，其中M是行星的质量，R是火箭与行星质心的距离，而G是 被称为牛顿引力常数的自然常数。重力作用总是将火箭拉向行星或恒星的中心，朝向被称为的点。不过，逃逸速度的取值与火箭的质量完全无关。 因此，不论其内部载荷是几根羽毛还是几台三角钢琴，从距离地球质心约6400千米的卡纳维拉尔角发射的火箭都具有相同的逃逸速度，也就是11千米/ 秒多一点或约为声速的34倍(可以写为34马赫)。现在假设我们可以压缩地球 的全部质量，使它占据更小的体积，假定它的半径变为其当前的四分之一。 如果火箭发射处距离质心6400千米，其逃逸速度将保持不变。然而，如果它重新放到距质心1600千米的压缩后的地球的新表面，那么逃逸速度将会是原始值的两倍。