Chapter 4 Falling Into a Black Hole

落入黑洞

How close is too close?

太近是多近

Before we can consider in detail what would happen if you or your belongings had the misfortune to fall into a black hole, it is important to understand the effect of an observer's particular perspective, or frame of reference. This means that different observers see very different things. Exactly what your perspective is on an object falling into a black hole depends on how far away you are from that object (and indeed whether you are that object!). Consider a particle of light, a photon, that is outside the event horizon of a black hole: since it is outside the horizon, it can in principle escape. Inside the event horizon it would be a different story-the photon could not escape the gravitational field of the black hole. But even outside the event horizon, a photon that is travelling away from the black hole will not escape completely unscathed. The photon suffers a loss in its energy due to the work it has to do against gravity. This is an example of a gravitational potential well; just as energy would be needed to haul yourself upwards out of a deep well, so the photon needs to expend energy to pull itself away from the region near a massive object. The effect has even been measured for photons moving in the Earth's gravity.The energy of a photon is inversely related to its wavelength: a high-energy photon has a short wavelength whereas a low-energy photon has a long wavelength. The photon loses energy as it retreats away from the black hole, so its wavelength increases.This changes the colour of the light, moving from the blue (short wavelength) towards the red (long wavelength) end of the spectrum (this effect is called redshift). This sort of redshift, known as gravitational redshift, arises where spacetime itself stretches out, or is curved, for example by the effect of a massive body such as a black hole. Note that John Michell, despite having significant original thoughts about dark stars, was incorrect in thinking that the velocity of light decreases as it climbs out of the potential well.We now know that it is the wavelength(hence frequency) of light that is affected by the presence of a massive star. 

如果你或者你的东西不幸落入黑洞会发生什么?在我们详细考虑这个问题之前，了解特定观测者的特定视角或参考系的影响是很重要的。这意味着不同的观测者会看到非常不同的事情。你如何看待落入黑洞的物体完全取决于你离这个物体有多远(以及你是否就是那个物体)。在黑洞的事件视界之外的一个光子，因为它在视界之外，所以理论上可以逃离。而在事件视界内， 故事将会不同了——光子无法逃离黑洞的引力场。但即使在事件视界之外， 离开黑洞的光子也不会毫发无损地逃脱。光子会由于要克服引力做功而损失 能量。这是一个引力势阱的例子，就像你将自己从深井中拉出来需要能量一 样，光子也需要消耗能量以使自己远离大质量天体附近的区域;甚至从在地球引力中移动的光子当中，人们也已经测量到了这种效应。光子的能量与其波长成反比:高能光子波长短，而低能光子波长长。光子在从黑洞逃离时会失去能量，因此其波长会增加。这会改变光的颜色，使其在光谱上从蓝色端 (短波)向红色端(长波)移动。这种移动被称为引力红移，产生于时空本身的延展，或由于黑洞之类的大质量物体作用而弯曲的地方。要注意的是约翰· 米歇尔虽然在暗星问题上给出了重要的原创想法，却错误地认为当光从势阱中爬出时速度会降低。我们现在知道了受大质量恒星所影响的是光的波长(也就是频率)。 

What happens to time near a black hole?

黑洞附近的时间会受到什么影响

In Chapters 1 and 2 I described how spacetime is distorted by the presence of a mass (i.e. something which produces its own gravitational field) and this means that not just space, but also time is affected close to a black hole. 

在第1章和第2章中，我描述了时空是如何因质量(也就是自身会产生引力 场的事物)的存在而变形的，而这意味着在黑洞附近不仅是空间，时间也会受到影响。 

Imagine you want to keep a safe distance from a Schwarzschild black hole but you want to learn more about how time behaves nearby. Thus you have arranged for twenty-six fixed observers to be stationed close to the black hole's event horizon but definitely safely outside it. These observers are named A to Z, and are arranged in a line with A closest to the event horizon and with Z being nearest to you, safely far away. Each observer from A to Z has a good clock with which to measure their local time, at their particular location. As part of the deal to persuade A to Z to participate in this experiment, you had offered them each an inducement in the form of a gift of an additional, unusual clock that had been adjusted so that the time on it would read the same as the time on your clock at your safe distance. Participant Z, closest to you, would find that the two clocks in his possession read slightly different times because his own clock, which measures local time (`proper time' in the jargon),would be running slightly more slowly than the gift clock which matches the time you measure at your rather safer and more remote distance.The collated results of participants Z to A would display a remarkable effect: closer to a black hole, a clock measuring time ‘runs more slowly' compared with the distant time as reported on the participants' specially adjusted gift clocks. This effect, described by Einstein's theory of general relativity, is known as time dilation. The effect would be greater and greater for the observers nearer the start of the alphabet who are nearer to the black hole. The greater the proximity to a black hole, the more slowly a local clock (of any kind: atomic, biochemical) will run compared to a clock used by a distant observer. 

想象一下，你想与史瓦西黑洞保持安全的距离，但是你又想了解在其附近时间是如何表现的。因此，你安排了26名固定的观察者安全驻扎在黑洞外靠近事件视界的地方。这些观察者按照从A到Z的顺序命名，并且排成一条直 线，其中A最接近事件视界，而Z最接近安全地待在远处的你。从A到Z的每个 观察者都有一个精确的时钟，来测量他们所在的特定位置的时间。为说服A到 Z参与这个实验，你还为他们每个人额外提供了一个不同寻常的时钟作为礼 物。这些时钟经过调整，与你所在的安全位置的时钟读数相同。最接近你的参与者Z会发现他所拥有的两个时钟所读取的时间略有不同，因为他自己的时钟测量的是当地时间(术语叫作“ 固有时”)，其运行会比与你在更远更安全的距离所测量到的时间相同的礼物时钟更慢一点。参与者Z到A所整理出的结果将显示出一个显著效应:与他们特别调整过的礼物时钟上所显示的远处的时间相比，越接近黑洞测时的时钟“运行得越慢”。爱因斯坦的广义相对论所描述的这种效应被称为时间膨胀。对于更加靠近黑洞的字母表开头的观察者来说，效应会越来越明显。与远处观测者使用的时钟相比，本地时钟(不管是原子钟还是生化钟)越靠近黑洞，运行速度就会越慢。 

Suppose you were multi-tasking your experiments with a different set of twenty-six observers at the same distances from a different black hole. They are arranged in just the same way as their namesakes near the first black hole. However, in this second case, the black hole has twice the mass of the black hole in your first experiment. The unusual clocks you had prepared as gifts for this second set of observers would need to be radically altered as for your original experiment, but the rate at which each unusual clock has to be adjusted is exactly double that of the rate needed for the corresponding clock in the first set of gift clocks at the exact same distance from the centre of the first black hole which has half the mass of the second. These time dilation effects are larger if the black hole mass is larger, and also become more extreme the closer you get to the event horizon.