What is a black hole? Well, it's difficult to answer this question, since the terms we would normally use to describe a scientific phenomenon are inadequate here. Astronomers and scientists think that a black hole is a region of space (not a thing) into which matter has fallen and from which nothing can escape - not even light. So we can't see a black hole. A black hole exerts a strong gravitational pull and yet it has no matter. It is only space - or so we think. How can this happen?

The theory is that some stars explode when their density increases to a particular point; they collapse and sometimes a supernova occurs. From earth, a supernova looks like a very bright light in the sky which shines even in the daytime. Supernovae were reported by astronomers in the seventeenth and eighteenth centuries. Some people think that the Star of Bethlehem could have been a supernova. The collapse of a star may produce a White Dwarf or neutron star - a star, whose matter is so dense that it continually shrinks by the force of its own gravity. But if the star is very large (much bigger than our sun) this process of shrinking may be so intense that a black hole results. Imagine the earth reduced to the size of a marble, but still having the same mass and a stronger gravitational pull, and you have some idea of the force of a black hole. Any matter near the black hole is sucked in. It is impossible to say what happens inside a black hole. Scientists have called the boundary area around the hole the "event horizon." We know nothing about events which happen once objects pass this boundary. But in theory, matter must behave very differently inside the hole.

For example, if a man fell into a black hole, he would think that he reached the center of it very quickly. However an observer at the event horizon would think that the man never reached the center at all. Our space and time laws don't seem to apply to objects in the area of a black hole. Einstein's relativity theory is the only one which can explain such phenomena. Einstein claimed that matter and energy are interchangeable, so that there is no "absolute" time and space. There are no constants at all, and measurements of time and space depend on the position of the observer. They are relative. We do not yet fully understand the implications of the relativity theory; but it is interesting that Einstein's theory provided a basis for the idea of black holes before astronomers started to find some evidence for their existence. It is only recently that astronomers have begun specific research into black hole. In august 1977, a satellite was launched to gather data about the 10 million black holes which are thought to be in the Milky Way. And astronomers are planning a new observatory to study the individual exploding stars believed to be black holes.

The most convincing evidence of black holes comes from research into binary star systems. Binary stars, as their name suggests, are twin stars whose position in space affects each other. In some binary systems, astronomers have shown that there is an invisible companion star, a "partner" to the one which we can see in the sky. Matter from the one which we can see is being pulled towards the companion star. Could this invisible star, which exerts such a great force, be a black hole? Astronomers have evidence of a few other stars too, which might have black holes as companions.

The story of black holes is just beginning. Speculations about them are endless. There might be a massive black hole at the center of our galaxy swallowing up stars at a very rapid rate. Mankind may one day meet this fate. On the other hand, scientists have suggested that very advanced technology could one day make use of the energy of black holes for mankind. These speculations sound like science fiction. But the theory of black holes in space is accepted by many serious scientists and astronomers. They show us a world which operates in a totally different way from our own and they question our most basic experience of space and time.

什么是黑洞呢？这个问题很难回答，因为我们通常用来描述一种科学现象的术语用在这里来解释是不够的。

天文学家和科学家们认为黑洞是个空间区域，物体会掉进去，而没有物体能从中逃逸出来――即使是光也不能，所以我们看不到黑洞。黑洞产生很强的引力，而它却没有物质。它只是空间――或者我们认为是空间。这是怎样发生的呢？

理论是一些星球的密度增长到特定的时刻就会爆炸。它们崩溃时会产生超新星。在地球上看去，超新星就像天空中非常耀眼的灯，即使在白天也能看到其闪光。在17、18世纪，天文学家就有关于超新星的记录，一些人认为圣诞星可能是一颗超新星。一颗星崩溃可能产生吸引力，你就了解黑洞的力量。黑洞附近任何物质都会被吸进去，根本不可能说出黑洞里面发生了什么。科学家们把这个洞的边缘区域称为"事界"。一旦物体经过这个边界，我们对所发生的事一无所知。但是在理论上，在黑洞里的物质的运动肯定与洞外有很大区别。

例如：如果一个人掉进黑洞，他会认为自己很快就达到了其核心，但在"事界"的观察者则认为这个人根本不会到达黑洞的核心。我们的时空规律看起来不适于黑洞里的物体。爱因斯坦的相对论是唯一可以解释这种现象的理论。爱因斯坦声称物质和能量是可以互相转化的，因此就没有绝对的时间和空间，根本不存在永恒，时间和空间的衡量取决于观测者所在的位置，它们是相对的。我们还没有完全理解相对论的含义，但有意思的是，在天文学家的着手发现黑洞的存在证据之前，爱因斯坦就提供了黑洞这种想法的基础。只是近来科学家才开始对黑洞的具体研究。1977年8月，发射了一颗卫星去收集关于被认为是在银河系的一千万个黑洞的数据。天文学家正在计划一个新的天文台以研究个别的正在爆炸的被相信要变为黑洞的星球。

黑洞最有说服力的证据来自对双星系的研究。双星，正如它们的名字所表明的，是两颗在空间位置上互相影响的星球。在一些双星系里，天文学家已经表示这里有一颗看不到的伴星，即我们可以在天空中看到的一颗星球的伙伴。来自我们的看到的星球的物质正被吸引到伴星去。这颗产生如此巨大力量的看不见的星球会是黑洞吗？天文学家还有其它一些星球的证据，这些星球可能与黑洞相伴。

对黑洞的研究刚刚开始，各种推测会层出不穷。在我们银河系的中心很可能存在着一个巨大的黑洞正以极快的速度吞食着星球。人类有一天也会面临被吞食的命运。而科学家提出，有一天高科技利用轩洞的能量为人类服务。这些设想听起来像科幻小说，但空间中黑洞的理论被许多严谨的科学家和天文学家接受。他们向我们展示了一个以完全不同于我们理解的方式运行的世界，并对我们最基本的时空经验提出了疑问。