接着聊一聊神奇的时空科学吧。上回说到爱因斯坦解决了牛顿的困惑，引力并不是瞬间即时传播的。在爱因斯坦的广义相对论当中，它是在引力场当中，由于质量弯曲了时空而产生的几何效应，并且给出了引力场的方程。这个方程是一个微分方程，求解起来非常复杂。
一战期间，一位德国数学家施瓦希与被征召进入德国军队，为炮兵计算炮弹的弹道。在工作之余，他将研究爱因斯坦的场方程，并求解作为自己消磨闲暇时光的方式，并且求出了相对论场方程的第一个解，就是考虑的是最简单的情况，广袤的宇宙中空无一物，出来一颗有质量之外。而这个解有一个特性，就是当质量集中在中心的时候，就会形成一个奇点，同时在外部也会形成一个产生无穷大的奇点，这个就是事件视界。显然这个时候施瓦西描述的是一个黑洞。也就是说，爱因斯坦相对论的场方程预言了——黑洞这种奇怪的天体存在。
不过无穷大这种东西在数学上是可以接受，但是在物理上却是不可以接受的。所以当时的科学家们认为宇宙中肯定不存在黑洞，而宇宙的法则一定会禁止黑洞的形成。而在爱因斯坦发表广义相对论，也就是一九一五年之后不久，在二十世纪二十年代，量子力学蓬勃地发展了起来。人们根据其中的泡利不相容原理找到了阻止黑洞形成的理由。也就是说，电子不可能占据完全相同的量子态，也就是说虽然可以挨得很近，但是不可能聚拢到一个奇点上面。不过，一位印度科学神童钱德拉塞卡，在他坐船前往英国求学的中，为了打发时光，他也在琢磨这个问题。虽然虽然电子不能被压缩到一个起点里面去，但是随着彼此距离的缩小，根据海参堡的不确定性原理，这时电子的抖动速度会变得越来越快。而我们知道有质量的物体运动速度是有上限的，那就是光速。那一旦电子的抖动速度超过光速，那么它就无法继续抵抗引力的压缩而进入原子核的质子，形成一个中子。也就是说，在一些比太阳质量更大的恒星，最后会因为自身的引力坍缩成一个完全由中子组成的中子星。而这一理论后来也得到了天文观测的证实。终于到了大名鼎鼎的奥本海默上场了。奥本海默是作为美国原子弹之父，闻名于世的，但他在接手曼哈顿工程之前，是一位彻头撤尾的物理学家，而他最大的学术成就就是在理论上证明呢没有什么已知的物理定律能够阻止黑洞的形成。黑洞是可以存在的。
如今我们已经拍摄到了黑洞的图片，这已经不是什么新闻了。所以爱因斯坦的场方程的预言，现在并没有被推翻，它仍然是我们人类所设想出来的最好的关于宇宙的物理理论之一。如果从爱因斯坦场方程中推出的结论是值得信赖的，那么对于微分方程有一个特性，如果你找到了一个解，那么正在时间上将这个解反过来，它也是满足原来的微分方程的，也就是说让时间倒流，也可以满足爱因斯坦的场方程。很容易理解黑洞的引力如此强大，就连光也无法从中逃出，一旦越过黑洞的事件视界就再也出不来。那如果在时间上倒转过来，它们会得到一个什么呢？就是一个白洞，它和黑洞是完全对称。白洞也有一个事件视界，如果你处于视界内，那么你就会被弹射出来，再也回不去了。既然描述黑洞的解被证实正确的。那么白洞也有很大程度是是真实存在的。不过目前科学家还没有找到确切的证据，不过，你仔细思考的话，会发现宇宙大爆炸时的状态和白洞的描述非常的相似。宇宙从一个奇点急剧扩，张物质从中产生，并向四周扩散，非常符合白洞的描述。而且黑洞和白洞都存在于都存在一个奇点，很可能奇点一边连接着一个黑洞一边连接着一个白洞，这样的解也是符合爱因斯坦场方程的。当然，这样的虫洞并不用来进行星际航行，或者在平行宇宙之间穿梭。因为没有人能够活着通过奇点，就算可以活着通过。时间也会在接近起点的时候变得无限长。
不过到目前为止，我们讨论的都是施瓦洗当时想象出的最简单的状态，一个静止没有旋转的天体所形成的黑洞。在宇宙当中，大部分黑洞都是由大质量的恒星形成的。把这些恒星本身都是带有自转，也根据角动量守恒定律。形成的黑洞也是在旋转的。一个旋转的黑洞基本上就变得更加复杂，而且更加有趣。到了二十世世纪六十年代，一位叫做克尔的科学家，找出了描述旋转黑洞的场方程的解。而在这个解当中奇点不再是点状，变成了一个环形，只要这个环足够大，人类是可以从中间穿过去的。如果我们不可能从黑洞里面出来，那穿过去之后可能就会到达另一个宇宙中的白洞，通过旋转黑洞中心的奇环，我们就进入了另外一个宇宙。而如果那个宇宙中也存在转黑洞的话，那我们就可以如法炮制进入下一个宇宙。这个似乎说明爱因斯坦场方程暗示着存在着无穷多的平行宇宙。
当然，就像当初施瓦西提出黑洞的概念一样，科学家们相信这些一定不是真的。某些物理定律会阻止这样的事情发生。比如，有的科学家相信在奇环的附近会形成一个能量自己放大的过程，从而让奇环坍缩成为一个起点，让人无法穿越。又或者要维持一个可穿越的从洞需要具有负能量的奇异物质将奇点撑开。而现在的物理规律定禁止存在这样的物质。但是谁知道呢？就像黑洞刚刚的提出来一样，科学家们也都是持怀疑态度，毕竟最后还是证明了爱因斯坦的场方程是对的。毕竟我们现在已经有一个宇宙了，为什么不能有第二个呢？
Let's talk about the amazing science of time and space. Last time we talked about Einstein solving Newton's puzzle, gravity doesn't propagate instantaneously. In Einstein's theory of general relativity, it is the geometric effect of mass bending space-time in a gravitational field, and the equations for the gravitational field are given. This equation is a differential equation and is very complicated to solve.
During World War I, a German mathematician named Schwab was recruited into the German army to calculate the trajectory of artillery shells for artillery. In his spare time, he will study Einstein's field equations and solve them as a way to kill his leisure time, and found the first solution of the relativistic field equations, which is to consider the simplest case, the vast universe empty of nothing, out of a mass. And one of the properties of this solution is that when the mass is concentrated in the center, a singularity will be formed, and at the same time, an infinite singularity will be formed outside, and this is the event horizon. Apparently at this point Schwarzey was describing a black hole. In other words, the field equations of Einstein's theory of relativity predicted the existence of such strange objects as black holes.
But infinity is something that is mathematically acceptable, but physically unacceptable. So scientists at that time believed that there must be no black holes in the universe, and the laws of the universe must prohibit the formation of black holes. And in the 1920s, shortly after Einstein published his general theory of relativity, in 1915, quantum mechanics took off. The Pauli exclusion principle explains why black holes should not be formed. In other words, electrons cannot occupy exactly the same quantum state, which means that although they can be close together, they cannot converge on a singularity. But Chandrasekhar, an Indian science prodigy, pondered the same question as he passed the time on his boat to England to study. Although the electrons can not be compressed into a starting point, but as the distance between each other shrinks, according to the uncertainty principle of sea cucumerburg, the speed of the electrons will become faster and faster. And we know that there's an upper limit to the speed at which mass moves, and that's the speed of light. Then once an electron shakes faster than the speed of light, it can no longer resist the gravitational compression of protons into the nucleus, forming a neutron. In other words, some stars with more mass than the Sun will eventually collapse under their own gravity into a neutron star composed entirely of neutrons. This theory was later confirmed by astronomical observations. Finally, it's time for the famous Oppenheimer. Oppenheimer is best known as the father of the American atomic bomb, but before he took over the Manhattan Project, he was a complete physicist, and his greatest academic achievement was to prove that no known laws of physics could prevent the formation of black holes. Black holes can exist.
Now that we've captured images of black holes, it's not news. So the prediction of Einstein's field equations has not been overturned, and it remains one of the best physical theories of the universe that we humans have come up with. If the conclusions derived from Einstein's field equations are trustworthy, then there is a property of differential equations that if you find a solution, and you reverse the solution in time, it also satisfies the original differential equation, that is, if you go back in time, it also satisfies Einstein's field equations. It is easy to understand that the gravitational pull of a black hole is so strong that not even light can escape from it, once it crosses the event horizon of a black hole, it can never get out again. What would they get if they went backwards in time? It's a white hole, and it's perfectly symmetric to a black hole. The White hole also has an event horizon, and if you are within the horizon, then you will be ejected and never be able to return. Now that the solution describing the black hole has been proved correct. So white holes are also, to a large extent, real. So far, scientists have not found the exact proof, but if you think about it, the state of the universe at the Big Bang is very similar to the description of the white hole. The universe expanded dramatically from a singularity, from which tensor matter emerged and spread out, very much in line with the description of a white hole. Moreover, black holes and white holes both exist in a singularity, and it is possible that the singularity connects a black hole and a white hole on one side, which is also in line with Einstein's field equations. Of course, such wormholes are not used for interstellar travel, or to travel between parallel universes. Because no one can get through the singularity alive, even if they could. Time also becomes infinitely long near the beginning.
But so far, we've been talking about the simplest state that Schwarzschild could have imagined, a black hole formed by a stationary, non-rotating object. Most black holes in the universe are formed by massive stars. These stars themselves are rotated, also according to the law of conservation of angular momentum. The black holes that form are also spinning. A spinning black hole basically becomes more complex and more interesting. In the 1960s, a scientist named Kerr found a solution to the field equations that describe spinning black holes. In this solution, the singularity is no longer a point, but a ring, and as long as the ring is large enough, a human can pass through the middle. If we can't get out of the black hole, we might end up in a white hole in another universe by rotating the strange ring at the center of the black hole. And if there are rotating black holes in that universe, we could do the same to the next one. This seems to indicate that Einstein's field equations imply the existence of an infinite number of parallel universes.
Of course, just as when Schwarzschild came up with the idea of black holes, scientists believe that these cannot be true. Certain laws of physics prevent that from happening. For example, some scientists believe that a process of energy amplification occurs near the odd ring, so that the collapse of the ring becomes a starting point, making it impossible to cross. Or sustaining a traversable tunnel would require exotic matter with negative energy to push the singularity apart. The laws of physics now forbid such substances. But who knows? Just as black holes had just been proposed, scientists were skeptical, after all, Einstein's field equations were finally proved right. After all, we already have one universe now, so why not have a second?
