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续接昨天时间旅行的话题，我们先来回顾一下爱因斯坦的相对论。在相对论中，时间和空间是一个整体，被称为时空。空间有三维，而时间只有一维，加在一起就是四维的时空。
四维的物体我们人类无法想象，但是我们可以用《三体》小说中那个著名的降维打击的方法来获得一个直观的印象。就是把三维空间降维成二维的平面，然后时间就是垂直于这个平面的一个第三个坐标轴。这样我们就可以用三维的形象来理解四维时空了。
不过还有一个问题，那就是在这个图景中，利用我们中学的物理知识就知道距离除以时间就是速度，而光速又是所有速度的极限。那么就可以在这个三维空间中画出一个圆锥形，圆锥的顶点就是我们所处的时空位置。把所有这些时空位置连在一起，形成一条线就是我们的人生经历，或者叫做世界线。
因为所有的速度都不能超过光速，所以我们的移动范围只能在圆锥形的光锥之内。好像也是出自《三体》小说中的一句话，就是“光锥之内都是命运”。
如果从这张图上来看，如果我们要实现时间旅行，也就是说从未来回到过去，那么我们的世界线就必须组成一条环线。而要组成一条环线则必须越过光锥的限制达到超光速。这也就是狭义相对论限制我们进行回到过去的时光旅行的一个条件。
不过我们所说的这种四维时空，实际上是一种新的几何学，也就是闵可夫斯基几何。它和欧氏几何一样，都是时空是平直的。但是到了广义相对论中，时空不再是平直的，而是像软体动物一样可以被质量和能量弯曲。而要用一种全新的黎曼几何来进行描述。
在平直的时空中，任何一个点的光锥都是朝向同一个方向的。但是弯曲的时空就不一样了。由于不同地点的时空曲率不同，不同地点的光锥可以朝向不同的方向。时空也可以像橡皮泥一样揉捏。只要有合适的质量和能量配置，也能把时空像橡皮一样捏成任何形状。
那么形成一条封闭的世界线，同时也不越过光锥之外，也是可行的。于是科学家们就把爱因斯坦场方程当做玩具一样玩弄。想制造出形成封闭世界线的时空形状所需要的质量和能量配置。做出了许多很有意思的方案。
比如其中一个就是用一个圆柱体。用一个质量极大的圆柱体旋转起来带动周围的时空，就能够形成时空旅行的效果。不过这个圆柱体的长度必须和整个宇宙的大小相比拟。
不过也有一些比较现实的情况，其中一个就是旋转的黑洞。
现在人们已经认识到我们宇宙中存在的黑洞数量是非常惊人的。而且几乎所有的黑洞都在旋转。因为大部分黑洞都是由之前的恒星形成的。而恒星本身都带有自转。由于角动量守恒，当它变为黑洞的时候，这种自转仍然保持着。所以一个旋转的黑洞周围的时空就可以看成是一个漩涡。
这个扭曲时空中的一个世界线就可以形成封闭的环路。也就是说回到了过去。这一假设在理论上是完全成立的。而且就在几天之前正好有新闻报道说，一个有中国科学家参与的研究小组通过人类拍摄的第一张黑洞照片，就是M87活动星系核，它的喷流动态分析出这个黑洞确实是在旋转的。
那么这样就意味着掉入黑洞，也许并不意味着毁灭，而是回到了过去，时光倒流。
这样一来，这个黑洞和之前聊到的宇宙的目的也对上了。
在那篇讲宇宙意义的帖子里谈到过，其中一种理论认为宇宙存在的目的，就像生命一样是在繁殖自己。它们的繁殖方式就是通过产生黑洞。每一个黑洞内部都会产生一个新的宇宙。
如果旋转的黑洞可以让时光倒流，那么这里就在逻辑上形成了闭环了。黑洞会让自己周围的时空时光倒流，一直倒流到宇宙大爆炸之初的那个状态，然后就会发生新的大爆炸，产生一个新的宇宙。于是就实现了宇宙的繁殖。好吧这个只是我个人脑洞而已。
Continuing yesterday's discussion of time travel, let's take a look back at Einstein's theory of relativity. In the theory of relativity, time and space are a whole, called spacetime. Space has three dimensions, while time has only one, which adds up to four dimensions of space-time.
We humans can't imagine four-dimensional objects, but we can get an intuitive impression with the famous dimensionality reduction blow in the Three-Body Problem novel. It is to reduce the three-dimensional space to a two-dimensional plane, and then time is a third axis perpendicular to this plane. So we can use the three-dimensional image to understand the four-dimensional space-time.
But there's just one problem, and that's that in this picture, using all of our high school physics, we know that distance divided by time is the speed, and the speed of light is the limit of all speeds. Then you can draw a cone in this three-dimensional space, and the apex of the cone is where we are in space-time. Connecting all of these places in time and space into a line is our life experience, or world line.
Because nothing can travel faster than the speed of light, we can only move within the conical cone of light. It seems to be a sentence from the Three-Body Problem novel, that is, "Inside the cone of light is fate."
If we are to achieve time travel, that is to say, from the future to the past, then our world lines must form a loop. And to form a loop, you have to go beyond the cone of light and go faster than light. This is one of the conditions under which special relativity restricts us from traveling back in time.
But what we call four-dimensional spacetime is actually a new geometry, Minkowski geometry. Like Euclidean geometry, spacetime is flat. But in general relativity, space-time is no longer flat, but can be bent by mass and energy like a mollusk. We're going to use a new Riemannian geometry to describe it.
In flat spacetime, the light cone at any point is oriented in the same direction. But curved spacetime is different. Because of the different curvature of space-time at different locations, light cones at different locations can be oriented in different directions. Space-time can also be kneaded like plasticine. Given the right mass and energy configuration, space-time can be shaped like a rubber into any shape.
Then it is possible to form a closed world line without going beyond the light cone. So scientists are playing with Einstein's field equations like a toy. To create the mass and energy configuration needed to form the space-time shape of a closed world line. And made a lot of interesting solutions.
For example, one of them uses a cylinder. The effect of time travel can be created by rotating a cylinder with a large mass to drive the surrounding space-time. But the length of the cylinder has to match the size of the entire universe.
But there are a few more realistic scenarios, one of which is a spinning black hole.
It is now recognized that the number of black holes that exist in our universe is staggering. And almost all black holes are spinning. Because most black holes are formed by former stars. And stars themselves spin. Due to the conservation of angular momentum, this rotation is still maintained when it becomes a black hole. So space-time around a spinning black hole can be seen as a vortex.
A world line in this warped space-time can form a closed loop. Which means going back in time. This hypothesis is perfectly valid in theory. And just a few days ago, it was reported that a research team with Chinese scientists participated in the first photo of a black hole taken by humans, that is, the M87 active galactic nucleus, and its jet dynamics analysis found that the black hole is indeed spinning.
So that means falling into a black hole, maybe not destruction, but going back in time, going back in time.
So this black hole fits in with the purpose of the universe we talked about earlier.
As I mentioned in my post on the meaning of the universe, one theory is that the purpose of the universe, like life, is to reproduce itself. The way they reproduce is by creating black holes. Inside each black hole, a new universe is created.
If a spinning black hole could turn back time, then there would be a logical closed loop. A black hole would rewind the space-time around it, all the way back to the beginning of the Big Bang, and then a new big Bang would happen, creating a new universe. And so the universe reproduces. Well, that's just my imagination.
