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As far as we know, time only moves in one direction – forward –
众所周知，时间只能朝一个方向移动—向前—
but if you could loop back to visit the universe at an earlier point in time, a famous paradox arises.
但如果你可以回去宇宙开始的时间点，一个著名的悖论就产生了。
What if you killed your grandfather when he was a child?
如果你杀死了还是孩子的祖父会怎样？
Then your father or mother wouldn't have been born, so you wouldn't have been born,
那么就不会有你的父亲或母亲，所以也就不会有你，
so you wouldn't have been able to go back in time to kill your grandfather in the first place. Paradox.
这样你就不能回到那时杀死自己的祖父。悖论。
The simplest resolution to the grandfather paradox is that when you go back in time,
祖父悖论最简单的解决方法就是当你回到那个时期时，
you're actually not going back into your own history but to a copy,
实际上你回到的不是你的过去而是一个复制版，
and everything you do there influences the new alternate future of that universe, not your own past.
并且你所做的一切只会影响其他的未来，不是你本身的过去。
But that's boring, because it just avoids the paradox.
但这样很无聊，因为这样只是逃避悖论。
If what you do when you go back in time actually influences your own past,
如果当你回到过去时你所做的事情实际上影响了你的过去，
and the effects of your time travel do loop back to the present...future...past– no problem.
而你的时间旅行的影响确实会循环回到现在……未来……过去—没有问题。
Let's just follow the paradoxical timeline through BEYOND its paradoxical conclusion.
让我们跟随这个矛盾的时间轴，超越它矛盾的结论。
You go back in time, kill your grandfather, thus you aren't born so you can't go back in time,
你回到过去，杀死自己的祖父， 那么你不会出生，所以你不能回到以前，
thus your grandfather isn't killed, thus you are born, so you go back in time and kill your grandfather, and so on...
也就是你的祖父没被杀死，那么你还是出生了，所以你回到过去，杀死自己的祖父，由此循环......
I'm showing this as a linear series of events but really it's two entangled histories happening in parallel.
我把它表示成一个线性的事件系列，但实际上它是两个相互纠缠的平行发生的历史。
Is that even possible?
这可能吗？
Well, I don't know about the time travel part, but subatomic particles regularly do multiple different things in parallel –
关于时间旅行那一部分，我不懂，但是亚原子粒子经常一条平行线上做许多不同的事情—
it's called quantum superposition and is responsible for the weirdness of the double slit experiment,
那被称为量子叠加，并且为双缝实验
many properties of atoms and molecules, fusion in the sun's core, and so on.
原子和分子的许多性质、太阳核心的聚变等等的怪异负责。
So if the universe were to exist in a superposition of two states – your grandfather is alive and your grandfather is dead –
所以如果宇宙存在于叠加的两种状态—你的祖父活着以及你的祖父死了—
then the natural result is a superposition of two states:
那么自然的结果也会处于叠加的两个状态：
you're born and able to go back in time to kill your grandfather, and you're not born.
你出生了并能回到过去杀死自己的祖父以及你没有出生。
And the natural result of these is a superposition of two states – your grandfather is dead and your grandfather is alive –
而这种情形的自然结果会是叠加的两种状态—你的祖父死了和你的祖父活着—
and so, at least from a logical perspective, this looping timeline is entirely consistent and there's no paradox.
所以至少从逻辑角度来看，这个循环的时间轴是完全一致且不矛盾的。
And a similar paradox-free solution can be obtained by viewing the problem as a steady-state
类似的无悖论方案可以通过把这个问题看作是马尔可夫链的稳态解来得到，
solution to a Markov chain – but I won't go into that here.
但我在这里就不深入讨论了。
Now of course nothing about these solutions to the grandfather paradox suggests that closed time loops are actually possible –
当然，关于祖父悖论的这些解决方案并不意味着闭合时间循环是可能的
in fact, some of the implications this kind of time loop have in the study of complexity theory
实际上，这种时间循环在复杂性理论研究中的一些含义似乎表明，
seem to suggest that time loops – and thus time travel into the past – must be impossible.
时间循环——回到过去的时间旅行——一定是不可能的。
But the main point is sometimes we think a situation creates a paradox when it doesn't,
但主要的一点是，有时我们认为一种情况会产生矛盾，但事实并非如此
and really the only paradox is how our thinking can be twisted enough to dream up time-traveling murderous grandsons,
并且唯一的悖论是，我们的思维是如何被扭曲到足以想象出穿越时空的杀人外孙，
but not twisted enough to think about twisting time.
而不足以思考扭曲的时间。
