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As far as we know, time only moves in one direction – forward –
据我们所知，时间只朝一个方向移动——向前——
but if you could loop back to visit the universe at an earlier point in time, a famous paradox arises.
但如果你能回到宇宙更早的时间点，一个著名的悖论就出现了。
What if you killed your grandfather when he was a child?
如果你在你祖父还是个孩子的时候就杀了他会怎样?
Then your father or mother wouldn't have been born, so you wouldn't have been born,
那么你的父亲或母亲就不会出生，所以你就不会出生，
so you wouldn't have been able to go back in time to kill your grandfather in the first place.
所以你就不能回到过去一开始就杀死你的祖父。
Paradox.
悖论。
The simplest resolution to the grandfather paradox is that when you go back in time,
对于祖父悖论最简单的解答是当你回到过去，
you're actually not going back into your own history but to a copy,
你实际上不是回到你自己的过去，而是回到一个副本中，
and everything you do there influences the new alternate future of that universe, not your own past.
你在那里所做的一切都会影响那个宇宙的新的不同的未来，而不是你自己的过去。
But that's boring, because it just avoids the paradox.
但这种说法很无聊，因为它只是避免了悖论。
If what you do when you go back in time actually influences your own past,
如果你回到过去时所做的事情真的影响了你自己的过去，
and the effects of your time travel do loop back to the present...future...past – no problem.
而你的时空旅行所造成的影响确实会循环回到现在……未来……过去——没问题。
Let's just follow the paradoxical timeline through beyond its paradoxical conclusion.
让我们沿着这条矛盾的时间线走下去，越过矛盾的结论。
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，杀死了你的祖父，所以你没有出生，所以你无法回到过去，
thus your grandfather isn't killed, thus you are born, so you go back in time and kill your grandfather, and so on...
所以你的祖父没有被杀死，所以你出生了，所以你回到过去，杀死了你的祖父，以此类推。
I'm showing this as a linear series of events but really it's two entangled histories happening in parallel.
我把这表现为一系列线性事件，但实际上这是两段纠缠在一起的平行时空。
Is that even possible?
这是可能的吗?
Well, I don't know about the time travel part, but subatomic particles regularly do multiple different things in parallel –
好吧，我不知道时间旅行的部分可不可能，但亚原子粒子经常同时做多件不同的事情——
it's called quantum superposition and is responsible for the weirdness of the double slit experiment, many properties of atoms and molecules, fusion in the sun's core, and so on.
这被称为量子叠加，它导致了双缝实验的怪异现象，原子和分子的许多性质，太阳核心的聚变，等等。
So if the universe were to exist in a superposition of two states – your grandfather is alive and your grandfather is dead –
因此，如果宇宙存在两种状态的叠加——你的祖父还活着，你的祖父已经死了——
then the natural result is a superposition of two states: you're born and able to go back in time to kill your grandfather, and you're not born.
那么自然结果就是两种状态的叠加：你出生了，并且能够回到过去杀死你的祖父，同时你并没有出生。
And the natural result of these is a superposition of two states – your grandfather is dead and your grandfather is alive –
这些的自然结果是两种状态的叠加——你的祖父去世了，同时你的祖父还活着——
and so, at least from a logical perspective, this looping timeline is entirely consistent and there's no paradox.
所以，至少从逻辑的角度来看，这个循环的时间线是前后一致的，没有悖论。
And a similar paradox-free solution can be obtained by viewing the problem as a steady-state solution to a Markov chain – but I won't go into that here.
类似的无悖论解也可以通过将这个问题看作马尔可夫链的稳态解来得到——但我在这里就不深入探讨了。
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没有任何迹象表明封闭的时间环是真的可能的——
in fact, some of the implications this kind of time loop have in the study of complexity theory seem to suggest
事实上，这种时间循环在复杂性理论研究中的一些可能结果似乎表明，
that time loops – and thus time travel into the past – must be impossible.
时间环——即能穿越时空回到过去——肯定是不可能的。
But the main point is sometimes we think a situation creates a paradox when it doesn't,
但主要的问题是，有时我们认为一种情况会产生悖论，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, but not twisted enough to think about twisting time.
而真正唯一的悖论是，我们的思维如何发展到足以幻想出时间旅行的谋杀故事，但又不足以思考扭曲时间的程度。
