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Time travel can be complicated and complicating,
时间旅行很复杂，
so let's ignore all the paradoxical stuff and talk about a few simple ways that you can time travel without leaving your home.
所以让我们忽略所有自相矛盾的东西，并讨论几个关于不用离开家就能时间旅行的简单方法。
First. Do nothing - you're already traveling through time!
第一种，什么都不用做，你就已经穿梭时空了！
I mean, here you are - fifteen seconds into the future since the start of this video!
我是说，现在你已身处于视频开始15秒后的未来了。
Easy, right? The point is, we're all always traveling forwards through time.
很简单，对吧。我们一直都在时间中旅行。
But that's boring time travel. What's interesting is time travel relative to other people.
但这种时间旅行很无聊。有意思的是相对于其他人的时间旅行。
To do that...Start walking, and you'll travel through time relative to someone standing still!
那么你需要开始行走，相对于站着静止的人来说，你将踏上时空之旅！
We've known for over a century that time and space are really just two components of a single 'spacetime'
一个多世纪以来，我们都知道时间和空间其实只是一个“时空”的两个组成部分
and the faster you move, the slower time will pass for you.
你移动速度越快，你的时间就过得越慢。
If you take a walk around the block, you'll be 3 femtoseconds younger than your friend who stayed home.
如果你绕着街区走一圈，你会比呆在家里的朋友年轻3飞秒的时间。
Except, in order to walk around the block, you had to...
为了绕着街区走，你必须……
Stand up. You're now further from the Earth, and so gravity is a tiny bit weaker for you.
站着。现在你距离地球远了一点，所以重力对你来说稍微弱一点。
Which means you've traveled through time relative to your friend who's sitting down.
也就是说，相对于你坐着的朋友，你已经穿越了时间。
That's right, more gravity makes time slow down, too.
没错，更多的重力也会让时间放慢。
If you stand up for a minute your feet will have aged 10 femtoseconds less than your head.
如果你站起来一分钟，你的脚会比你的头年轻10飞秒。
On the other hand, GPS satellites high in orbit experience less of earth's gravity
另一方面，在轨道上运行的GPS卫星受到的地球引力较小
and thus travel noticeably faster through time than we do, which is why their clocks are calibrated to run slow.
因此在时间上运行的速度明显快于我们，所以它们的时钟被校准为慢速运行。
But maybe you want to time travel more than a few femtoseconds.
但或许你想将时间旅行再延长几飞秒。
Get ready for your head to start spinning - I mean, the universe.
准备好开始旋转吧，我说的是宇宙。
Because if the whole universe were spinning really fast,
但如果整个宇宙都在快速旋转，
general relativity predicts there would be time-loops all over the place.
广义相对论预测到处都将存在时间循环。
Moving along one of these loops, you'd always feel like you were moving forwards in time,
沿着其中一个循环移动，你总会有有一种在时间中前进的感觉，
but overall you'd loop around and travel back to a time and place in your own past.
但总的来说，你会回到你自己过去的时间和地点。
It's a little like how you can keep moving forwards on the Earth,
有点像是在地球上的你如何保持前进，
but Earth's curvature brings you back to where you started.
但是地球的曲率将带你回到你出发的地方。
Unfortunately, our universe isn't spinning. Maybe it would be easier to build...
但是，我们的宇宙没有在旋转。或许更简单的方法是建造一个...
An infinitely long, super-dense spinning cylinder, which would also curve space-time enough to create a time-loop.
无限长且超密的旋转圆筒，这也会使时空弯曲，形成一个时间循环。
The problem, of course, is how do you build something that's infinite in size?
当然，问题是如何建造一个无限大的物体？
Maybe you could just make it really really really big? Mmm. No.
或许你可以把它做的非常非常大？嗯...不行。
If you tried to squeeze this time machine into finite space, you'd need negative energy -
如果你想把时间机器塞进有限的空间，你需要负能量——
something nobody knows how to create - to make it work.
一种没人知道怎么创造的东西——才能成功。
Otherwise you'd end up with a black hole. Wait, but what if instead of a black hole, we...
否则你得到的只是一个黑洞而已。但等等如果不是黑洞，我们...
Built a wormhole? Wormholes are hypothetical (but not physically impossible) bridges through space-time,
建造一个虫洞？虫洞是假想的（但在物理上不是不可能实现）穿越时空的桥梁，
shortcuts that can instantaneously connect two different places and times in our universe.
是可以瞬间连接我们宇宙中两个不同的地方和时间的捷径。
If you had a wormhole you might be able to use it to travel into the past or the future.
如果有虫洞，你或许可以用它来穿梭过去与未来。
The problem is that no-one knows how to build a wormhole. Or, once you've built it, how to keep it from collapsing:
问题是没人知道如何建虫洞。或者一旦建了虫洞，如何防止它崩塌：
as Sean Carroll has eloquently written, 'keeping wormholes open requires a form of negative energies.
正如肖恩·卡罗尔雄辩地写道：“保持虫洞畅通需要一种形式的负面能量。”
Nobody knows how to make negative energies,
没人知道如何制造这种负能量，
although they occasionally slap the name 'exotic matter' on the concept and pretend it might exist.'
虽然他们偶尔会贴上‘异物质’这一概念，并假装它是存在的。’
Well that's too bad... but as consolation, welcome to the future!
这太糟糕了，但是作为安慰，欢迎来到未来！
Almost three minutes have passed since the beginning of this video, and we have personal jet-packs now!
从这个视频开始到现在已经过去了将近三分钟，而且我们现在已经有了自己的私人喷气背包！
