Have you ever daydreamed about traveling through time,
你曾梦想着穿越时空吗，
perhaps fast forward in the centuries and seeing the distant future?
或者穿越到几百年之后及看到遥远未来？
Well, time travel is possible, and what's more, it's already been done.
让我告诉你，穿越时空是有可能的。而且，它已经被实现了。
Meet Sergei Krikalev, the greatest time traveler in human history.
谢尔盖·克里卡列夫是人类历史上最伟大的穿越时空者。
This Russian cosmonaut holds the record for the most amount of time spent orbiting our planet,
这位俄罗斯宇航员创了记录，他是环绕地球时间最久的人，
a total of 803 days, 9 hours, and 39 minutes.
总共803天9小时39分钟。
During his stay in space, he time traveled into his own future by 0.02 seconds.
当他在太空的时候，他时空穿越到自己的未来，且用了0.02秒。
Traveling at 17,500 miles an hour, he experienced an effect known as time dilation,
当时，他在太空时的行驶速度为每小时17500英里，他体验了一种名为'时间膨胀'的效应，
and one day the same effect might make significant time travel to the future commonplace.
而且，有一天这种效应将会让穿越时空变得更平常、时间更持久。
To see why moving faster through space affects passage of time,
如果想了解为何在空间里的快速运动会影响时间流逝的速率，
we need to go back to the 1880s, when two American scientists, Albert Michelson and Edward Morley,
我们得回到19世纪80年代，当时两个美国科学家，阿尔伯特·迈克尔逊和爱德华·莫雷，
were trying to measure the effect of the Earth's movement around the Sun on the speed of light.
尝试测量地球绕太阳的运动对光速有什么影响。
When a beam of light was moving in the same direction as the Earth, they expected the light to travel faster.
当一束光与地球往相同方向移动时，他们预测光速会变快。
And when the Earth was moving in the opposite direction, they expected it to go slower.
当两者方向相反时，光速会减慢。
But they found something very curious.
结果令他们很惊讶。
The speed of light remained the same no matter what the Earth was doing.
不论地球往哪边走，测出的光速都相同。
Two decades later, Albert Einstein was thinking about the consequences of that never-changing speed of light.
20年后，爱因斯坦开始思考这一实验的结果：光速保持恒定不变。
And it was his conclusions, formulated in the theory of special relativity, that opened the door into the world of time travel.
他的结论写在他的相对论当中，开启了时空旅行的大门。
Imagine a man named Jack, standing in the middle of a train carriage, traveling at a steady speed.
试想，一个名为杰克的人，站在一辆火车上，以固定的速率移动，
Jack's bored and starts bouncing a ball up and down.
杰克觉得很无聊，开始在地板上拍球。
What would Jill, standing on the platform, see through the window as the train whistles through?
这个时候，站在站台上的吉尔，当火车鸣笛经过，透过窗户会看到什么？
Well, between Jack dropping the ball and catching it again,
在杰克扔下球又接住球这一过程之间，
Jill would have seen him move slightly further down the track, resulting in her seeing the ball follow a triangular path.
吉尔会看到杰克和火车一起向前移动，导致她看到的球是沿三角形的路线移动。
This means Jill sees the ball travel further than Jack does in the same time period.
这意味着，吉尔看到的球在同一时段里比杰克所看到的移动得更远。
And because speed is distance divided by time, Jill actually sees the ball move faster.
因为速率等于距离除以时间，所以吉尔所看到球比杰克看到的移动得更快。
But what if Jack's bouncing ball is replaced with two mirrors which bounce a beam of light between them?
但如果我们把杰克的球用两面镜子代替，有一束光在镜子之间来回反射。

Jack still sees the beam dropping down and Jill still sees the light beam travel a longer distance,
杰克依然会看到光束射下，而吉尔还是会看到该光束只是移动得更远，
except this time Jack and Jill cannot disagree on the speed because the speed of light remains the same no matter what.
不同的是，这次两人会测出相同的光速，这是因为光速无论如何都是固定不变的。
And if the speed is the same while the distance is different, this means the time taken will be different as well.
如果速率相同而距离不一样，那花的时间也将不相同。
Thus, time must tick at different rates for people moving relative to each other.
所以，当人们相对他人运动时，时间会以不一样的速率流逝。
Imagine that Jack and Jill have highly accurate watches that they synchronize before Jack boards the train.
假设杰克和吉尔都有非常精确的手表，并在杰克上火车前校对好时间。
During the experiment, Jack and Jill would each see their own watch ticking normally.
在实验当中，杰克和吉尔会看到自己的手表正常地走。
But if they meet up again later to compare watches,
但是如果他们在实验结束后再相遇，比对彼此的手表时，
less time would have elapsed on Jack's watch, balancing the fact that Jill saw the light move further.
他们会发现杰克的手表走得较慢一些，因此解释了吉尔看到的光移动得更远这一事实。
This idea may sound crazy, but like any good scientific theory, it can be tested.
这听起来很诡异，但是，就像任何一个完善的科学理论一样，它是可以被检验的。
In the 1970s, scientists boarded a plane with some super-accurate atomic clocks
在20世纪70年代，科学家们校对了一些非常精确的原子钟，
that were synchronized with some others left on the ground.
一些带到飞机上，一些留在地面。
After the plane had flown around the world, the clocks on board showed a different time from those left behind.
在飞机环球飞行之后，他们发现飞机上的原子钟显示的时间与地面的不一样。
Of course, at the speed of trains and planes, the effect is minuscule.
当然，以火车与飞机的速率，这一效应微乎其微。
But the faster you go, the more time dilates.
但是行驶的速率越快，时间膨胀越明显。
For astronauts orbiting the Earth for 800 days, it starts to add up.
如果航天员环绕地球800天，其影响就很明显了。
But what affects humans also affects machines.
而且，影响人类的也会影响机器。
Satellites of the global positioning system are also hurdling around the Earth at thousands of miles an hour.
全球卫星定位系统的卫星，持续环绕着地球飞行，且时速高达数千英里。
So, time dilation kicks in here, too.
因此，在此速率下也会出现时间膨胀效应。
In fact, their speed causes the atomic clocks on board to disagree with clocks on the ground by seven millionths of a second daily.
事实上，GPS卫星的飞行速率，会导致飞行中的原子钟与地上的原子钟之间每天产生7百万分之一秒的差异。
Left uncorrected, this would cause GPS to lose accuracy by a few kilometers each day.
如果不进行校正的话，将会使GPS因此失去准确性，每天产生几公里误差。
So, what does all this have to do with time travel to the far, distant future?
那么，这些与穿越时空回到未来有什么关系呢？
Well, the faster you go, the greater the effect of time dilation.
当你的速率越快，时间膨胀的效应也越明显。
If you could travel really close to the speed of light, say 99.9999%,
如果你的速率真的能够接近光速的话，比方说达到光速的99.9999%，
on a round-trip through space for what seemed to you like ten years, you'd actually return to Earth around the year 9000.
绕太空来回一圈，对你来说可能像过了10年，但是当你回到地球时，将是差不多9000年后了。
Who knows what you'd see when you returned?!
到那时候，谁知道你回来后会看到什么？
Humanity merged with machines, extinct due to climate change or asteroid impact, or inhabiting a permanent colony on Mars.
人类与机器分不开，人类因全球变暖或小行星撞击而灭绝，又或是人类已在火星定居了？
But the trouble is, getting heavy things like people, not to mention space ships,
但问题是，先不提太空飞船，单要把像人一样重的东西
up to such speeds requires unimaginable amounts of energy.
加速到达接近光速，将需要无法想象的巨大能量。
It already takes enormous particle accelerators like the Large Hadron Collider
庞大的粒子加速器，如大型强子对撞机，
to accelerate tiny subatomic particles to close to light speed.
已能把次原子粒子加速到接近光速了。
But one day, if we can develop the tools to accelerate ourselves to similar speeds,
如果有一天我们能发明一些工具，可以让我们自身加速到如光速般，
then we may regularly send time travelers into the future, bringing with them tales of a long, forgotten past.
那么到那时候，我们可能随时可以把时空穿越者送到遥远的未来，并带着我们古老、被遗忘的故事。