What would Jill, standing on the platform, see through the window as the train whistles through?
这个时候站在月台上的吉尔会透过窗口所看到是什么？
Well, between Jack dropping the ball and catching it again,
在杰克扔下球和接着球之间，
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，结果她看到的球是以三角形的路线移动的。
This means Jill sees the ball travel further than Jack does in the same time period.
意思是，在同一个时间段里吉尔看到的球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.
因为速度等于距离除以时间，事实上，吉尔所看到球比杰克所看到的移动得更快。
But what if Jack's bouncing ball is replaced with two mirrors which bounce a beam of light between them?
但是，如果我们把杰克的球用两面镜子作代替，而在镜子之间有一束光来回反弹。
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反弹，吉尔还是会看到该光束移动得更远，
except this time Jack and Jill cannot disagree on the speed because the speed of light remains the same no matter what.
例外的是，这次杰克和吉尔却会对光的速度有分歧。這是因为光的速度。无论如何还是保留不变。
And if the speed is the same while the distance is different, this means the time taken will be different as well.
如果速度是一样的话，而距离却不一样，那花的时间将也不一样。
Thus, time must tick at different rates for people moving relative to each other.
所以当人们对他人相对地移动时，时间一定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.
试想如果杰克和吉尔都有非常精确的手表。他们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.
当这个实验中，杰克和吉尔会看到自己的手表正常的走。
But if they meet up again later to compare watches,
但是如果他们在实验结束后相遇，比对彼此手表时，
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会走慢一些，因此解释了这事实：吉尔看到的光移动得更远。
This idea may sound crazy, but like any good scientific theory, it can be tested.
这听上去很诡异，但是，像任何一个完好的科学理论，它是可以被测试的。
In the 1970s, scientists boarded a plane with some super-accurate atomic clocks
在1970年左右，科学家们校对了一些超级准确的原子钟，一起带到飞机上去，
that were synchronized with some others left on the ground.
而数个原子钟则留在陆地上。
After the plane had flown around the world, the clocks on board showed a different time from those left behind.
当飞机环球飞行之后，他们发现飞机上的原子钟和地球上的原子钟上显示的时间是不一样的。
Of course, at the speed of trains and planes, the effect is minuscule.
当然，以火车与飞机的速度，对时间膨胀的影响是很微小的。
