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We take navigation for granted these days.
现在人们对GPS已经习以为常。
GPS receivers guide airplanes, cars, and even cell phones!
GPS可以为飞机,汽车甚至手机导航！
But did you know that the Global Positioning System is basically a big clock in space?
但你知道全球定位系统就像是宇宙中的一个巨大的时钟吗？
There are thirty GPS satellites in orbit,
绕地轨道上共有30颗GPS卫星，
and they just broadcast where they are and what time it is.
它们会随时发送自己的位置和时间。
All your phone GPS has to do is receive signals from four satellites
你的手机GPS所要做的就是接收来自四个卫星的信号，
and it can triangulate its location in the four dimensions in which we live,
就能建立一个你所在点的四维坐标，
three space and one time.
三条空间轴和一条时间轴。
But actually it's not that simple! In order for navigation to work,
但其实没那么简单！为了精确导航，
the satellites carry atomic clocks accurate to the nanosecond,
卫星的原子钟必须精确到纳秒。
otherwise your GPS receiver might tell you you're halfway across town when you're still in the driveway.
否则你的车还没启动，GPS就报告你已经上路了。
And special relativity tells us that moving clocks run slow,
狭义相对论告诉我们运动中的时钟会变慢，
while general relativity tells us that clocks run faster higher in a gravitational field.
而广义相对论则告诉我们引力场会使高处的时钟走得更快。
These effects don't quite cancel, general relativity wins out
这些影响并不能抵消，而广义相对论的影响更大，
and time indeed runs faster up in orbit with the satellites!
所以卫星的时间确实会变快！
But some of the engineers working on the first GPS satellite
但是，一些负责设计第一颗GPS卫星的工程师
couldn't bring themselves to believe that their clock would actually run fast just from being higher up,
固执的认为，时钟不会因为离引力场越远而走得更快，
so they sent it up uncorrected!
所以他们把错误时间的卫星送了上去！
Within minutes it was off by enough to impair GPS navigation,
几分钟后，GPS已经不能定位，
and by the end of the day, GPS receivers would have been wrong by tens of kilometers!
一天下来，卫星偏离了数十公里！
Needless to say, the engineers turned the correction back on,
不用说工程师们又重新校正了时间，
and these days they trust general relativity!
现在他们相信广义相对论！
Oh, and one last thing - GPS is also a nuclear weapons detector:
哦，还有最后一件事，GPS还可以监测核武器：
there are always at least four GPS satellites visible from any point on earth,
地球上任何一点都有四颗卫星同时监视，
and because of this, any nuclear detonation will be seen by enough satellites
正因如此，这些卫星足以观测到任何核爆，
to pinpoint exactly where and when it took place.
从而精确地确定爆炸发生的地点和时间。
