We take navigation for granted these days - GPS receivers guide airplanes, cars, and
even cell phones!
But did you know that the Global Positioning System is basically a big clock in space?
There are thirty GPS satellites in orbit, and they just broadcast where they are and
what time it is.
All your phone GPS has to do is receive signals from four satellites and it can triangulate
its location in the four dimensions in which we live – three space and one time.
But actually it's not that simple!
In order for navigation to work, the satellites carry atomic clocks accurate to the nanosecond
– otherwise your GPS receiver might tell you you're halfway across town when you're
still in the driveway.
And special relativity tells us that moving clocks run slow, while general relativity
tells us that clocks run faster higher in a gravitational field.
These effects don't quite cancel – general relativity wins out and time indeed runs faster
up in orbit with the satellites!
But some of the engineers working on the first GPS satellite couldn't bring themselves to
believe that their clock would actually run fast just from being higher up, so they sent
it up uncorrected!
Within minutes it was off by enough to impair GPS navigation, and by the end of the day,
GPS receivers would have been wrong by tens of kilometers!
Needless to say, the engineers turned the correction back on, and these days they trust
Oh, and one last thing - GPS is also a nuclear weapons detector: there are always at least
four GPS satellites visible from any point on earth, and because of this, any nuclear
detonation will be seen by enough satellites to pinpoint exactly where and when it took