## Relativity and GPS

About 12,500 miles above the Earth, orbit 24 special satellites with super-accurate clocks are ticking away, if you can call them ticks at a billion per second. Image: NASA

Does your car talk to you? Does it say things like “In half a mile, turn left”?

How on Earth does it know where you are? Is it magic or science? Science, of course, and the answer is not “on Earth” but “in the sky.”

About 12,500 miles above the Earth, orbit 24 special satellites 24 special satellites with super-accurate clocks are ticking away, if you can call them ticks at a billion per second.

The satellites beam those ticks, and coded information to identify themselves, down to Earth by radio at the speed of light (which is the speed of radio waves, and X-rays too). If the GPS (Global Positioning System) receiver in your car or cellphone can detect the ticks from three or more of those satellites, your tiny clever computer can figure out (wow, science!) where you are to within a few yards from the time differences of the ticks. It does this even though the satellites up there are all zipping along in different directions at about 7,000 miles per hour, a hundred times faster than you are supposed to be doing in your car.

While we all make mistakes, this system is not allowed to, or you might arrive at the wrong party, and planes might land at the wrong airports. But the GPS would go terribly wrong were it not for a couple of corrections proposed by Professor Einstein a hundred years ago. He was thinking (very hard!) about falling elevators in space and knew nothing about artificial satellites, because he died two years before the first: Sputnik One.

But Einstein predicted that fast-moving clocks tick more slowly, while clocks falling freely tick faster, than those on the ground. That is, relative to you, standing still on the ground. Without those corrections the GPS would be useless.

That’s relativity for you, magic or science, or maybe both.