GPS, Relatively speaking

[rionapo]

From a NY Times article on Einstein:

Twenty years later, theoretical physicists briefed United States Air Force generals on a subtle complication with a new military technology, the Global Positioning System. Effects from Earth’s gravity would be stronger on the ground than in orbit, the physicists explained, and hence clocks on the ground would tick more slowly than those aboard satellites. If the clocks disagreed on time, they would also disagree on space, and that could spell trouble for this technology. If left uncorrected, the tiny differences in clock rates would snowball into enormous errors in determining distances. With GPS, the warping of time that Einstein imagined assumed operational significance. (Later, GPS was opened to the commercial market, and now billions of people rely on general relativity to find their place in the world, every single day.)

Don