Each of the GPS satellites that allow me to navigate to a new restaurant carries an atomic clock that needs to be accurate in order to triangulate the speed and position of my moving car. But there are a couple of problems. Einstein’s Theory of Special Relativity predicts that clocks hurtling through space at satellite speed will appear to tick more slowly than earthbound clocks by about 7,000 nanoseconds each day (a nanosecond is a billionth of a second). His Theory of General Relativity, on the other hand, predicts that clocks farther from a massive object (the Earth), will advance faster than clocks on the ground, in this case by a little more than 45,000 nanoseconds.
If the GPS system fails to compensate for the 38,000-nanosecond difference predicted by an eccentric German physicist, errors in global positioning would increase by about 6 miles each day. In 23 days, my restaurant in Washington, D.C., might mistakenly show up in Philadelphia. This is the oddness of modern physics invading the everyday world.