Everybody has a GPS nowadays right? You know how GPS works? The GPS you have is just a receiver that receives timing signals from several satellites. On those satellites, there are extremely accurate atomic clocks sending out time signals, as well as position of that satellite. When your GPS receives several signals from at least 3 different satellite, it can compare the signals and based on the time label on each signal and it's satellite location, and speed of light, calculate your exact location on earth surface. Sounds simple enough right?
Well, when USA first launched these satellites and try this concept on their military projects such as cruise missles, they found the error is huge. Base on the theory it should be able to pin point the location of the GPS receiver within a few meters, but in reality the result is off by a few Kms. It's so inaccurate that the military don't want to use it, and called it a failure. Later they found the problem. Based on General relativity, time passes more slowly at a more intense gravity field. So the clock on the GPS on earth surface has a clock runs more slowly than the atomic clocks on those satellites, which runs on earth orbit and is thus further away from earth's gravitation center. The differences are really small though, and hard to notice until you try to use the precise time to calculate the exact position of a missle, or vehicle. After modifying the clocks on the satellites based on the calculation done with General Relativity, the GPS system is now very accurate.
So next time you go on a trip, take one more look at your GPS because it's one of the many proofs that General Relativity is CORRECT.
