I read recently that “time . . . passes more quickly when gravity is reduced.” Assuming gravity is reduced at higher altitudes, that means time goes by faster in Santa Fe than in Poughkeepsie. What’s up with that? —Chris in SF, NM

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    You heard right, Chris. But unless you’re way more anal than anybody I want living in my reference frame, you won’t have to reset your watch. Due to the “warpage of time,” clocks run slower in Poughkeepsie than Santa Fe by about a millisecond. Per century. (Get friendly with a black hole and it’s another story—gravitational time dilation approaches infinity as you near the event horizon. However, notwithstanding sporadic distortions of space-time due to Taos, Los Alamos, Roswell, etc, we’ll assume that’s not a problem for you in New Mexico.) Time dilation affects not just ordinary clocks but any measure of time, including how long it takes to say “one Mississippi, two Mississippi.” So you’ll never notice anything odd about SF time, only about that of people living under different gravity conditions, e.g., Poughkeepsie, Hoboken, or other low burgs.



    1. Imagine you’re in a spaceship far from any source of gravity. The ship is moving in a straight line at constant speed, so you float in the center of the cabin. Now imagine your idiot brother at the controls unexpectedly turns on the rocket booster, accelerating the ship. The rapidly approaching back wall is now indistinguishable from a floor you’re falling toward under gravity.



    Mere nanoseconds, you say—who gives a flying clock? You do, if you use the global positioning system. Because of their altitude, the clocks on GPS satellites run about 30 nanoseconds fast per minute due to gravitational effects. Since the system works by timing light signals and the distances involved are great, an uncorrected time error would mean a distance error growing at about 9.5 meters per minute. You may think it’s amazing you can hike in the Sangre de Cristos with a $300 GPS receiver that tells you exactly where you are. What’s more amazing is that the geniuses who designed it needed a rough knowledge of general relativity to get it to work.