According to Einstein's theory of relativity, a clock on the floor ought to run very slightly slower than an identical one on top of a step stool because the lower clock nestles deeper into Earth's gravitational field. Now, physicists have demonstrated this effect using two super-accurate clocks and hoisting one several centimeters above the other. It's the first time scientists have used clocks to show that time flies faster for your nose than for your navel. “The demonstration of the gravitational shift by elevating a clock about one foot is quite stunning,” says Daniel Kleppner, a physicist at the Massachusetts Institute of Technology in Cambridge, who was not involved in the work. He adds, however, that the demonstration “does not change anyone's view on relativity.”
Einstein realized that time passes at different rates depending on the circumstances. For example, suppose you stand on a train platform with a Rolex on your wrist while a friend wearing an identical watch zooms by in a train. Your friend's watch runs slower than yours simply because he is moving relative to you, Einstein predicted in his theory of special relativity. And according to his theory of general relativity, gravity comes about because massive things like Earth stretch the fabric of space and time. As a result, a clock at lower altitude and, hence, lower gravitational energy, should run slower than one at higher altitude—by about 3 microseconds per year per kilometer of elevation.