According to "Physics for Engineers and Scientists by Ohanian and Markert, a second is the time needed for 9,192,631,770 vibrations of a cesium atom. In 1971, Hafele and Keating put four cesium-beam atomic clocks aboard commercial air liners which were flown around the world - two eastward, two westward and compared their times with two clocks which remained at the airport. After the test, the three pairs of clocks disagreed with one another. These results are supported by the experiments done with muons. How long is a second, really? (if the question has a real[?] meaning)? Wouldn't clocks on Mars keep different times from those on earth? What about the length of a meter, which is based on the measurement of the speed of light?