Page 11 - Gravity and Time: It’s All Relative You know the old saying ‘a watched pot never boils’. This refers to how a certain amount of time can seem long or short, depending on what you’re doing. We all know this from experience but we also know (or think we know) that in reality time moves along at the same exact rate for everyone, no matter where they are or what they are doing, right? Wrong. According to Einstein’s general theory of relativity, the deeper an object is inside a gravitational field, the slower time will move for that object! This idea may sound completely bogus. After all how could such a thing be true that so blatantly contradicts common sense? But if we trusted common sense, remember, we would still believe that the Earth is flat and that the Sun orbits around it. In truth, your location inside a gravitational field does affect the speed at which time passes for you when compared to things farther inside or outside that field. To illustrate this point, lets take a pair of twins separated at birth. One lives his life on top of a mountain while the other stays at sea level. If they meet again many years later, the twin who lived on the mountain, farther away from Earth’s gravity will actually be older than the twin who lived at sea level (but not by too much)! This, of course, is a version of the classic twins paradox which is more often used to illustrate the effects that traveling at high speeds has on the passage of time. It is only a paradox, however, if you believe in absolute time. Scientist first proved the prediction of the slowing down of time inside a gravitational field by conducting this simple experiment. To begin with they took two extremely accurate clocks and set them to be exactly the same. Then, they took one clock to the top of a high tower. When they brought it down again and compared it to the clock that had stayed on the ground, the readings were different. This could only mean that time had passed at different speeds for the two clocks! Believe it or not, this strange concept is actually used in practical science. For example, when putting together super-accurate navigational systems that require the use of satellites high above the Earth, engineers must take into account this slowing down of time. Otherwise, the results that the navigational systems give could be off by as much as a few kilometers! For more on time and black holes see Large Scale Effects Page 9.