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stronger field. Near the surface of Earth the frequency of an atomic clock increases about one part in 10'° per meter, and, thus, a clock 100 meters higher than a reference clock will have a frequency greater by one part in 1014. Clocks carried eastward around the world in an airplane will differ (very slightly) from a clock at rest on Earth, or one carried westward, since they are in rotation at different speeds about the center of Earth and there is also a difference in gravitational potential. In 1971, in experiments with atomic clocks by the U.S. Naval Observatory, atomic clocks were actually sent around the world in airplanes, one eastward and one westward. The eastward-flying clock lost 0.06 microsecond with respect to one at rest, and the westward one gained 0.27 microsecond, confirming the predicted relativistic effects. I once served on the board of directors of a company that was in the process of acquiring a company which specialized in making cesium clocks. I remember when the president proudly announced that their key product was an atomic clock "accurate to within one second in 100,000 years." I couldn't resist asking two questions: "How do you know?" and "Who cares?" It turns out that the accuracy of such a device can be predicted from its molecular behavior. Furthermore, the accuracy of time measurement is a critical factor in the precision of navigation. The precision of their unique product now makes the Global Positioning Satellite navigation system possible. Most discussions of the physics of time will also mention the interesting case of two hypothetical astronauts born at the same instant. One remains on Earth; the other is sent on a space mission to the nearest star, Alpha Centauri, about 4.5 light years distant. If his vehicle travels at a speed of half the velocity of light, when our traveler returns to Earth he will be more than two years younger than his twin brother! This example is often used to illustrate the dilation of time. Since God has the technology to create us in the first place, he certainly has the technology to get a message to us. But how does he authenticate his message? How does he assure us that the message is really from him and not from a fraud or a contrivance? One way is to demonstrate that the message has its source from outside our time domain. God declares, "I alone know the end from the beginning."*** His message includes history written in advance. We call this "prophecy." An illustrative example is that of a parade. We sit on the curb and observe the many bands, marching units, floats, and other elements coming around the corner and passing in front of 132 A time measurement device in a weaker gravitational field runs faster than one in a , maa wre a ° ert at 8 Pe dete At aa WHO CARES? THE TWIN ASTRONAUTS OUR COMMON MISCONCEPTION Is God subject to gravity? Is he subject to the constraints of mass or acceleration? Hardly. God is not someone "who has lots of time." He is outside the domain of time altogether. That is what Isaiah means when he says, "It is he who inhabits eternity. "