TAKING THE EARTH’S PULSE

In looking for aberrations, sometimes it’s wise to start with the familiar. To determine whether or not the gravitational constant is changing, we need look no farther than Earth itself. Aspects of Earth’s dynamics, such as its rate of spin, are sensitive measures of gravity’s strength. Hence, geophysics provides us with another credible means of testing Dirac’s hypothesis and comparing its various incarnations.

By virtue of its rotation, Earth represents a kind of cosmic clock that has been ticking for about 4.5 billion years. During that time it has been gradually slowing down by a rate of about two milliseconds per century. By considering the various processes that could possibly contribute to this lag, we could theoretically deduce information about long-term cosmological effects—such as changes in G. In practice, this is a complicated problem because there are numerous mundane effects that contribute to the slowdown. Scientists believe that much of the deceleration stems from ocean tides, caused by the Moon’s gravitational attraction. Over time these tugs dissipate energy and gradually decrease Earth’s rotational speed.

Newton’s laws, applicable to the Earth-Moon system, mandate that angular momentum (the mass of each body times its rotational velocity times its distance from the center of rotation) must be conserved. Hence, as Earth has slightly slowed down, the Moon has compensated by speeding up a bit. This has resulted in the Moon receding, ever so slightly, over the eons. Consequently, by measuring the Moon’s orbital motion, we can obtain a precise record of Earth’s rotational slowdown, which we can then use to measure any change in the strength of gravity.

There are a number of ways to track the Moon’s behavior. The most direct method goes back to the APOLLO project, mentioned earlier, and its predecessors. By beaming a laser pulse to a mirror on the Moon (placed there by astronauts in 1969) and measuring the return time, scientists have developed precise records of the Moon’s



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