No. 116: Mar-Apr 1998
Of the four fundamental forces of nature, gravity was the first to be discovered. Even the Neanderthals knew of it! That's hardly surprising; it's everywhere. Unfortunately, we don't know much more about it than the Neanderthals. Though it seems powerful when you trip and fall, gravity is the weakest of the fundamental four. In a helium nucleus, the force of repulsion between two protons is 1040 times the gravitational attraction between them. Weak though it may be, gravity controls the trajectory of a baseball, the motion of the planets, and the shape of our Galaxy.
Physicists describe gravitation with Newton's Law of Gravitation, which incorporates the Gravitational Constant G. Here's where the embarrassment arises. Many other constants of nature, such as the charge on the electron, are known to eight significant figures. We only know G to three. What's worse, modern attempts to refine the measurement of G come up with wildly different answers. Torsion-pendulum experiments in the U.S., Germany, and New Zealand are far apart in their G-measurements. And physicists are perplexed -- to put it mildly.
Of course, G is hard to measure. Seismic waves from ocean surf hundreds of miles away can affect the experiments. If a colleague a few offices away brings in some boxes of books for his library, the experiment is compromised. Better instrument science may eventually resolve the too-large discrepancies between the measured values of G, but some physicists worry that perhaps the problem runs deeper.
"If experiments find that G is changing slowly over time, for example, physicists would have to rethink how space and time are stitched together in a single fabric. Einstein would groan in his grave."
(Kestenbaum, David; "The Legend of Big G," New Scientist, p. 39, January 17, 1998.)