News Feature: The curious case of the gravitational constant.

In 1763, surveyors Charles Mason and Jeremiah Dixon set off from Philadelphia to demarcate the border between Pennsylvania and Maryland. On the other side of the Atlantic, British scientist Henry Cavendish worried about the effect of the Allegheny Mountains on Mason’s and Dixon’s measurements, which would eventually establish the famous Mason–Dixon line. Cavendish reasoned that the gravitational attraction of the mountains would pull at the theodolite plumbbob that Mason and Dixon were using, enough to throw off their measurements of local latitudes by as much as 200 meters (1). Such analysis set Cavendish down a path to performing one of the first high-precision experiments in physics. Using a newly designed device called a torsion balance, Cavendish measured the almost-infinitesimal gravitational attraction between two spheres of lead. His experiment allowed physicists to calculate a value for the gravitational constant—often called Big G to differentiate it from little g, the acceleration due to gravity— for the first time since IsaacNewtonwrote down his law of gravity approximately a century earlier. Cavendish’s brilliant insights are more obvious in light of modern experiments. More than 200 years and 350 experiments later, scientists know the gravitational constant to a precision only about 1,000-times better than could be calculated from Cavendish’s data. “For any other experiment in physics, this would be a real surprise or shame,” says physicist Guglielmo Tino of the University of Florence in Italy. That’s because the majority of physical constants are known with extreme precision—often out to 9, 10, or even 12 digits. But G stops at five digits. Worse, many of the measurements disagree with one another. It’s no wonder that physicists are continuing to develop new techniques to accurately measure this fundamental constant. “In some sense gravity is the most basic force that humans experience, and yet it’s the hardest to measure,” says physicist Holger Müller of the University of California, Berkeley. “As a physicist it makes me uncomfortable that we can’t measure it better.” Does our inability to pin down the gravitational constant result from subpar means of measurement, or might it actually signal something deeper about the nature of reality? There’s the possibility that new physics waits to be uncovered, perhaps a variable gravitational constant, hidden extra dimensions, or even the long sought after unification of Einstein’s general relativity with quantum mechanics.