Explanation for G’s imprecision stumbles

Tiny changes in Earth’s rotation rate could explain physicists’ inability to precisely measure a key fundamental constant of nature, a study in the April EPL proposes. Physicists say the idea would be extremely compelling — if not for some confusion with dates that probably derails the findings.

A graph in the paper shows that the measured values of Newton’s gravitational constant, G, peak and trough in lockstep with a nearly six-year cycle of fluctuations in the duration of Earth’s roughly 24-hour days. The researchers don’t suggest that the value of G is actually changing but say an unknown geophysical mechanism could influence experiments. An unpublished reanalysis of the data, however, makes the researchers’ argument far less convincing. It’s the latest twist in a long, frustrating effort to pin down one of the universe’s cagiest constants.

The gravitational constant is a measure of gravity’s strength. As defined by the Committee on Data for Science and Technology, a pair of one-kilogram spheres whose centers are a meter apart exert a gravitational force of 66.7384 trillionths of a newton on each other.

Despite more than two centuries of measuring G, though, nobody is confident in the current value. Since 2001, experiments tracking the tiny gravitational attraction between objects have produced values like 66.7234 trillionths and 66.7559 trillionths. Those are big discrepancies when compared with measurements of other constants.

John Anderson, a retired astronomer formerly at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., was reading an article about G when he noticed a pattern in 13 measurements taken over three decades: The values rose and fell in a 5.9-year cycle. Then he uncovered a recent finding that Earth’s daily rotation rate deviates by as much as 0.15 milliseconds from its expected value, also in a 5.9-year cycle (SN: 8/24/13, p. 11). Plotting the two phenomena on the same graph revealed a seemingly slam dunk relationship.

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The study impressed Stephan Schlamminger, a metrologist at the National Institute of Standards and Technology in Gaithersburg, Md. But when he looked for his 2001 G-measuring experiment on the graph, he found the dot placed in 2006, the year he published a paper about the result. Anderson says he did his best to determine the dates of each experiment, but many studies don’t specify when the work was done.

Over the last few weeks, Schlamminger and colleagues have corrected the dates of some experiments and added other published values of G to create a more complete and accurate dataset. In a revised graph, which will appear in an upcoming paper, the connection between G and Earth’s spin largely disappears. “When you replot it, it’s not as compelling anymore,” Schlamminger says.

If an association remains, physicists would have to prove a cause-and-effect relationship by devising a physical mechanism that could manipulate the G experiments. Study coauthor Virginia Trimble, an astronomer at the University of California, Irvine, notes that the rate at which Earth or any object spins depends on the distribution of its mass. She speculates that migrating material in Earth’s core impacts rotation rate and causes small changes in gravitational pull on the surface, which could influence sensitive experiments. Scientists would also have to explain why measurements of G would be attuned to the 5.9-year rotation cycle but not to larger variations in Earth’s spin rate that occur over a year and over decades.