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It is one of nature’s most fundamental numbers, but humanity still doesn’t have an accurate value for the gravitational constant. And, bafflingly, scientists’ ability to pinpoint G seems to be getting worse. This week, the world’s leading gravity metrologists are meeting to devise a set of experiments that will try to set the record straight. This will call for precision measurements that are notoriously difficult to make — but it will also require former rivals to work together.

In his 1687 Philosophiæ Naturalis Principia Mathematica, Isaac Newton outlined his theory of gravity: that the force between two objects is proportional to the product of their masses and inversely proportional to the square of the distance between them. G is the constant by which the masses must be multiplied to put an absolute value on that attraction. But more than 300 years later, the constant — known as Big G to distinguish it from little g, the acceleration due to gravity on Earth — is known for sure to only 3 significant figures (6.67384(±0.0008) × 10−11 m3 kg−1 s−2).

Independent groups of physicists have been trying to pin down the true value of the constant for decades, but in recent years, rather than converging on an ever more precise figure, the results of their experiments have been diverging, causing the uncertainty in the official figure to rise (see ‘Trouble with Big G’). “There’s no other fundamental constant of physics where we’ve had such a wide dispersion of results,” says Terry Quinn, former director of the International Bureau of Weights and Measures (BIPM) in Paris. Some or all of the experiments must either grossly underestimate their uncertainty or miss significant systematic errors, he adds.

On 9 and 10 October, metrologists will meet at the US National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland, to devise a plan to resolve the issue.

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