The gravitational constant might not be that… constant (Image: Noël Gaspard/Millennium Images)

Did gravity, the force that pins us to Earth’s surface and holds stars together, just shift? Maybe, just maybe. The latest measurement of G, the so-called constant that puts a figure on the gravitational attraction between two objects, has come up higher than the current official value.

Measurements of G are notoriously unreliable, so the constant is in permanent flux and the official value is an average. However, the recent deviation is particularly puzzling, as it is at once starkly different to the official value and yet very similar to a measurement made back in 2001, not what you would expect if the discrepancy was due to random experimental errors.

It’s possible that both experiments suffer from a hidden, persistent error, but the result is also prompting serious consideration of a weirder possibility: that G itself can change. That’s a pretty radical option, but if correct, it would take us a step closer to tackling one very big mystery – dark energy, the unknown entity accelerating the expansion of the universe.


“If G has changed by this tiny amount then we would expect that G depends on a new field,” says cosmologist Tony Padilla of the University of Nottingham, UK. “One could imagine a scenario in which this field plays a role in dark energy.”

Twisting wires

According to Isaac Newton, the gravitational attraction between two objects is proportional to their masses and inversely proportional to the square of distance between them. G puts an absolute value on the attraction.

It was first measured in a laboratory in 1798 by British scientist Henry Cavendish using a device that determines the twisting of a wire due to the gravitational attraction of two pairs of precisely known masses.

Since then, other methods have produced a multitude of different values. This is assumed to be due to various experimental errors and the official value of G is routinely updated to reflect this, with the assumption that the values will eventually converge.

Now a team led by Terry Quinn of the International Bureau of Weights and Measures (BIPM) in Paris, France, and Clive Speake of the University of Birmingham, UK, has measured G using two methods: a modern version of the Cavendish experiment and one that relies on electrostatics. The resulting value for G is 240 parts per million bigger than the official one, set in 2010.

Violets in springtime

The figure alone is not the weird part – one recent measurement came up 290 ppm below today’s official value. The strange thing about the latest one is that it is just 21 ppm off the value Quinn’s team got using the same set-ups in 2001. Since the team took care this time around to remove every source of error that might have been at play back then, you would not expect the two results to be identical.

Quinn has arranged a special conference on G at the Royal Society in London in February to discuss the problem.

“This meeting is going to be very exciting,” says James Hough, an experimental physicist from the University of Glasgow, UK. But he suggests carrying out the experiment a third time. “My own view is that the BIPM experiment needs to be copied exactly in another laboratory on a different continent by different experimenters initially to see if the same result is obtained,” he says.

However, James Faller of the University of Colorado at Boulder, who tested G in 2010, is holding out for an error: “Errors are like violets in the springtime: they can spring up in any group’s experiment,” he says.

Fifth force

But the latest result could also be evidence that gravity itself may be changing.

“Logically, either some of the experiments are wrong, or G is not constant,” says Mark Kasevich of Stanford University.

An oscillating G could be evidence for a particular theory that relates dark energy to a fifth, hypothetical fundamental force, in addition to the four we know – gravity, electromagnetism, and the two nuclear forces. This force might also cause the strength of gravity to oscillate, says Padilla. “This result is indeed very intriguing.”

A further, less radical option is that G is still a constant but that Quinn’s team has hit upon its true value. That would mean the actual value of G is higher than the official figure, which is interesting in itself, says cosmologist Clare Burrage of Nottingham University.

“If the value of G is slightly bigger, then we have to go back and redo all the calculations,” she says. “Stars would burn up quicker than we previously thought because it takes more energy to push against a stronger gravitational force.”

Journal reference: Physical Review Letters, doi.org/nqw