Hidden extra dimensions are causing measurements of the strength of gravity at different locations on Earth to be affected by the planet's magnetic field, French researchers say.

The two most accurate measurements of Newton's constant (G) contradict each other

This is a controversial claim because no one has ever provided experimental evidence to support either the existence of extra dimensions or any interaction between gravity and electromagnetism. But lab measurements of Newton's gravitational constant G suggest that both are real.

Newton's constant, which describes the strength of the gravitational pull that bodies exert on each other, is the most poorly determined of the constants of nature. The two most accurate measurements have experimental errors of 1 part in 10,000, yet their values differ by 10 times that amount. So physicists are left with no idea of its absolute value.

Now Jean-Paul Mbelek and Marc Lachieze-Ray of the French Atomic Energy Commission near Paris say they can resolve the contradiction by taking into account the location of the labs where the experiments were carried out.

The pair suggest that electromagnetism and gravity influence one another enough for gravity's pull to be noticeably affected by the Earth's magnetic field.

String theory

Their work is based on theories such as string theory that try to unify all the forces, including electromagnetism and gravity, by invoking the existence of several extra spatial dimensions.

In a paper submitted to Classical and Quantum Gravity and presented at a meeting of the European Astronomical Society in Porto, Portugal, the researchers calculated the values they would expect G to have at different locations around the world. They say it should be greater where the Earth's magnetic field is stronger, with the highest measurements at the north and south magnetic poles.

The values of G measured so far seem to fit with that idea. But the researchers say the best way to test their theory would be to take accurate measurements of G at locations such as the magnetic poles and particular longitudes on the equator, and then check those values against the predictions.

Studies of the Sun also support the theory. To make mathematical models of the star's interior tally with experimental data, physicists have to use a lower value of G than is traditionally agreed. Mbelek says his calculations predict that electromagnetism would not boost gravity as much at higher temperatures, so you would expect G to be lower inside the Sun.

Exotic physics

But other researchers are not convinced. Clifford Will, a gravity theorist at Washington University in St Louis, Missouri, believes improvements in terrestrial experiments will eventually do away with the need for explanations that rely on such exotic physics.

"In many ways it's a scandal that we don't have an agreed value for G, but if you look at the experiments, the values have been converging," he says. "In five years or so, we'll have an agreed value."

But Mbelek does not think so. Although the precision of individual measurements is improving, he says, the values are not converging.

Michael Brooks, Porto
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