Scientific Method —

Gravity’s strength still an open question after latest measurement

Little agreement among measures of Newton's G.

You might expect that, all these years after Newton, we might have a good measure of his gravitational constant, G. As the authors of a new paper on the topic note, there are plenty of reasons to want a good measure of G "given the relevance of the gravitational constant in several fields ranging from cosmology to particle physics, and in the absence of a complete theory linking gravity to other forces."

Yet most of our measurements of G come from an updated version of a device designed by Henry Cavendish back in the 1700s. And rather annoyingly, these measurements don't agree with each other—they're all close to a single value, but their error bars don't consistently overlap. Now, researchers have made a new measurement of G using a method that certainly wasn't available in the 1700s: interference between clouds of ultracold atoms. And the value that they have come up with doesn't agree with many of the other measurements, either.

The gravitational attraction being studied here is that between a cloud of cold rubidium atoms and a 500 kg tungsten weight. The tungsten was arranged in a cylinder that surrounded the device that contained the rubidium atoms. It could be shifted up to pull the atoms back against the downward force of the Earth's gravity or shifted down to accelerate the atoms further.

Using light, a single cloud of atoms was split into two populations and propelled upward through a vacuum chamber—the authors refer to "using the ‘moving-molasses’ technique" to shift the atoms. The two populations move to different heights and then return to the bottom of the chamber. There, due to the quantum nature of the atoms, they create an interference pattern. If anything alters the trajectory of the atoms, such as the gravitational attraction of the large chunk of tungsten used in these experiments, it will show up as changes in the interference pattern.

This sort of device, called an atomic interferometer, is extremely sensitive, meaning it's also very sensitive to environmental noise. So most of the hard work went into controlling for this noise, which involved repeated measurements using two different devices and shifting the tungsten weights above and below the devices. Even so, there were still significant uncertainties left from things like the evenness of the chamber and the weight of the bench the devices sat on.

In the end, the experimental uncertainty is 150 parts-per-million, a value the authors think they can reduce considerably in future work. The value of G ends up being 6.67191 x 10-11 cubic meters per kilogram per second-squared.

That's in keeping with fewer than half of the previous measurements, most of which were made using torsion bar devices. Of course, those measurements don't all agree with each other, so this isn't as much of a problem as it might seem.

The advantage of an entirely new type of measurement is that it should have a completely different set of experimental errors. And that's rather important, because as far as we know, there's no reason to expect that G would have any particular value—it's just something we have to measure. And because gravity is such a weak force, those measurements have consistently proven challenging.

Nature, 2014. DOI: 10.1038/nature13433  (About DOIs).

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