How is gravity measured

Measurement of the gravitational constant with cold atoms

Gravitation is the most important force in the cosmos - but there is still no agreement about its exact strength. That could change soon: A research team has developed a new method, independent of previous methods, to measure the gravitational constant G. Using laser-cooled atoms in a quantum interferometer, the scientists obtained a value of 6.67191 × 10-11m3kg-1s-2, which is slightly smaller than the current standard value of 6.67384 × 10-11m3kg-1s-2. With a further improvement, the new method could provide an explanation for the discrepancy in previous measurements, according to the scientists in the journal "Nature".

Atomic interferometer as a gravitational balance

"Improving the accuracy of G is not only of metrological interest," emphasize Guglielmo Tino from the University of Florence and his colleagues. "The exact value is important because G plays a key role in particle and astrophysics, in cosmology and even in geophysical models." are not moving towards an ever more accurate value. On the contrary: in some cases they differ significantly more than the measurement errors would suggest.

Most of the experiments are based on the principle of the torsion balance or the torsion pendulum, in which the movement of a test mass is examined under the influence of the attraction of a second, larger mass. For the first time, Tino and his colleagues are not using a macroscopic test mass, but rather individual atoms. With the help of a laser, they cool rubidium atoms down to almost absolute zero. At such a low temperature, the atoms no longer behave like particles but, according to the laws of quantum mechanics, like waves. Tino and his team use the wave properties to measure the energy states of atoms above and below a mass - a total of 516 kilograms of tungsten - in an atomic interferometer with high accuracy. Because the gravitational field of the mass changes the phase relationship between certain energy states in the atom. And from this shift one can deduce the gravitational constant.

The lack of convergence of the earlier measurements suggests that the experimenters overlook or underestimate systematic errors. "A conceptually different experiment like ours can help track down these systematic errors," said Tino and his colleagues. Within six years, the team managed to improve the accuracy of the new method tenfold. Now they could catch up with classic torsion tests. A further improvement by a factor of ten can be achieved, according to the researchers - and thus a “trustworthy value of G” is within reach.