Gravity keeps us on the ground and keeps the planets orbiting the sun. But measuring this basic force exactly is extremely difficult; there are still large deviations in the values for the gravitational constant G. Physicists have now repeated an experiment from 2014 using the same apparatus and checked it using new analytical methods. The result: The newly measured gravitational constant deviates significantly from the old result – but also does not agree with the international reference value set by CODATA. The new measurement does not solve the problems surrounding the gravitational constant, but it advances measurement technology and provides insights into possible sources of error, as the researchers explain.
Of the four fundamental forces, gravity is the most mysterious. Because this attraction between masses is the only one for which no mediating particle has yet been found. Isaac Newton already described this natural constant in his law of gravity and thanks to Albert Einstein’s general theory of relativity we can describe the effect of gravity as a curvature of space-time. But to this day, science has difficulty determining the gravitational constant G precisely. “Despite its central role, G has so far only been determined with an unusually high level of uncertainty,” explain Stephan Schmelzinger from the US National Institute of Standards and Technology (NIST) in Maryland and his colleagues. Although scientists have been trying to measure the exact value of this force for 225 years, using increasingly sophisticated methods, the results are far apart – farther apart than for any other basic force.
(Video: NIST)
There has been a confusion of values since Henry Cavendish
This raises the question of whether there are only measurement uncertainties behind the deviations in the gravitational constant or whether there are fundamental gaps in our understanding of gravity. “One way to expand our knowledge is to develop ever new, more precise experiments,” explain Schmelzinger and his team. “However, these alone provide little benefit: the previous, inconsistent results continue to dominate the weighted average and additional data alone cannot resolve the inconsistencies between existing measurements.” A better strategy is therefore to check outliers among the previous results – for example by repeating the measurements using, if possible, the same apparatus.
This is exactly the approach that was decided at a metrology workshop in 2014. Researchers from the central metrology organization, the Bureau International des Poids et Mesures (BIPM) in France, had just presented new measurement results for G, which they had determined using a special torsion measuring apparatus. This measuring apparatus is based on an experiment by the British researcher Henry Cavendish from 1797. The core of his apparatus was a torsion pendulum – a thin, horizontally suspended rod with weights at both ends. This pendulum hung over two lead balls measuring around 30 centimeters in size and weighing 160 kilograms. When one of these weights came into the sphere of influence of the lead ball, it was slightly tightened and this movement was transmitted via the rod to the other weight. Its deflection could be measured.
The BIPM torsion balance also works according to a similar principle. Four cylinders on a rotating outer ring serve as measuring masses. In the inner area, four smaller measuring masses stand on a disk suspended from a hair-thin copper-beryllium thread. The entire experiment is housed in a vibration-damped and insulated vacuum chamber. As in the Cavendish experiment, the minimum deflection of the inner disk caused by the measuring masses is determined.
Repeated experiment provides another “outlier”
The value for the gravitational constant measured with this apparatus at the BIPM in 2014 was significantly higher than the reference value of the CODATA Committee, the body that sets the globally valid reference values for natural constants and base units. The researchers therefore decided to bring this measuring device to the USA and repeat the measurements at NIST. “Replicating the experiment gave us the chance to check it and find possible systematic errors,” explain Schmelzinger and his colleagues. However, they not only used the same apparatus, they also supplemented the procedure with some additional measurements. In a measurement variant, they applied an electrical voltage to the internal masses, which twisted the copper-beryllium thread – counter to the rotation caused by the gravitational effect of the measuring masses. By determining how high the voltage needs to be to precisely balance this attraction, physicists were able to measure the value of the gravitational constant in an alternative way. They also carried out the test with measuring weights using both copper weights and sapphire weights.
After a good ten years of measuring and evaluating, Schmelzinger and his team have now published their results. Accordingly, they arrive at a value of 6.67387 x 10 for the gravitational constant G-11 meter3/ kilogram-1/Second2. “Our result therefore confirms neither the previous BIPM measurement nor the current CODATA recommendation,” the physicists state. While the BIPM value for G was around 1.7 ten thousandths above the CODATA reference value, the new result is now 6.4 hundred thousandths lower. Although the team identified some systematic measurement uncertainties, these only reduce the deviation slightly. “In conclusion, we note: Even if the diagram of the values determined for G appears even more confusing due to our results, we hope that we have at least brought more clarity with regard to the approach, technology and implementation,” conclude Schmelzinger and his team. This means that the gravitational constant once again proves to be a tough nut to crack, even for metrology measurement specialists.
Source: Stephan Schmelzinger (National Institute of Standards and Technology (NIST), Gaithersburg, USA) et al., Metrologia, doi: 10.1088/1681-7575/ae570f