A decade-long experiment at NIST has yielded a new value for G that doesn't fully match a previous French measurement, highlighting how difficult it is to measure gravity accurately.
One of the most fundamental numbers in physics is also one of the most difficult to measure. It is the universal gravitational constant, denoted by the letter G, which determines the strength of the gravitational force between bodies in the universe. Despite its enormous importance, scientists still cannot agree on it with high accuracy, and even today it is the least precise of the constants associated with the four fundamental forces in nature.
Not to be confused with The Big G And little gWhile G is a universal constant that should be the same everywhere in the universe, the small g describes the local acceleration of gravity. On Earth, its value is approximately 9.8 meters per second squared, while on the moon it is only 1.62 meters per second squared, because the moon's mass is smaller and therefore its pull is weaker. G, on the other hand, is the number that allows us to calculate the gravitational force between any two bodies, from a person and a planet to two weights in a laboratory.
A new study by the United States National Institute of Standards and Technology, NIST, published in the journal Metrology, did not solve this mystery. On the contrary: after a decade of measurements and analyses, the team of researchers obtained a value that does not fully match one of the previous precise measurements, made in 2007 at the International Bureau of Weights and Measures in France, BIPM. The new value the researchers obtained is 6.67387×10⁻¹¹ cubic meters per kilogram per second squaredand is low in0.0235% From the value measured in the French experiment.
On the surface, this is a very small difference, one that doesn’t affect everyday life, weighing food, or measuring body weight. But in science, such small discrepancies can hint at a deeper problem: either there are systematic errors in the experiments that haven’t been detected, or all the sources of uncertainty in the gravity measurement itself haven’t yet been fully understood. The researchers note that the differences between the measurements are about one part in 10,000—small, but too large to be attributed simply to normal experimental noise.
The difficulty stems from the fact that gravity is very weak compared to other forces in nature. The article explains, for example, that a magnet the size of a pinhead can lift a paper clip with a force greater than the Earth's gravitational pull on that paper clip. Therefore, when trying to measure the force of attraction between relatively small bodies in the laboratory, the measured signal is incredibly weak. Researchers are forced to work with tiny masses, about 500 billion trillion times smaller than the Earth, so any small disturbance can disrupt the result.
Torsion balances
To see if the French measurement could be replicated, physicist Stefan Schleminger and his colleagues at NIST reconstructed a precise experiment based on Torsion balances — A classic method that dates back to Henry Cavendish’s experiment of 1798. This method examines how a very thin fiber rotates or twists in response to a tiny gravitational force between masses. In the modern experiment, eight metal cylinders were placed: four large masses on a rotating structure and four smaller masses on an inner disk, connected to a very thin copper-beryllium strip, about the thickness of a human hair.
The team didn't stop with just one method. In addition to measuring the torsion caused by gravity, the researchers also applied a counter-electrostatic force using electrodes and an electric voltage. When they adjusted the voltage so that the system stopped rotating, they were able to calculate G from the electrical balance as well. They also repeated the measurements with masses made of copper and with masses made of sapphire, to see if the composition of the material affected the result. According to the report, the two sets of measurements gave almost identical results.
One of the interesting points in the study is the way the researchers tried to prevent psychological bias. Schleminger asked his colleague Patrick Abbott to change the data by using a secret number to subtract from some of the measurements, so that he himself would not know the real result until the last moment. Only after years of work, when he opened the envelope containing the secret number, was he able to calculate the final value. At first he felt relieved, but later it turned out that the number was larger than expected, and therefore the final result also did not match that of the French experiment.
225 years and still surprising
The new study does not settle the debate, but it adds an important piece of data to the database of measurements and highlights the fact that even after more than 225 years of experiments, the gravitational constant still refuses to give in. Schleminger himself sums it up simply: Every measurement is important, because the truth is important. According to him, precise measurement is a way to bring order to the universe, even if the resulting number is not what they hoped for. After a decade of work, he announced that he would pass the baton to the younger generation of physicists, who will continue to tackle the problem.
for the scientific article DOI: 10.1088/1681-7575/ae570f
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