New experiment resulted in lower value for ‘Big G’, one of physics’ key constants
A recent experiment to measure the fundamental constant of gravity, or G, used two sets of cylindrical masses that attract each other. An optical device made it possible to measure the deviation caused by gravitational attraction, allowing scientists to calculate G.
S. Kelley/NIST
Physicists have just proposed a new precise measurement of gravity.
The recently published value for the force of gravity, known as “G” or “Big G”, is significantly smaller than some previous measurementsthe researchers report in the April report Metrology. This disagreement reflects a long-standing trend and could mean that there are hidden factors affecting these types of gravitational experiments.
Since Isaac Newton published his theory of gravity in the 17th century, researchers have attempted measure the force of gravity. But because it is the weakest of the four fundamental forces of nature, it is the most difficult to measure accurately. A dozen precision experiments over the past 50 years found a range of values.
“All the other fundamental constants are measured very precisely, and the big G is something of an outlier,” says physicist Michael Ross of the University of Washington in Seattle, who was not involved in the new study. For example, the fundamental constant that defines the intensity of the electromagnetic force is known with approximately 100,000 times less uncertainty.
Limiting yourself to G will not affect how we measure the weight of objects in our daily lives. But knowing precisely the fundamental constant is important to ensure that nothing crucial is missing from our understanding of gravity. If disagreements between G measurements turned out to be a reflection of nature, Ross says, it would break physics completely. “That’s why we spend so much time trying to figure out these numbers, because they actually control the entire universe.”
The most common way to measure G is to suspend masses by fibers or threads to measure the gravitational attraction between them. In 1798, English scientist Henry Cavendish developed such a setup, called a torsion balance, and scientists have continued to refine the method ever since.
For this new study, physicist Stephan Schlamminger and his colleagues recreated a torsion balance experiment first carried out in France in the early 2000s. In this setup, four large masses on a rotating ring encircled four smaller masses on a suspended disk. G was calculated by measuring the minute movement of small masses as gravitational forces pulled them toward larger masses.
By focusing on an existing technique, the team hoped to avoid simply adding another data point from an entirely new approach.
“The experiment took me about 10 years,” says Schalamminger, of the National Institute of Standards and Technology in Gaithersburg, Maryland. “The results highlight how difficult it is to measure this gravitational constant.”
The recreated experience followed the French configuration as closely as possible. To avoid biasing the result, the researchers hid part of the calibration until the end. Along the way, they also discovered previously unexplained effects, including atmospheric pressure. The team finally arrived at their new value for G: 6.67387 × 10−11 cubic meters per kilogram per second squared.
This value is 0.0235% lower than the results of the original French experiment – a notable difference given the precision of the measurement – but closer to the value officially recommended by the Data Committee of the International Scientific Council, which evaluates measurements of fundamental constants and publishes recommended values.
Although the result does not end the debate, it adds an important new data point and the researchers hope it will help scientists continue the quest for a reliable G.
