The Gravitational Constant
The gravitational constant G determines the intensity of gravity, the force that pulls the Earth in its orbit around the sun or causes apples to fall to the ground. It is a component of Isaac Newton’s law of universal gravitation, which was developed almost 300 years ago. The constant must be determined by experimentation since it cannot be mathematically calculated.
The value of G has been the subject of several tests throughout the years, but the scientific community remains unsatisfied with the result. In comparison to the values of all the other important natural constants, such as the speed of light in a vacuum, it is far less accurate.
Gravity is a very weak force that cannot be separated, which makes it incredibly challenging to measure. When you measure the gravity between two bodies, you also have to estimate the impact of all other bodies in the universe.
“The only option for resolving this situation is to measure the gravitational constant with as many different methods as possible,” explains Jürg Dual, a professor in the Department of Mechanical and Process Engineering at ETH Zurich. He and his colleagues conducted a new experiment to redetermine the gravitational constant and have now published their work in the prestigious journal Nature Physics.
A novel experiment in an old fortress
Dual’s team set up their measurement equipment at the former Furggels fortress, which is located close to Pfäfers above Bad Ragaz, Switzerland, in order to exclude sources of interference as much as possible. Two beams hung in vacuum chambers make up the experimental setup. After the researchers set one vibrating, gravitational coupling caused the second beam to also exhibit minimal movement (in the picometer range – i.e., one trillionth of a meter). The researchers used laser equipment to measure the motion of the two beams, and by analyzing this dynamic effect, they were able to estimate the gravitational constant’s magnitude.
The value the researchers arrived at using this method is 2.2 percent higher than the current official value given by the Committee on Data for Science and Technology. However, Dual acknowledges that the new value is subject to a great deal of uncertainty: “To obtain a reliable value, we still need to reduce this uncertainty by a considerable amount. We’re already in the process of taking measurements with a slightly modified experimental setup so that we can determine the constant G with even greater precision.” Initial results are available but haven’t yet been published. Still, Dual confirms that “we’re on the right track.”
The researchers run the experiment remotely from Zurich, which minimizes disruptions from personnel present on site. The team can view the measurement data in real-time whenever they choose.
Insight into the history of the universe
For Dual, the advantage of the new method is that it measures gravity dynamically via the moving beams. “In dynamic measurements, unlike static ones, it doesn’t matter that it’s impossible to isolate the gravitational effect of other bodies,” he says. That’s why he hopes that he and his team can use the experiment to help crack the gravity conundrum. Science has still not fully understood this natural force or the experiments that relate to it.
For example, a better understanding of gravity would allow us to better interpret gravitational wave signals. Such waves were detected for the first time in 2015 at the LIGO observatories in the US. They were the result of two orbiting black holes that had merged at a distance of about 1.3 billion light-years from Earth. Since then, scientists have documented dozens of such events; if they could be traced in detail, they would reveal new insights into the universe and its history.
A career-crowning achievement
Dual began working on methods to measure the gravitational constant in 1991, but at one point had put his work on hold. However, the observation of gravitational waves at LIGO gave it new momentum, and in 2018 he resumed his research. In 2019, the project team set up the laboratory in the Furggels fortress and began new experiments. In addition to the scientists from Dual’s group and a statistics professor, the project also involved infrastructure personnel such as cleanroom specialists, an electrical engineer, and a mechanic. “This experiment couldn’t have come together without years of team effort,” Dual says. He is becoming a professor emeritus at the end of July this year. “A successful experiment is a nice way to end my career,” he says.
Reference: “Dynamic measurement of gravitational coupling between resonating beams in the hertz regime” by Tobias Brack, Bernhard Zybach, Fadoua Balabdaoui, Stephan Kaufmann, Francesco Palmegiano, Jean-Claude Tomasina, Stefan Blunier, Donat Scheiwiller, Jonas Fankhauser, and Jürg Dual, 11 July 2022, Nature Physics.