The gravitational constant G determines the strength of gravity—the force
that makes apples fall to the ground or pulls the Earth in its orbit around
the sun. It is part of Isaac Newton's law of universal gravitation, which he
first formulated more than 300 years ago. The constant cannot be derived
mathematically; it has to be determined through experiment.
Over the centuries, scientists have conducted numerous experiments to
determine the value of G, but the scientific community isn't satisfied with
the current figure. It is still less precise than the values of all the
other fundamental natural constants—for example, the speed of light in a
vacuum.
One reason gravity is extremely difficult to quantify is that it is a very
weak force and cannot be isolated: when you measure the gravity between two
bodies, you also measure the effect of all other bodies in the world.
"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 presented their work
in the scientific journal Nature Physics.
A novel experiment in an old fortress
To rule out sources of interference as far as possible, Dual's team set up
their measuring equipment in what used to be the Furggels fortress, located
near Pfäfers above Bad Ragaz, Switzerland. The experimental setup consists
of two beams suspended in vacuum chambers. After the researchers set one
vibrating, gravitational coupling caused the second beam to also exhibit
minimal movement (in the picometre range—i.e., one trillionth of a meter).
Using laser devices, the team measured the motion of the two beams, and the
measurement of this dynamic effect allowed them to infer the magnitude of
the gravitational constant.
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.
Dual is becoming a professor emeritus at the end of July this year and has
already given his farewell lecture. "A successful experiment is a nice way
to end my career," he says.
Reference:
Jürg Dual, Dynamic measurement of gravitational coupling between resonating
beams in the hertz regime, Nature Physics (2022).
DOI: 10.1038/s41567-022-01642-8.
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Physics