A new experiment deepens the mystery over gravitational constant, Big G

Scientists have announced the results of a decade-long search to measure Newton’s gravitational constant, the force that keeps our feet on the ground and planets in orbit.

The follow-up was more or less a fiasco. The most ambitious effort to date to determine the fundamental constant that determines the strength of attraction between two masses anywhere in the universe has resulted in a number that contradicts previous findings, including the results of an experiment it attempted to replicate.

Stephan Schlamminger, the scientist who meticulously conducted the latest experiment that began in 2016, described it as a “life-sucking” experience. “It was like walking through a really dark valley,” added Schlamminger, a physicist at the National Institute of Standards and Technology in Gaithersburg, Maryland.

However, he has since managed to put a positive spin on his efforts. “I kind of put that in my rearview mirror now,” he said. “I think every measurement is a learning opportunity and every measurement sheds light on this darkness.”

What is the gravitational constant?

The fundamental constants of nature are the key values ​​that define its behavior. Physical events in the universe – and they don’t change no matter where you are in time or space. These include the speed of light and Planck’s constant. It plays an important role in quantum physics.

These constants are “woven into the fabric of the universe,” Schlamminger said. “It’s pretty cool, because they’re the same across generations. If you talked to an extraterrestrial, they’d have the same concept.”

Scientists have been trying to measure the gravitational constant, nicknamed Big G, for more than 225 years. British scientist Henry Cavendish conducted the first experiment to measure it in 1798, more than 100 years after Isaac Newton first discovered the force of gravity.

But scientists have been unable to agree on a measurement with a level of precision comparable to constants such as the speed of light (299,792,458 meters per second) or the Planck constant, which is known to eight decimal places.

The Data Committee of the International Council for Science, or CODATA, publishes recommended values. fundamental physical constants. The recommended numerical value for large G is a four-digit number with a measurement uncertainty of 22 parts per million.

He said this value was “an embarrassment to the active metrologist,” a scientist who specializes in measurements, given that other constants in nature are known and accepted as exact to six or more significant digits.

“If you had a clock that was running 22 pages per minute late, you would measure the year 12 minutes too long,” he added.

He stated that the field of metrology (the science of measurement) is important because it creates confidence in science, economy and trade. “This is the kind of science that underpins so much of our society, and no one realizes it,” he said.

“When you pay your electric bill, you want to make sure you’re paying the right amount, right? There are people who know how to measure voltage, how to measure current, how to measure power.”

Why is it so difficult to measure?

Christian Rothleitner, a physicist at Germany’s National Metrology Institute Physikalisch-Technische Bundesanstalt, who was not involved in the research, said measuring gravity accurately is quite difficult for three reasons. First, it is a relatively weak force.

“We perceive the force of gravity as a very strong force because we have to apply a lot of force to lift something from the earth,” he said via email.

In fact, he said, it is much weaker than the other three fundamental forces (electromagnetic, weak nuclear and strong nuclear forces) that hold atoms and nuclei together.

“You can easily see this when you look at a magnet that, although relatively small, exerts a very strong force on magnetic objects.”

Another reason why determining the gravitational constant is difficult is that the masses used in the experiment in the laboratory must fit into a relatively small, restricted space: “And small masses only produce small gravitational forces.”

Moreover, since the gravitational force is created by every object, it is “extremely difficult” to be sure that the force you measure in the laboratory is actually coming from the intended mass.

“The problem with large G measurements is that the values ​​are very dispersed, so the measurement results are not consistent with each other,” Rothleitner said. “This leaves a lot of room for speculation about the origin of the discrepancy.”

secret envelope

Over four decades, there have been at least 16 more attempts to measure Big G. Instead of adding a new measurement to an already inconsistent data set, Schlamminger and his colleagues attempted to replicate an experiment conducted by the International Bureau of Weights and Measures in Sèvres, France.

If he could independently produce the same results, the mystery surrounding the exact value of Big G could be solved.

The experiment relied on sensitive equipment known as a torsion balance, which detects small forces by measuring the bending angle, or torsion, of metal masses suspended on a thin fiber, which must be operated in a vacuum. The bending cannot be detected with the naked eye, but it can be detected with sensors, thus making it possible to understand the gravitational force.

Throughout the experiment, Schlamminger spent years calibrating the equipment and removing the physical effects of properties such as temperature and pressure that might confound the measurements and prove that these factors did not affect the results.

Given that the team was repeating an earlier experiment, they took another precaution to avoid any personal bias, conscious or unconscious, that might move the experiment towards the answer they thought it should get, and to prevent them from stopping the study too early.

A colleague not involved in the study added a random offset number to the masses to prevent Schlamminger from seeing the actual measurement he had taken. This number was kept in a secret envelope that was kept from Schlamminger until the study was completed.

After the honeymoon period of research, Schlamminger found the work demoralizing at times. “It looks like a random number generator to me,” he said. “I felt like I was going to the casino every day to work.”

The envelope containing the secret number was opened at a conference stage in July 2024, and Schlamminger and his team finally learned the real results of their work. His initial elation (the final numerical value of the Capital G was on the right) was later dashed, and he said he felt “a little unhappy.”

Team Big G’s measured value is 6.67387×10^-11 Square meters per kilogram per second. The unit reflects distance, mass and motion: how gravity works. This is 0.0235% lower than the result the researchers attempted to replicate and contradicts the CODATA figure.

This is a remarkable difference, Schlamminger said; such as measuring a person’s height and being a millimeter or two off. “It’s a small thing in the grand scheme of things, but it’s pretty embarrassing when it comes to these basic concepts,” he said. A scientific paper detailing the study was published on April 16. Metrologia magazine.

Schlamminger’s efforts could give scientists the tools to make precise measurements in other areas where extremely small forces are involved, said Ian Robinson of the National Physical Laboratory in the United Kingdom. Although Robinson attended the meeting where Schlamminger’s data were announced, he was not involved in the research.

“Some extremely obscure problems were found, addressed, and a new result was produced,” Robinson said.

Unknown physics?

What could explain the discrepancy in measurements of Big G?

It is possible that there is something unknown about the universe that prevents a definitive value from being determined. But while this unknown is an exciting possibility, Schlamminger, Robinson and Rothleitner said this hypothesis is an exaggeration.

“It’s unlikely that some fundamental physics we don’t understand is causing the discrepancy in results,” Robinson said. “It is much more likely that an undiscovered, extremely small and uncertain effect or effects will influence some outcome.”

Schlamminger suggested that better designed equipment could have improved the situation, or perhaps some human error was involved.
However, he said that he did not waste the last 10 years.

““Precision metrology is not just about converging on a number, it is also about meticulously uncovering unknowns,” his study concludes.

Schlamminger’s passion for the field has not waned. On his forearm are the numbers in Planck’s constant, which were fixed in the work he was involved with in 2019.

Schlamminger said he hopes young researchers interested in the Big G will not be discouraged from embarking on the quest. But even if an exact numerical value were found, Big G would never get a tattoo: “He’s very particular about a number.”

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