The gravitational constant, known as Big G, still eludes scientists - CNN

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

The pursuit was more or less a bust. The most ambitious effort to date to pin down the fundamental constant, which determines the strength of the attraction between two masses anywhere in the universe, resulted in a number that disagreed with previous findings, including the results of an experiment it sought to replicate.

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

But he has since been able to put a positive spin on his endeavors. “Now, I’ve put it a little bit in my rearview mirror,” he said. “I think every measurement is an opportunity to learn and every measurement brings light into this darkness.”

What is the gravitational constant?

Fundamental constants of nature are key values that define the behavior of physical phenomena in the universe — and they don’t change regardless of where you are in time or space. They include the speed of light and Planck’s constant, which plays a key role in quantum physics.

These constants are “baked into the fabric of the universe,” Schlamminger said. “It’s quite beautiful, because they are the same over generations. If you ever talked to an extraterrestrial, they would have the same concept.”

For more than 225 years, scientists have tried to measure the gravitational constant, nicknamed Big G. British scientist Henry Cavendish performed the first experiment to measure it in 1798, more than a 100 years after Isaac Newton first discovered the force of gravity.

Scientists have not, however, been able to converge on a measurement with a level of precision comparable to that of constants such as the speed of light (299,792,458 meters per second) or Planck’s constant, which is known to eight decimal places.

The Committee on Data of the International Science Council, or CODATA, issues recommended values of fundamental physical constants. Its recommended numerical value for Big G is a four digit number with a measurement uncertainty of 22 points per million.

Given that other constants in nature are known to six or more significant digits and are considered exact, this value, he said, is an “embarrassment for the active metrologist,” a scientist who specializes in measurements.

“If you had a watch that runs 22 ppm late, you would measure the year 12 minutes too long,” he added.

The field of metrology — the science of measurement — is important, he noted, because it creates trust in science, the economy and trade. “It is the kind of the science that undergirds a lot of our society, and nobody notices,” he said.

“When you pay your electricity bill, you want to make sure that you pay the right amount, right? There are people who know how to measure voltages and how to measure currents and how to measure power.”

Why it’s so difficult to measure

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

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

In reality, he said, it is much weaker than the other three fundamental forces — electromagnetic, weak nuclear and strong nuclear forces — which hold atoms and nuclei together.

“You can easily see this if you look at a magnet, which is relatively small, but nevertheless exerts a very strong force on magnetic objects.”

The other reason it’s hard to determine the gravitational constant is that in a laboratory, the masses used in the experiment must fit inside a relatively small, constrained space: “And small masses in turn only generate small gravitational forces.”

What’s more, because the gravitational force is generated by every object, it’s “extremely challenging” to make sure the force you measure in the laboratory really comes from the intended mass.