Physicists Just Made the Most Precise Measurement Ever of Gravity's Strength

By Ryan F. Mandelbaum on at

Gravity might feel strong if you drop a bowling ball on your feet, but is in fact the weakest force. Compare it to electromagnetism: the pull of all the Earth’s gravity can’t stop you from picking up a paperclip with a refrigerator magnet. That weakness makes gravity incredibly difficult to measure.

A team of scientists in China are reporting that they have now performed the most precise measurement of gravity’s strength yet by measuring G, the Newtonian or universal gravitational constant. G relates the gravitational attraction between two objects to their masses and the distance between them. The new measurement is important both for high-powered atomic clocks as well as the study of the universe, earth science, or any kind of science that relies on gravity in some way.

The values measured by the team “have the smallest uncertainties reported until now,” according to the paper published in Nature.

Before we get to the experiments, a little more about what G is. Isaac Newton realised that gravity’s strength depends on the masses of two objects (like a planet and a moon) and weakens in proportion to the distance between those masses squared. But in order to calculate the actual gravitational force between two things, you have to multiply the combined masses divided by the distance squared by a very, very, very tiny number. Henry Cavendish first indirectly measured that number, G in the late 18th century.

Given how small it is, determining the true value of G is very difficult, and the agreed-upon value set by the international Committee on Data for Science and Technology (CODATA) is much less precise than the values agreed upon for other numbers that scientists use. Today, scientists use a value of .0000000000667408, give or take 47 parts-per-million. Essentially, calculating with this G is like trying to paint with a fat paint brush versus other experiments whose constants are like painting with skinnier paintbrushes.

In the new study, scientists performed two independent calculations of G using a pair of pendulums in a vacuum, one pendulum setup for each test. Each pendulum swings back and forth between a pair of massive objects whose positions can be adjusted.

The pendulums measure the force of gravity in two ways. First, they measure the difference between how quickly the pendulum swings to the “near,” or parallel position, versus the “far,” or horizontal position. They also measure how the direction of the pendulum’s swing changes based on the pull of the test masses.

Obviously, the experiments require super sensitive detectors and super well-controlled setups in order to determine accurate values for G. Each part must be specially fabricated and considered so the only possible force acting on the pendulum could come from the test masses. Additionally, the lab is in a special room in a cave to control for possible effects from changing temperatures.

The researchers ended up measuring 6.674184 and 6.674484 hundred billionths (10-11) for the time-of-swinging and angular acceleration methods, respectively. These measures were both very precise, but are still different from one another for unknown reasons. This might have had something to do with the string used for the pendulum.

And these are just two of many attempts to calculate G, many of which disagree. As the paper’s reviewer, Stephan Schlamminger from the National Institute of Standards and Technology wrote in a commentary:

“Li et al. carried out their experiments with great care and gave a detailed description of their work. The study is an example of excellent craftsmanship in precision measurements. However, the true value of G remains unclear. Various determinations of G that have been made over the past 40 years have a wide spread of values. Although some of the individual relative uncertainties are of the order of 10 parts per million, the difference between the smallest and largest values is about 500 parts per million.

Scientists hoping to make the most accurate measurements of masses, or to understand how things move under the effects of gravity, need to nail down G’s value. So, they’re going to keep working until that value is certain beyond a doubt. [Nature]