Decade-Long Experiment Yields New Measurement of Gravitational Constant

A 10-year experiment conducted by a team at the National Institute of Standards and Technology (NIST) in the United States has produced a new measurement of the gravitational constant, referred to as big G. The constant quantifies the strength of gravitational attraction between objects.

The team's leader stated that the effort serves as a reminder of gaps in scientific understanding. The gravitational constant influences phenomena such as objects falling, personal weight measurements, and the moon's orbit around Earth. It applies to both small-scale events like an apple dropping and large-scale motions of galaxies.

Despite over two centuries of attempts, measurements of big G continue to vary slightly between experiments.

The experiment replicated a prior study from the International Bureau of Weights and Measures in France to assess consistency. The setup used a torsion balance, based on a design by English scientist Henry Cavendish in 1798. This involves a thin wire suspending weights, which rotate minimally when attracted by nearby masses.

In the NIST version, metal cylinders were positioned to exert gravitational pull on the suspended weights. Instruments measured the wire's twist to calculate the gravitational force and derive big G. To ensure accuracy, the team refined the apparatus over the decade.

The team leader implemented a blinding method by having a colleague add an unknown offset to key measurements, sealed in an envelope. This prevented bias from influencing refinements. The envelope was opened at the experiment's conclusion during a conference.

The resulting value for big G was 6.67387 × 10−11 cubic metres per kilogram per second squared. This differed from the French measurement by about 0.02 percent. While small, this discrepancy exceeds acceptable limits for such a fundamental constant.

Challenges Big G is the least precisely known fundamental constant in physics. In practice, combinations like big G multiplied by Earth's or the sun's mass are determined accurately through observations. Isolated measurements of big G are rarely used in engineering or technology.

Disagreements may stem from measurement difficulties, such as lab vibrations or static electricity interfering with weak signals. Alternatively, they could indicate incomplete aspects of current physical theories. The Standard Model explains electromagnetism and nuclear forces but not gravity.

A precise big G value could test future theories incorporating gravity. The team leader noted that the uncertainty underscores areas of unknown knowledge in science.