Stephan Schlamminger and his colleague, Vincent Lee, examine the torsion balance they used to measure the gravitational constant
R. Eskalis/NIST
Physicists have long sought to accurately measure the gravitational constant, known as “big G.” Despite numerous efforts, these measurements have often been inconsistent, leading to questions about our understanding of either the experiments or gravity itself. The most recent experiment, characterized by exceptional precision, may finally help researchers find common ground.
Gravity’s weakness compared to other fundamental forces makes it particularly challenging to measure with precision. Stephan Schlamminger from the US National Institute of Standards and Technology in Maryland explains, “As children, we were fascinated by magnets and their attractive forces. With gravity, there is a force between two coffee cups in each hand, but it is so slight that it goes unnoticed, which is why it is not as captivating.” This inherent weakness complicates the measurement of gravity’s true strength.
Unlike other forces, gravity cannot be shielded in experiments. In 1798, physicist Henry Cavendish circumvented this issue with a torsion balance, allowing him to measure gravity for the first time, albeit with limited precision.
To visualize a torsion balance, imagine a horizontal toothpick suspended from a thread at its center, with a small marble at each end. When another object approaches one of the marbles, its gravitational pull causes the toothpick to rotate slightly. By measuring this rotation, the gravitational force between the marble and the object can be calculated, independent of Earth’s gravity, which the thread counteracts.
Schlamminger and his colleagues conducted a more advanced version of this experiment, using eight weights on two precisely calibrated turntables, all suspended by ribbons as thin as a human hair. This intricate setup replicated an experiment first conducted in France in 2007. The team spent a decade identifying and minimizing every potential source of uncertainty. Jens Gundlach of the University of Washington, who was not involved in the study, commented, “This is experimental physics at its finest.”
Kasey Wagoner from North Carolina State University, also uninvolved in the research, described it as a “game-changer” due to the meticulous care taken to explore various effects. The resulting value of big G, 6.67387×10-11 metres3 per kilogram per second2, is slightly lower than the 2007 measurement but aligns more closely with other historical tests.
Schlamminger noted, “Big G is more than just a measure of gravity; it’s a gauge of our ability to measure gravity across different periods of physics. We can compare our work to Cavendish’s experiment from 230 years ago, and future generations will compare theirs to ours.” The precise determination of previously unknown uncertainties has improved consensus, according to Gundlach, making the landscape more reliable and trustworthy.
The groundwork laid by Schlamminger’s team could enable future experiments to measure big G with even greater precision, which is crucial as cosmological measurements—many reliant on gravity’s strength—become more precise. Wagoner added, “Any minor anomaly here could impact scales from the laboratory to the universe, leading to significant implications when expanded to cosmic scales.”
While many researchers suspect the remaining discrepancies are due to unrecognized biases and uncertainties in the experiments, there’s a possibility that gravity behaves differently than expected, hinting at new, exotic physics. Schlamminger emphasizes, “There is a gap in our scientific understanding, and we must explore these gaps. There might be nothing, but ignoring them would be a mistake.”
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