This year marks 340 years since the gravitational constant, affectionately called “Big G,” was introduced. Created by Isaac Newton in 1687, Big G plays a crucial role in understanding gravity, yet it remains the least precisely known constant in physics.
Despite advancements in science, measuring Big G has proven challenging. Researchers, including Stephan Schlamminger from the National Institute of Standards and Technology (NIST), have dedicated years to solving this mystery. Schlamminger calls Big G the “oldest fundamental constant,” noting its persistent uncertainty is a discomfort for scientists. “It’s one of the great unresolved embarrassments of physics,” he shares.
Currently, about 17 different measurements of G exist, but they vary widely. “Nobody knows why,” Schlamminger adds, expressing the frustration shared by metrologists—those who study measurements—and the scientific community at large. Gravity, being the weakest of the fundamental forces, makes precise measurement difficult. “Everything pulls on everything all the time,” he points out.
Big G plays a significant role in calculations that shape our understanding of the universe. For gravity’s strength in Newtonian physics and the elasticity of spacetime in Einstein’s theory, its value impacts everything. When Newton introduced it, gravity was seen only as a force. Einstein later showed that gravity results from the curvature of spacetime caused by mass. This shift in understanding did not reduce G’s importance; it remained central to both theories.
The first measurement attempt came from Henry Cavendish in 1798. By measuring the gravitational force between lead spheres, he laid the foundation for calculating G. Yet, even with modern tools, scientists still grapple with its elusive nature.
Schlamminger and his team approached the latest measurement by replicating a previous experiment from the International Bureau of Weights and Measures (BIPM). To avoid biases, they sealed a secret value related to their measurement in an envelope, revealing it only once they were confident in their findings. This method aims to prevent subconscious influences on their results.
The suspense built up as they opened the envelope in July 2024, a delay from the original 2022 plan due to overlooked calculations. The new value of G they calculated was slightly lower than previous estimates, suggesting a potential discrepancy in Earth’s mass, which could be greater than previously accepted by an astounding amount—around 360 quadrillion tons.
Despite achieving a new measurement, Schlamminger warns that the fundamental disagreement among various experiments remains unresolved. “This mystery isn’t solved yet,” he says. He plans to step back from studying fundamental constants and turn his attention to precision measurements of electrical elements instead. “These measurements take a lot out of you,” he notes, reflecting on the years spent on Big G.
The findings were published in the journal Metrologia. These ongoing challenges with G remind us of the complexities of understanding our universe and highlight the intrigue that continues to drive scientific exploration.
