After a Decade in a Locked Envelope, Physicists Still Cannot Explain Gravity’s Secrets

For over a decade, Stephan Schlamminger had chased one number, and he had just placed it in an envelope. A physicist at the National Institute of Standards and Technology, Schlamminger devoted years to measuring the universal gravitational constant — a figure so central that it determines the strength of gravity throughout the entire cosmos. Inside that envelope was the figure that would unlock the results of his long years of work.

One of the most familiar forces in everyday life is gravity. It holds people to Earth, guides planets around the Sun, and gathers stars into galaxies. Scientists have not yet determined its fundamental strength with the precision they have reached for other basic forces of nature. That strength is represented by big G. Scientists have been attempting to measure it for more than 225 years, roughly a century after Isaac Newton introduced his law of universal gravitation. Despite generations of increasingly sophisticated experiments, the gravitational constant remains less precisely known than comparable constants associated with electromagnetism and the strong and weak nuclear forces.

Gravity’s Weak Problem

The force of gravity is remarkably feeble. A magnet no larger than a pinhead can lift a paper clip against the gravitational pull exerted by the whole Earth, since the electromagnetic force easily surpasses its strength. Within the laboratory, the issue is even more severe: researchers cannot move planets to study gravity, so they instead measure the pull between much smaller objects that can be weighed and precisely positioned.

Researchers are attempting to detect gravitational forces that are unbelievably weak, and the experimental masses they use are roughly 500 billion trillion times less massive than Earth.

Measurements That Refuse to Agree

Today’s measuring tools are highly sensitive, yet repeated measurements of big G still turn up slightly different results. These differences are small, roughly one part in 10,000, but they are bigger than what researchers would normally expect from ordinary experimental error. One plausible explanation is that a subtle experimental flaw has been missed. But there is a far more captivating possibility as well: scientists may be overlooking something about gravity’s very nature.

Blind Data and a Sealed Envelope

Schlamminger and his colleagues aimed to settle the disagreement over big G by replicating a precision experiment carried out by the International Bureau of Weights and Measures (BIPM) in Sèvres, France, in 2007. They wanted to show that an independent team at NIST’s campus in Gaithersburg, Maryland, could also produce the same measurement using essentially the same approach.

Schlamminger, however, worried about a different source of error: himself. Researchers can subconsciously allow their expectations to shape how they read or work with measurements.

He asked colleague Patrick Abbott to scramble part of the experiment by removing a secret number from the weights of some of the masses used. Abbott took out the number from the carefully measured weights, so Schlamminger was unable to recognize the true value of big G his team had produced. Because Abbott alone knew the number, he could analyze the experiment in ignorance of the correct result. The correction needed to recover the real answer was sealed inside an envelope.

The Moment of Truth

Before opening the envelope in 2022, Schlamminger nearly did so. He paused at the last moment, though, when he realized the team had not fully accounted for a subtle effect involving air pressure. That realization, combined with the fact that even tiny disturbances can matter in an experiment this sensitive, persuaded him to postpone the reveal and return to the analysis.

The day was finally here, more than two years in the making. At 3 p.m. on July 11, 2024, Schlamminger was supposed to report the findings at the annual Conference on Precision Electromagnetic Measurements in Aurora, Colorado. His nerves were so high that he skipped the morning sessions entirely, instead reviewing in his mind how temperature and pressure fluctuations could have thrown off the measurement.

“I had really dotted all the i’s and crossed all the t’s of the experiment,” he said.

Schlamminger finally disclosed the concealed number during his afternoon presentation, and he felt an instant sense of relief.

The experiment’s intended outcome depended on Abbott’s secret correction being sizable and negative, and it was.

1. The morning brought a sign that looked favorable at the time.
2. Only later did Schlamminger come to see that there was a drawback attached to it.

What This Means

A small difference exists, but for nature’s most important constant, it is enough to leave scientists questioning whether buried errors in the experiment or something stranger still are at fault. The large and negative correction adds to a puzzle that has puzzled physics for 225 years instead of settling it.

Key Facts

• Measurement discrepancy: roughly one part in 10,000
• Experiment length: 10 years
• Correction sealed in envelope revealed at 3 p.m. on July 11, 2024
• Experiment replica: BIPM measurement from Sèvres, France, in 2007
• Presentation venue: Conference on Precision Electromagnetic Measurements, Aurora, Colorado
• Measurement target: universal gravitational constant (big G)

Scientists are still debating even the most basic principles of nature. A physicist devoted ten years of work to securing a number, placed that correction inside a sealed envelope to remove bias, and then skipped morning sessions to hold the tension before finally opening it. The envelope revealed that the experiment had succeeded, but it also raised a question that will take further experiments to resolve.

Source material: “Scientists opened a sealed envelope after 10 years. Gravity still didn’t make sense,” ScienceDaily.

big G · gravity · NIST · physics · science