After 10 Years, Physicist Finally Opens Envelope With Gravity's Secret

The Envelope That Changed Everything

For a decade, physicist Stephan Schlamminger chased one of science’s most stubborn numbers. On July 11, 2024, sitting in an auditorium in Aurora, Colorado, he was finally ready to find out if he’d caught it.

The answer was sealed inside an envelope.

Schlamminger, working at the National Institute of Standards and Technology (NIST), had devoted years to measuring big G, the universal gravitational constant that determines how strongly gravity pulls throughout the cosmos. It sounds straightforward, but it’s anything but. Despite over 225 years of increasingly sophisticated experiments, scientists still cannot measure this fundamental constant with the precision they’ve achieved for nature’s other three basic forces.

The problem is deceptively simple: gravity is extraordinarily weak. A pinhead-sized magnet can lift a paper clip against the gravitational pull of the entire Earth. In laboratories, researchers must measure gravitational attraction between masses roughly 500 billion trillion times smaller than Earth itself, making the forces they detect incredibly faint.

A Blinded Approach to Unbiased Science

Schlamminger knew his biggest enemy might not be experimental equipment or cosmic interference. It could be himself. Scientists often unconsciously influence how they interpret measurements when they expect a particular answer. So he devised something clever: he asked colleague Patrick Abbott to scramble part of the data by subtracting a secret number from the measured weights of the experimental masses.

Only Abbott knew the correction. Schlamminger could analyze everything without knowing what value for big G his team was producing. The real answer stayed hidden in that sealed envelope until the very end.

But even blinding couldn’t overcome nature’s stubbornness. Just before his presentation, Schlamminger realized the team hadn’t fully accounted for how air pressure affected their measurements. Sensitivity at this level means everything matters. He postponed the reveal and dove back into the analysis.

The Disappointing Discrepancy

When Schlamminger finally opened that envelope during his afternoon presentation, he discovered a problem. The correction was too large. Once the blinded data were restored, the NIST measurement didn’t match the French experiment conducted by the International Bureau of Weights and Measures in 2007.

The NIST value came in at 6.67387 × 10-11 meters3/kilogram/second2, roughly 0.0235% lower than the French result. In everyday life, such a tiny difference would be meaningless. It wouldn’t change your bathroom scale reading or affect how much peanut butter fills a 16-ounce jar.

For fundamental physics, however, it matters enormously. Other fundamental constants are known to six or more significant digits. Big G remains stubbornly imprecise.

The technique both teams used dates back over two centuries to Henry Cavendish’s 1798 experiment. Using a torsion balance, suspended by a fiber thinner than human hair, researchers measure how gravity causes tiny twists and rotations. The BIPM and NIST experiments employed eight cylindrical metal masses arranged on a rotating carousel, with measurements precise enough to detect gravitational whispers.

What This Means for Science

History teaches physicists to pay attention to tiny discrepancies. Small mismatches between measurements and expectations have repeatedly revealed that scientists were missing something fundamental about how nature actually works. The disagreement persisting across multiple measurement attempts keeps a tantalizing question alive: Are we overlooking something about gravity itself?

To eliminate one possible culprit, Schlamminger’s team performed their measurements twice, first using copper masses, then sapphire. Both produced essentially identical results, ruling out material-based explanations but not solving the larger mystery.

Experimental error remains the most likely explanation for the continued disagreement. Yet after devoting a decade to perfecting his measurements, Schlamminger has handed the challenge to younger scientists. “Every measurement is important, because the truth matters,” he reflected. “For me, making an accurate measurement is a way of bringing order to the universe, whether or not the number agrees with the expected value.”

After more than two centuries of precision experiments, the most fundamental force holding galaxies together remains one of physics’ most elusive constants to pin down, leaving scientists to wonder whether nature is hiding a secret about gravity we haven’t yet discovered.

Source: National Institute of Standards and Technology (NIST)