The time had come to open the envelope. But Stephan Schlamminger, a physicist at the National Institute of Standards and Technology (NIST), wasn’t sure he wanted to know the secret number that was inside.

For the past 10 years, Schlamminger had spent most of his working hours trying to measure a single quantity, known as the universal gravitational constant, which determines the strength of gravity everywhere in the universe. The secret number would enable him to unscramble his data and get his answer.

Gravity keeps our feet on the ground, holds planets in orbit around the sun, corrals stars and other matter to create galaxies, and shapes galactic clusters to weave the web of the universe. But its strength, expressed as Big G, is not exactly known.

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