For more than two centuries, scientists have tried to determine one of the most important numbers in physics: the universal gravitational constant, known as "big G." It defines the strength of gravity throughout the universe, influencing everything from falling objects on Earth to the motion of galaxies. Yet despite its importance, researchers still cannot agree on its exact value. That uncertainty weighed heavily on Stephan Schlamminger, a physicist at the National Institute of Standards and Technology (NIST), as he prepared to open a sealed envelope containing a crucial secret number. For nearly 10 years, Schlamminger had devoted much of his...