For 10 years, physicist Stephan Schlamminger had been chasing one of the most stubborn numbers in science. Now, after a decade of experiments, corrections, and painstaking analysis, the answer was sitting inside a sealed envelope. He was not entirely sure he wanted to open it.
Schlamminger, a physicist at the National Institute of Standards and Technology (NIST), had spent much of the previous decade trying to measure the universal gravitational constant. Known to physicists as big G, this fundamental number determines the strength of gravitational attraction throughout the universe. The number hidden in the envelope was the key that would finally unscramble his experimental data and reveal what his team had measured.
Gravity is one of the most familiar forces in everyday life. It keeps people anchored to Earth, guides planets around the Sun, helps gather stars into galaxies, and plays a central role in shaping the enormous cosmic web of galaxy clusters that stretches across the universe. Yet scientists still do not know its fundamental strength with the precision they have achieved for other basic forces of nature.
Scientists have been attempting to measure big G for more than 225 years, beginning 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 nature's other three fundamental forces: electromagnetism and the strong and weak nuclear forces. Part of the problem is surprisingly simple.
Gravity is extraordinarily weak. A tiny magnet can demonstrate the problem. A magnet roughly the size of a pinhead can lift a paper clip against the gravitational pull of the entire Earth.
In that simple contest, the electromagnetic force produced by the magnet easily overcomes gravity. The challenge becomes even greater in the laboratory. Scientists cannot move planets around to perform controlled experiments, so they have to measure the gravitational attraction between much smaller objects that can be weighed and precisely positioned.
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