We've been measuring gravity for 200 years, but we're still not sure how strong it is. Meet the metrologists striving to find out

Harold Park's belongings were already leaving for France when he realised gravity had given him the slip. "The movers were in my apartment taking my stuff away," he says. He was in his lab at the research institute JILAin Boulder, Colorado, making the final checks on an experiment that had taken up the past two years of his life - to precisely measure the strength of gravity. "The signal shouldn't have changed," he recalls. "But it did."

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