For over a century, the kilogram was defined by a single, shiny lump of metal locked in a vault near Paris. But what happens when your definition of mass starts...losing mass? In this episode, Tom Whipple, Science Editor at The Times, unpacks the surprisingly dramatic story of the kilo - from the French Revolution’s quest for order, to a quantum reimagining of what “weight” really means. Enter the kibble balance: a machine so precise it can weigh light itself. Sort of.

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