For more than a century, the kilogram was based on a hunk of metal. The world just redefined it using quantum physics.
Since the 19th century, the world had based the kilogram on a prototype, which was stored in a safe in France. When that prototype was examined in the 1980s, it weighed less than it should, creating an international uproar.
Now representatives of more than 60 countries have approved a new definition for the kilogram by tying it to an unchanging feature from quantum physics called Planck��s constant. The new definition, unlike a physical object, should be constant across space and time.