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May 20, 2019 05:25 pm

The Definition of a Kilogram Just Changed Worldwide

For over a century, the kilogram was defined by a metal cylinder in a French vault. Now, this key unit of mass is defined using the Planck constant, a fundamental figure in physics. From a report: On Monday -- World Metrology Day -- Le Grand K lost its special status as the international prototype kilogram (IPK) and it will no longer represent this base unit of mass to the world. From now on, the kilogram -- along with the ampere, kelvin, mole, and candela -- will be defined by fundamental physical and atomic properties instead of tangible human-made objects. "The Metric System was envisioned to be 'for all people for all time,'" said Barry Inglis, president of the International Committee for Weights and Measures, in a statement. "From its outset it sought to ensure long-term stability by defining the units in terms of an internationally agreed 'constants of nature' instead of an arbitrary reference." To that end, the "arbitrary" Le Grand K has been deposed by the Planck constant, a fundamental quantity related to the energy of photons, the elementary particles that make up light. Defined as 6.626 x 10-34 joule-seconds, the constant fixes the kilogram to the speed of light and a temporal unit of measurement -- the second. The kilogram is now equal to the weight of 1.4755214 x 1040 photons with frequencies matching a cesium atomic clock. It may sound like a less relatable system of measurement, but what the change loses in familiarity it makes up for in precision. Even though Le Grand K is one of the most carefully protected objects on the planet, it is not immune from physical interactions that can alter its weight.

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