The Kilogram’s Makeover Is Almost Complete

From Scientific American:

This is a serious problem for modern science, given that many technologies depend on an accurate measurement of the kilogram, says Stephan Schlamminger, a physicist at the National Institute of Standards and Technology. That is why scientists, for decades now, have wanted to redefine the kilogram in terms of constants found in nature—an achievement that would provide a more stable (and accessible) unit of measurement.

Schlamminger and his team recently reported that they have paved the way for such a feat by using Planck’s constant, a mathematical value that describes the link between the energy of a photon and its frequency and that can be related to mass through Einstein’s famous equation E = mc2. As detailed in the Review of Scientific Instruments, the NIST team measured Planck’s constant with a high-tech scale called a watt balance.

The researchers placed a known mass on one end of the scale and then counterbalanced it by sending an electric current through a movable coil of wire suspended in a magnetic field. They then used that electromagnetic force to measure Planck’s constant down to an accuracy of 34 parts per billion.

Before the world redefines the kilogram based on Planck’s constant, however, multiple teams must publish independent measurements by July 2017. At the 2018 General Conference on Weights and Measures, the data from each group will then be evaluated, including a constant calculated by counting the atoms in a silicon sphere. A complex computer program will subsequently sift through the numbers to arrive at a final value. Only then may Le Grand K be retired to the Louvre, next to the old meter and other artifacts.

6.626 069 83 × 10−34 kg m2/s

Planck’s constant as measured using the NIST-4 watt balance

0.000 000 22 × 10−34 kg m2/s (or 34 …

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