KILOGRAM GETS HISTORIC UPDATE.

Redefinition of the kilogram

The redefinition of the kilogram, the globally approved unit of mass, was the mostly hotly anticipated change. For more than a century, the kilogram has been defined as the mass of a cylinder of platinum-iridium alloy kept in a high-security vault in France.

While scientists will still monitor and study Le Grand K, it no longer has its former scientific significance. Now, it’s just a cylinder with a lot of history. Starting in May, the kilogram will be defined in terms of Planck’s constant, a number that relates a radio wave’s energy to its frequency.

Nobel prize winner William Phillips called the update "the greatest revolution in measurement since the French revolution," which ushered in the metric system of meters and kilograms.

The Grand K and its six official copies, kept together in the same safe on the outskirts of Paris and collectively known as the "heir and the spares," will be retired but not forgotten. Scientists want to keep studying them to see whether their masses change over time.

The update will have no discernable impact for most people. Bathroom scales won't suddenly get kinder and kilos and grams won't change in supermarkets.

But the new formula-based definition for the kilogram will have multiple advantages over the precision-crafted metal lump that set the standard from the 19th century to the 21st, through periods of stunning human achievement and stunning follies, including two world wars.

Unlike a physical object, the formula for the kilo, now also known as "the electric kilo," cannot pick up particles of dust, decay with time or be dropped and damaged, but will be easier to share.

But the kilogram, the last remaining unit defined by an artifact, has stubbornly resisted redefining a number that relates the frequency of a wave of light to the energy of a photon in that wave. 

Planck’s constant is a very small number: 6.62607015 x 10-34, to be exact,  Planck calculated the number to fit models of light coming from stars, matching the energy and temperature of the stars to their spectrums of electromagnetic radiation (collectively known as blackbody radiation). At the time, experimental data suggested that energy is not free flowing at any value, but rather contained in bundles or quanta—from which quantum mechanics takes its name—and Planck needed to calculate a value for these bundles to fit his blackbody radiation models.

But before you can convince the world to change the definition of the standard unit of mass, your measurements better be the best ever taken in the history of science. And as Newell puts it, “measuring something absolute is damn hard.”