Kilogram to be based on physical absolute instead of single, physical object

The kilogram is no longer defined by a physical object. Instead, from here on out, the unit of measurement will be based on fundamental constants, atomic properties and physical absolutes.

Scientists around the world will be able to reproduce the mass constant.

Until now, a kilogram unit was based on the mass of a cylinder made of platinum-iridium alloy. Housed in Paris, the cylinder has been the standard-bearer for the base unit of mass for 130 years.

Of course, physical objects change. Each time the cylinder was hauled out of storage to calibrate an instrument, the object shed a handful of atoms. Over the last 130 years, the cylinder lost 50 micrograms.

The abandonment of the physical kilogram was made official on May 20, 2019, which is World Metrology Day, a celebration of the Meter Convention held in 1875, where scientific leaders agreed upon the International System of Units.

With the official change in the definition of the kilogram, as well as changes to the definitions for the base units of charge, temperature, and mole, all international units are now defined by physical constants. Other common units, like the meter, made the switch years ago.

"The [International System of Units] is now based on a set of definitions each linked to the laws of physics and have the advantage of being able to embrace further improvements in measurement science and technology to meet the needs of future users for many years to come," scientists at the Meter Convention announced in a news release.

From now on, the definition of a kilogram will be based on the Planck constant, which is based on the energy of a photon to its frequency. A single kilogram is equal to 6.62607015 times 10^34 kilograms times square meters per second.

Nobel laureate Wolfgang Ketterle, a professor of physics at MIT, explained the change during a lecture on Monday, commemorating World Metrology Day.

"Conceptually, the explanation is that 1 kg is now the mass of a defined number of photons, 1.4755214*10^40, at the frequency of the cesium atomic clock," according to Ketterle.

The change doesn't mean scientists will now have to count photons. As Ketterle explained, there is a multi-step process for using physics and math to precisely define a kilogram.

"If you win a million dollars, and it is paid in pennies, you don't want to count pennies. You will first exchange the pennies into dollar bills, and then the dollar bills into 100 dollar bills, and then you count them," Ketterle told MIT News.

"In metrology, something analogous is done by comparing the atomic clock frequency of the cesium atoms to a much higher atomic frequency. Then you use this frequency to measure the mass of the electron or of a single atom, and only then you start counting," he said.

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