The kilogram, or kilo as it's typically known, is a household term today. But not many people would know that for the last 130 years, a cylinder made of a platinum-iridium alloy has been the internationally-accepted standard for the kilogram. Known as the International Prototype of the Kilogram (IPK), copies of this artifact are used as reference weights by countries across the world to calibrate scales and weights in places like factories, supermarkets and bakeries.

However, this lump of metal has lived its life. Kept in a vault in a suburb of Paris called Saint Cloud, it will no longer be relevant beginning Monday. Instead, the kilogram will be officially defined in terms of atomic properties and fundamental physics constants to make it a more accurate measurement unit. While this change will not impact the way we measure the gain or shedding of kilos around our waists, or weigh potatoes on a scale, it will be provide a more precise unit of measurement for scientists.

Objects on which physical standards are based, such as the standard meter, were already replaced years ago. However, the kilogram turned out to be a harder unit to define in absolute terms. Physicists and engineers have been frustrated by the imprecision of a unit based on a single physical object. For instance, every time the standard kilo was handled to compare it to another unit that could then be used to calibrate instruments, it would shed some atoms and its mass would be slightly changed. Over its lifetime, that standard kilo is estimated to have lost about 50 micrograms, MIT researchers noted in a press statement.

It was on 16 November that the General Conference on Weights and Measures (CGPM) in Versailles voted to officially make the change. Hence, beginning Monday which is also World Metrology Day, a kilo will be defined by fixing the numerical value of a fundamental constant of nature known as the Planck constant. This constant relates the energy of a photon to its frequency, and is referred to by the letter h. It is now defined as 6.62607015 times 10-34 kilograms times square meters per second, thereby defining the kilogram in terms of the second and the meter.

Wolfgang Ketterle, a Nobel Prize winner and the John D. MacArthur Professor of Physics at MIT, notes that the new definition of a kilogram corresponds to the mass of an exact number of particles--a very large number of particles. According to his calculations, it is 1.4755214 times 1040 photons (particles of light) of a particular wavelength, which is that of cesium atoms used in atomic clocks.

“Ideally, every high school teacher would tell his or her science class about this historic change," suggests Ketterle. Explaining how the other basic units have been defined through basic physical constants is a bit more straightforward than it is for the kilo: The second, for example, is defined as a specific number of vibrations of an atom of cesium. The meter, no longer a metal bar in Saint Cloud, is now defined as the distance that light travels (in a vacuum) in a specific interval of time, namely 1/299,792,458 of a second, the press statement explains.

Defining the kilo through the mass of photons has to address the fact that photons are constantly whizzing around at the speed of light. Since they are light, getting them to sit still on a balance scale is not possible. Instead, they can be trapped between a pair of mirrors, which form an 'optical cavity' that keeps them confined. Then, that cavity and its trapped photons can be placed on a balance and measured. The difference between an empty cavity and an identical one full of photons thus provides the mass of the photons themselves. So that’s the concept behind measuring a kilo according to the new definition.

However, collecting 1040 photons (1 followed by 40 zeros) is not practical. So measurements are made of a much smaller number and scaled up step by step. “How do you count to 1040? Well, you can’t," Ketterle notes, adding, “However, you can do it by using multiple steps." For instance, he explains, that if you win a million dollars and it is paid in pennies, you will not want to count pennies so will first exchange the pennies into dollar bills, and then the dollar bills into 100 dollar bills, and then you count them.

That’s essentially the principle used for measuring mass, according to Ketterle. “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 says.

In practice, the Kibble balance and the single-crystal silicon sphere are the two techniques that laboratories around the world can now use to provide a precise standard for mass and measurements of weights, without ever again having to correlate their measurements with a specific physical object at some central repository. The new definitions have great power, according to Ketterle because “every new method to count photons or atoms or measure frequencies will further improve the measurements of mass, since mass is no longer connected to an imprecise, man-made artifact".

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