The Kilogram is Not Dead Weight Anymore

Redefining a unit of measurement that was invented in 1799 has required the entire metric system to change.
24 May 2019


Scales reading 5kg


As of the 20th of May, 2019, a fundamental constant of the universe has redefined the kilogram. But do not fret, the average person will not see a change in their day-to-day lives; your food portions and scales will still be as they always were. Nonetheless, this is a huge leap for human advancement because, for the past 129 years, the kilogram has been defined by a physical hunk of platinum and iridium stored in one of the most secure locations in the world. “Le Grand K” as it is called, is held in Paris, with multiple other copies scattered throughout the world, and has been avoiding a unit altering scratch for too long now.

The past years of waiting for the redefinition have been especially crucial, as the conditions for precision had to meet a certain level of quality to be deemed worthy of pursuing. The very clean and over simplified plan of action; use a constant (rather than an actual weight) to define mass. And so, scientists voted to use something called the Planck constant as a more precise measurement system. Seems pretty easy right, but what is the Planck constant?

As Richard Brown, of the National Physical Laboratory (NPL) explains: “The Planck Constant is a physical constant which relates the energy carried by a photon, or light or electromagnetic radiation, and relates the energy of that to its frequency.”

Despite its name, the value of the constant has been fluctuating ever so slightly over the last century. The idea of a changing constant should not settle well in the mind, and it has not in the science community. Thus, the Avogadro Project was created to rid the world of the troublesome oxymoron. 

The project aimed to redefine the avogadro constant, or the number of atoms in 12 grams of carbon-12, to a resolution less than 2*10^-8. The reason being that the avogadro constant has a direct relationship with Planck’s constant.

The project spent millions of dollars to create the roundest ball of silicon-28 isotopes in the world, weighing in at one kilogram. In 2015, the project was able to define the new avogadro's constant. The near atomic precision of the sphere allowed scientists to redefine the value to a much higher level of precision which, in turn, makes Planck's constant more precise.

We now know what Planck’s constant is, and that it is precise enough to use, but how can there be a relationship between the energy in a photon and mass?

“The relationship is quite a complex one and I think in order to explain that we have to think about the experiment that’s being used to redefine the kilogram. And that experiment is something called the The Kibble Balance, named after Brian Kibble, who invented the experiment in the 1970s at NPL. The Kibble balance is an experiment which balances mechanical power, from a mass, with electrical power, from an electromagnetic. We calculate the electrical power from that electromagnet using quantum electrical effects, and those equations are governed by fundamental constants. One of which is the planck constant.” says Richard

The final measurements produced a final value of h  with an uncertainty of 10 parts per billion. Rearrange the equation for Planck’s constant and one is able to define a kilogram, a definition that can be understood by any intelligent, interstellar species.

“One of the best analogies  I’ve heard about the revision of the SI is it’s a bit like strengthening the foundations of your house; you don't notice any difference immediately, but what it means is that your house will last a lot longer and you're going to be able to do more things to it in the future.”

That foundation is supporting human ingenuity, and we have now set one that is incredibly strong to fuel our future endeavors.

“...[A]s technology advances and we see things like 5G communication, quantum computing, sensor networks, it’s the better measurements we can make with the revised SI that will make that technology be implemented all the faster.”


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