How Much Does a Kilogram Weigh?

etteHow much does a kilogram weigh? Well…it’s a kilogram, right? But how do we know how much that is? Maybe a better answer is: 1000 grams.

A gram was originally defined as the mass of one cubic centimeter of water at the melting point of ice, and well, that’s something we can actually measure, rather than some ephemeral idea of “a kilogram”.

So, now we can say “a kilogram is 1000 cubic centimeters of water at the melting point of ice”. That’s almost dead on accurate, but that’s not how much a kilogram weighs. The real answer is that a kilogram is defined by the weight of one single object in the entire universe: Le Grand K, or “the Big K”.

On the outskirts of Paris, in the basement of the International Bureau of Weights and Measures, is an environmentally controlled, impenetrable safe, in which is the official definition of the kilogram. It is called the International Prototype of the Kilogram or IPK. It is a cylindrical piece of metal, a platinum alloy to be specific, about the size of a film canister or a golf ball, and it weighs exactly one kilogram. And there’s no way it can’t, because the official metric definition of a kilogram is equal to whatever Le Grand K weighs.

So if you sneak into the Pavillion De Breteuil where it’s kept, and shave one gram off the top, every single “formerly” one-kilogram weight in the world will now officially be one gram heavier than a kilogram.

I cannot stress this enough: the entire world’s scientific understanding of what a kilogram is based on the exact weight of a physical object stored in a French basement. And it’s been this way since 1889.

Now, that’s not to say that the IPK isn’t a fairly reliable source. It is made up of 90% platinum and 10% iridium, making it super hard, resistant to oxidation, nearly twice as dense as lead, and not very susceptible to magnets. There are a number of exact copies all around the world to allow for comparison and calibration.

Now, it’s great that they’re going to such lengths to make sure that nothing happens to the IPK, because if something were to happen to it, science would change — mostly physics.

Think about how often we use the kilogram, and how much of physics is built on itself. Take, for example, the newton, which is defined by the amount of force necessary to move a kilogram at one meter per second squared. Then, take the pascal, which is defined as one newton per square meter, or one kilogram per meter per second squared.

The kilogram is a base SI unit, so it affects any measurement that uses mass in kilograms. And that’s a really bad precedent.

Already, despite every precaution, the IPK is changing. Since 1889 there have only been two other verifications of its weight, in 194817 and 1989. What they found is that the Le Grand K has lost a petit amount of weight. Roughly 50 micrograms, or 50 billionths of a kilogram. Worse yet, scientists don’t know why that’s happening.

So, does this change in mass means that now all our calculations have to be redone? Of course not, but it did force the people who make those sorts of decisions, The General Conference on Weights and Measures, to try and figure out a better system of defining the kilogram, rather than something you can accidentally lose in the washing machine.

In 2011, they generally agreed to redefine the kilogram based on Planck’s constant. Planck’s constant is a number representing the relationship between the energy of a particle and its frequency. You don’t really have to understand how that works, except that it never changes, and it relates to energy. That’s useful, because we can then relate energy to mass thanks to Einstein’s E=mc2 equation. This means we can define mass, and therefore, the kilogram, using a constant figure.

But the Conference on Weights and Measures decided to postpone the decision to make that change until 2014, and in 2014 they postponed it to 2018. Which means that if you’re having trouble losing weight, you still have a year or two to fly to France and stick a piece of gum on the Grand K. A heavier kilogram means a smaller you!

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