Scientists are about to redefine how much a kilogram weighs

For 129 years, the kilogram has been defined by a lump of metal in a French vault -- but that's about to change.

Jackson Ryan Former Science Editor
Jackson Ryan was CNET's science editor, and a multiple award-winning one at that. Earlier, he'd been a scientist, but he realized he wasn't very happy sitting at a lab bench all day. Science writing, he realized, was the best job in the world -- it let him tell stories about space, the planet, climate change and the people working at the frontiers of human knowledge. He also owns a lot of ugly Christmas sweaters.
Jackson Ryan
3 min read
Research on the new prototype kilogram
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Weight is important. My burgeoning, post-holiday waistline gives me enough evidence of that, but that's not enough for science. 

On Nov. 16, the General Conference on Weights and Measures -- which, look, I'm going to admit doesn't sound like the most riveting event -- begins in France. It's expected that at the conference, scientists will vote to change the definition of a kilogram, affixing it to one of the universe's immutable phenomena: The Planck constant.

That all sounds a little confusing, so let's take a few steps back.

The most widely used form of measurement in the world is based on the metric system, and is officially known as the International System of Units (SI). Seven "base units" make these up, including the ampere, the second and the mole. Some of those measurements were once defined by physical phenomena, such as the second, which was based on the Earth's rotation. Now, the second is defined by periods of radiation in a caesium 133 atom.

The kilogram is the last base unit linked to a physical object.

That physical object is a chunk of metal perpetually housed underground at the "Bureau international des poids et mesures", the International Bureau of Weights and Measures, in Sèvres, France. The platinum-iridium alloy, also known formally as the International Prototype Kilogram and informally as Le Grand K, is sort of like the ring in Lord of the Rings -- it's the one weight to rule them all. Every weight is calibrated against Le Grand K, standardising the measurement of a kilogram across the globe.

But Le Grand K, forged in 1889, has lost 50 micrograms in the last 129 years. The kilogram has become the "999.99995-gram." Except, even though it's lost that tiny fraction of its mass, the kilogram is still defined by Le Grand K, changing over time -- and scientists don't like that. They want it to be constant, forever.

So a kilogram may come to be defined by an unchanging universal phenomenon known as the Planck constant, rather than a hunk of metal in an underground vault. But how does that work?

The new definition of a kilogram, should the vote pass, is admittedly horrifying to read because it makes me feel like I understand almost nothing about the universe. Here it is:

It is defined by taking the fixed numerical value of the Planck constant h to be 6.626 070 15 x 10-34 when expressed in the unit J⋅s, which is equal to kgm2s-1, where the metre and the second are defined in terms of c and ∆νCs.  


The Planck constant is a phenomenon in quantum mechanics that allows mass to be related to electromagnetic energy. It is one of the smallest measures of the physical world and to find it -- to measure it -- was a great challenge. The journey spanned decades and involved a machine known as a Kibble balance that tries to equate not mass but energies.

"We find ourselves at a landmark moment in our journey," said Dr. Barry Inglis, inaugural CEO of the National Measurement Institute Australia.

"After years of research, it's now possible to justify a major revision of the SI. This decision, if taken, will represent a significant scientific achievement."

Most importantly for the kilogram, the Planck constant is just that: constant. Unlike Le Grand K, it won't be 50 micrograms different in 129 years time. That means the kilogram can be forever tied to it -- and no matter how far we wander in across the cosmos, the Planck constant is expected to remain the same. Thus, so will the kilogram.

And if all of that still confuses you, the too-long, didn't-read of it all is actually quite simple: After decades of hard work and hypothesizing, scientists -- those genius men and women -- have found a way to define the kilogram based on a universal truth. 

From Friday onward, the kilogram will always weigh a kilogram. 


The conference will also see votes taken to change the definition of the ampere, the mole and the kelvin. If you're interested, you can watch the world-changing event live on YouTube.

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