April Mini Post 2: Can the new kilogram really help us communicate with extraterrestrials?

For 130 years, there was one golfball-sized hunk of metal to rule (and measure) them all. Now there's a new kilo in town.

Welcome to another April mini-post, Amateurs! In case you missed it last week, I’m toying with a different (shorter? better?) format for a bit:

In the spirit of springtime renewal, I’ve decided to try out a different format for the [next few] newsletters. Instead of a couple of longer deep dives on a single [large] subject, I thought it would be fun to do some shorter mini-posts that focus in on one bite-sized idea at a time. There are just too many topics out there to choose from! Can’t we just do them all? You’d better believe we’re gonna try.

Give me a shout if you have opinions about what format you like best. You can leave a comment on this post, reply directly via email on any newsletter, or reach out on social media if we’re connected there.

Big thanks to you all who have shared your feedback so far, and hello to the new subscribers on the block! Here’s a post that works as a great primer for who I am and what you’re in for at the Amateurist.

And since no preamble would be complete without mildly debasing myself in the pursuit of readership and support, I’ll just say this: if you enjoy this publication, consider sharing it with someone new and/or supporting it with a (one mid-range latte per month) paid subscription. It would mean the world to me!

Now on to the main event: the universal kilo

Until 2020, every kilogram was measured against a single physical object: the original metal chunk from 1889, otherwise known as Le Grand K or the International Prototype of the Kilogram (IPK). This small cylinder made of 90% platinum and 10% iridium is twice as dense as lead and 21 times denser than water. For as long as it’s existed it’s lived under a double bell jar deep in an underground vault near Paris, only emerging every forty years for cleaning and calibration.

The International Bureau of Weights and Measures (BIPM) was founded in 1889. At that time, the Bureau codified the second, the kilogram, and the metre based on measurements members believed to be constant, since they were based on the size of the earth. Metal kilo and metre prototypes were cast and became the official physical standards. Member nations all got one, just in case.


  • The standard kilogram was originally based on the mass of 1000 cubic centimetres of water at four degrees Celsius.

  • The standard second was 1/86,400 of the time it takes for Earth to complete a single rotation on its axis.

  • The standard metre was one one-millionth the distance between the North Pole and the equator (specifically measured along the Paris meridian). This one, it’s since been found, is about two millimetres too long.

As it turns out, relying on the consistent application of inconsistent variables as the basis of an international unit of measure isn’t the winning strategy it appeared to be in 1889.

For example, time speeds up and slows down depending on the gravitational force in a particular location. The earth’s rotation is also notoriously wobbly, which leads time-keepers to tack on a leap year every four years just to keep up.

Having the single example of what a kilogram weighs be subject to physical conditions rather than being guaranteed by a formula of universal constants created issues for both its longevity and its application beyond our own planet. In fact, the fact that the cylinder even existed in the physical realm posed problems for the Bureau.

The IPK’s exposure to airborne particles began to add (small but still significant) weight to the official kilogram, even under layers of glass. The first calibration included a cleaning to lift contaminants from the surface - I bet you can guess the outcome there. They cleaned it too well and the IPK actually lost mass.

It reminds me of this scene from Brooklyn Nine-Nine:

Amy: I have great teeth! You can’t take that away from me!

Dentist: Have you heard of over-brushing? Your aggressive technique has stripped away the protective enamel and much of your gum. You have seven cavities.

Bureau technicians quickly established standard cleaning procedures that limited the impacts of handling during periodic calibrations. Even after those measures were in place, however, the IPK still netted a 50 microgram loss over its 130-year run.

Things were also starting to get funky with the replicas when the third calibration came around in the late ‘80s. In 1989, the U.S. replica was found to have lost 41 micrograms against the IPK. Many of the other replicas have inexplicably diverged from the pattern of loss demonstrated by the original, some adding mass and some losing despite the preventative measures. Chaos reigns!

Here’s where the story turns philosophical. If a unit of measure is only true under the conditions of our own planet, then by definition they aren’t a universal unit of measure. That’s pretty embarrassing for a so-called advanced civilization. But the whole reason it took so long to address the kilo issue is that the math required to prove a constant mass (otherwise known as Planck’s constant) wasn’t precise enough to do so until 2005. 2005!!

The mathematical work had been underway since the ‘70s, though, between the electromagnetic measurement called the Kibble balance and the ability to measure Avogadro’s constant (which relates the number of molecules or atoms to the mass of an object). Once the two could act as checks on each other, metrologists would no longer need a piece of metal to gauge what physical mass really was. The time was nigh to update the kilogram for good once the fundamental constants could be proven.

By 2019 the new kilo in town was here to stay. The BIPM officially adopted the foundational formula for mass under the International System of Units since it had the philosophical and mathematical backing to remain constant in any corner of the universe. The kilogram is now in line with the ampere (electric current), candela (light intensity), kelvin (temperature), metre (length), mole (amount of substance), and second (time) as fundamental constants. Physics is universal, after all.

National Institute of Standards and Technology (NIST) physicist Stephan Schlamminger said it best:

"If we make contact with aliens, what are we going to talk to them about? Physics. There is nothing else. But if you tell aliens that our units of measurement are based on a hunk of metal, you will be the laughingstock of the galaxy."

I’m sure humans will find something else to be a laughingstock about.