Seconds, meters, kilograms, ampere, kelvins, moles, and candelas: What do they have in common, and which one is the odd one out?

These are all SI base units, and until last Friday, the kilogram was the odd one out. Every other unit was defined in terms of natural constants. One second is defined as 9,192,631,770 cycles of radiation of a Caesium-133 atom. One meter is the distance light travels in vacuum in $\frac{1}{299792458}$ seconds. And so on.

Only the kilogram’s definition was not based on some sort of natural constant. Instead, it was defined by a reference kilogram, a cylinder made out of platinum-iridium alloy, and stored in an underground vault in Paris. This, along with several replicas distributed around the world, was the exact definition of how much mass one kilogram had. Whenever scales and weights had to be calibrated, one of these replicas was used to make sure that it had the correct value. This also impacted the US: Even though it uses pounds to measure weight, a pound was still defined in terms of a kilogram.

There was only one problem with that definition: The reference kilogram was losing weight. Every few decades, all replicas of the reference kilogram are brought back to Paris, and weighted against the reference. And last time this happened, there was a problem: The reference weight had lost about 50 micrograms, less than the weight of a hair. Or rather, since this is the reference kilogram, everything else got heavier by 50 micrograms. In our daily lives, this difference literally does not matter, but it is a serious problem for scientific experiments, where precision down to the smallest values is key.

So, the decision was made to move the definition of the kilogram off of a reference weight, and use some sort natural constant as the basis instead. This sounds simpler than it is; it involved decades of research. Eventually, it resulted in the Kibble balance, named after physicist Bryan Kibble, who invented it in 1975. It works like a regular scale, except one side uses electromagnetic force to balance the other side’s mass. This electromagnetic force can be measured extremely precisely. So, after about 4 decades of refinement and new scientific discoveries (including the results of 2 Nobel prices), this balance was sensitive enough to measure weight precisely down to the Planck constant¹.

As its name implies, it is a constant, but actually measuring it accurately had been difficult, in part because of the uncertainties introduced due to the old definition of the kilogram. With the Kibble balance, we can turn this on its head: Set Planck’s constant to a fixed value, and then, as the energy of a photon is defined as the Planck constant (denoted as h) times its frequency ($E=hf$), combined with Einstein’s famous $E=mc^{2}$ equation, we can define mass thus: $m=\frac{hf}{c^{2}}$. And since the Kibble balance is now precise enough to measure the value of Planck’s constant, we can use this equation to precisely define the kilogram.

And thus, on November 16th, an international committee of scientists voted to use these results to redefine the kilogram based on the Planck constant. Along with this, they also redefined the kelvin, the mole, and the ampere. The values don’t change, but their definitions have been updated to reflect that the kilogram is now also based on a natural constant.

These new definitions won’t go into effect until next May, and even when it happens, nothing around us will change, except that the kilogram in Paris will no longer be precisely one kilogram. This change will mostly matter to scientists and their experiments and measurements. But it is still an impressive achievement to be able to measure the world around us so precisely, in a way that is the same for everyone, not just here on Earth, but everywhere in the known universe.

Unforeseen Consequences

In Issue #82, I wrote about black boxes and their impact on flight safety. Since then, Boing has issued a bulletin, informing pilots about a safety feature that it hadn’t told pilots about previously, and whose malfunction might have contributed to the Lion Air Flight 610 crash.

Flat Believes

It’s generally accepted knowledge that the Earth is (mostly) a sphere. So why do Flat Earth believes still exist? (YouTube)

Diet Advice

It feels like every week, you’ll get new, and different, advice on what you should and shouldn’t eat to stay healthy. The Mediterranean Diet (with lots of olive oil, fish, and nuts) is the way to go. Too much oil kills you. Wine is good for you, except any alcohol is poison. But in reality, all these new studies add weight to what you already knew, and don’t throw it into doubt.

📖 Weekly Longread 📚

Big Tech wants to exploit our personal data, and the government wants to keep tabs on us. But “privacy” isn’t what’s really at stake: Why Do We Care So Much About Privacy?

🦄 Unicorn Chaser 🦄

A breathtaking storm time-lapse video: Monsoon V