They changed the kilo on us last week!

The most precise balance in the world.

The most precise balance in the world.

Last week there was a historic vote in Paris to redefine what the mass of a kilogram.

Does that mean you need to recalculate your percentages now? Well, how much mass is a kilogram?

Simple answer: The same as a liter of water.

Weightlifter’s answer: double it and add ten percent.

Last week’s answer: as much as this block of stuff in Paris weighs.

New answer: As much as it needs to be in order to make Planck's constant exactly 6.62607015×10^-34 kg m^2 s^-1.

Seriously, this is really cool. When I was in grad school I remember going to talks and visiting neighboring labs that were proposing building the watt (now Kibble) balance (pictured). It’s nice to have everything come full circle - I know the guys who define the mass standard and now we lift it.

This is a big deal because now every unit of measurement is defined in terms of a fundamental physical constant. Until this week, the unit of mass could only be defined by pointing to a hermetically sealed brick in Paris. That was the sole definition of a kilogram. It couldn’t be measured or derived from other measurements. But now we have a way to accurately relate the measurement of electrical current to mass through Plank’s constant.

Why is this so cool? It means that anyone (with the budget of NIST) can “measure” units using the properties of the universe. It doesn’t rely on a (single) physical object sitting in Paris.

Philosophically, we are closer to being in sync with the universe because everything we measure is based on properties of the universe - there’s nothing arbitrary anymore.

Don’t worry though, you don’t need to worry about recalculating your percentages. This definition only nails down that 9th significant digit. Our plates are probably only accurate to 1% at best, the kettlebells are probably only 5%.

Michael Deskevich