Author: Brandon Comella, B.S. Physics, Caltech

Have you heard the MASSive news out of France? The kilogram was just redefined.

(If you got that joke, might I suggest a career in Physics?)

First, let’s discuss measurements. Back in olden times, you couldn’t just go to Home Depot and buy a yardstick or a tape measure. The most readily at-hand tools to measure were things like hands and feet. While measuring objects based on the length of your thumb or the length of your foot might be fine for personal use, once you start engaging in commerce or scientific measurements, it’s harder to relate a “Dave foot” to a “Laura” foot.”

To make things easier, countries started to set their own standards for weights and measurements, but still, if you wanted to know how much a pound was, you’d have to buy a copy made from the original standard. Unscrupulous merchants would shave pieces off their weights to shortchange customers.

Science requires precision, so scientists created “Le Grand K” (The Big Kilogram), a cylinder of metal that was the standard for exactly 1 kilogram. Because it would be impractical to have everyone go to a basement in France each time they needed to calibrate their equipment, copies were made and given to the major countries.

Problems started happening when it was discovered that the masses of the copies were deviating from the original. Why? Microscopic bits of pollutants from the air were being deposited on the surface of The Kilogram. Why couldn’t the scientists just use a little Windex to clean the surface? Well, for starters, even though their commercials make it seem like Windex doesn’t leave anything behind, there is always a microscopic amount of residue left behind.

The most recent method for cleaning involved using ultraviolet light in an atmosphere of ozone to zap contaminants off the surface. This is an improvement over the old method of wiping the cylinder with a cloth soaked in various cleaning solvents. Still, the pervading issue with cleaning remains – if you clean for too short a time you leave behind some contaminants, and if you clean for too long, you risk wiping away pieces of the metal as well. Anyone that’s seen an antique door handle knows that repeated rubbings can cause the metal to wear down noticeably.

The solution scientists have come up with is to base the definition of a kilogram on a fundamental constant of the universe. Other fundamental units have been defined using universal constants. For example, the definition of a meter is the distance light in a vacuum travels in 1/299,792,458th of a second. Because the speed of light in a vacuum is a constant, and something that is (relatively) easy to measure, scientists everywhere can get accurate measurements.

A similar definition has just been approved for the kilogram using Planck’s constant. In order to determine the exact value of a kilogram, a scientist would need to use a Kibble balance, a machine that looks like something out of a steampunk-brewery and measures mass using electrical currents. 

Even though Kibble balances are a bit pricey and complicated to use right now, as the price for the balance drops, soon any lab will be able to accurately measure mass.

With the redefinition of the kilogram, all seven SI (International System) units are now defined in terms of fundamental constants built into the fabric of the universe.