Update 11/16/18: As expected, scientists have voted to dethrone le grand k in favor of measuring the kilogram based on the value of Planckâs constant. Itâs sad to see the metal hunk go, though surely itâs for the best. The next question is whether we Americans will ever figure out the metric system. Read the story below for context.

The future of mass depends on a conference vote next week.

A long-discussed resolution for next weekâs General Conference on Weights and Measures would toss out the international prototype of the kilogramâa hunk of platinum and iridium in Paris that everyone agrees weighs 1 kilogram. Instead, the International Bureau of Weights and Measures (BIPM) would redefine the International System of Units to ensure that kilograms are based on things that canât change over time. It would require an entire rethinking of how kilograms work.

As weâve previously reported, thereâs a reason scientists rely on a hunk of metal. King Louis XVI created a standardized weight to stop tradespeople from scamming customers, which was called the grave and was equal to a liter of water held at zero degrees Celsius. In 1795, French leaders decided instead to go with the gram, a cubic centimeter of water held at 4 degrees Celsius. The hunk of metalÂ replaced the inconvenient water measurement in 1799, and 17 countriesÂ signed a treaty in 1875 to standardize their mass to the international prototype of the kilogram, or le grand k. Today, over a hundred countries, including the U.S., have signed an agreement to use the BIPMâs services.

But even today, itâs all based on comparison. Le grand k remains secured under lock and key (three keys, actually) in Paris. Countries can get copies of le grand k, and then must use scales calibrated to the copy of the real thing. But itâs possible that something can get lost in the bucket brigadeâand of course, thereâs a chance the original goes missing or gets destroyed. Comparisons with copies have also revealed that the mass of the kilogram (or the mass of the copies) has slightly changed, albeit an incredibly tiny amount. Itâs possible the copies absorbed molecules from the air, or the original lost mass through washing or other physical processes, NPR once reported.Â

The proposal to be voted on next week would drop the kilogram as the last unit still based on a physical thing on the Earth. Instead, we would useÂ the universe itself as a measure. Built into the fabric of spacetime are fundamental constants that scientists can measure in experiments. The speed of light in a vacuum is always the same velocity, 299,792,458 meters per second. The ratio of a light particleâs energy to its frequency is equal to 6.62607015Ă10â34 Joule-seconds, where a jouleâs units are kilograms times meters squared over seconds squared; this number is called Planckâs constant. Meters and seconds have already been redefined by the BIPM to align with the speed of light and the vibration of a cesium atom, respectively. A kilogram would now be defined based on the fixed value of Planckâs constant.