*This post has been updated.*

Beneath three glass bell jars, in a locked vault in the basement of a highly secure facility outside Paris, sits the world's most important kilogram.

Ever since 1889, when the __General Conference on Weights and Measures __(CGPM) made the imperious pronouncement, "this prototype shall henceforth be considered to be the unit of mass," this platinum and iridium cylinder has served as the standard by which all other kilograms are measured, from the weights on a high-tech lab scale to the plastic discs high schoolers use in chemistry class. It's known as "le Grand K," and it's afforded the security and scrutiny befitting such a fancy title. Even the researchers who work with it can't touch it, lest their fingertips wipe away atoms or leave residue on the gleaming surface. The vault containing the cylinder can only be opened by gathering three custodians carrying three different keys, and that's happened fewer than a dozen times in the kilogram's 127-year history

Yet, despite all those precautions, "le Grand K" is getting slightly less grand. It seems to be shrinking — or at least, sister cylinders that used to be exactly its weight are now slightly heftier — and scientists don't know why. Perhaps microscopic bits of metal were wiped away during washing, or perhaps Le Grand K has stayed the same and all those other objects have gotten heavier. Or perhaps that's just the price of being a part of the physical world; nothing, not even "le Grand K," can exist unchanged.

Which is fine for philosophers, but not for scientists.

“This is simply not a satisfactory situation,” Terry Quinn, director emeritus of the International Bureau of Weights and Measures, told Wired in 2011. “You have an object made with the technology of the 19th century upon which a very large proportion of modern measurements are based — not just mass, but electrical measurements and measurements of force and heat and light.”

He concluded: "Something must be done."

And his colleagues agreed. For years, they've been working to find a measure of mass that is defined according to some unchanging universal constant, much the way a meter is defined in terms of the speed of light. But it's been tough going. The two most promising constants that could be applied to the definition of a kilogram haven't been measured with enough accuracy to be universally useful.

Until now. Scientists at the National Institute of Standards and Technology (NIST) reported in the Review of Scientific Instruments Tuesday that they now have one of the most precise measures yet of a quantum physics fundamental known as Planck's constant. It matches up with other calculations of the constant, and when the international community meets in 2018 to redefine the kilogram, the NIST hopes that scientists will turn to this more accurate Planck's constant for their new standard.

The new measurement of the constant was taken using the NIST-4 watt balance, a massive, highly-calibrated contraption that uses electrical and magnetic forces to gauge weight. It put the value for Planck's constant, known to physicists as "*h,*" at 6.62606983 times 10 the negative 34th power kilograms per square meter per second.

That is a very small, very weird number. And you're probably wondering what it means.

Planck refers to Max Planck, the Nobel prize-winning founder of quantum theory. It was Planck who discovered more than 100 years ago that energy is expressed in discrete units (that is, it's "quantized"). Darine Haddad, the lead author of the NIST study, frames it in terms of adding sweetener to your coffee: rather than being released in a continuous stream, like someone pouring sugar directly from the bowl, particles emit energy in small chunks comparable to sugar cubes.

The smallest chunks of energy come from a single unit of electromagnetic radiation — a photon. The amount of energy photons release depends on the frequency at which they're vibrating, and Planck's constant describes that relationship; a photon vibrating at a frequency of 1 hertz will release a Planck's constant worth of energy (you don't really want to read that number again, do you?)

When the General Conference on Weights and Measures (CGPM), which meets in Paris every four years, agreed to reconsider the definition of the kilogram, its members said that at least one instrument would need to calculate Planck's constant to an uncertainty of just 20 parts per billion. The NIST's finding is almost there — they say that their calculation comes within plus or minus 34 parts per billion — and they believe they can get it even closer. The CGPM also requires that four other instruments come to the same number, and that the final calculation of a kilogram be reconciled with another method of measuring weight based on a constant called Avogadro's number. (If nothing else, this should make clear how much 19th century scientists enjoyed naming universal principles after one another.)

If all that work pays off, scientists will have a measure of mass that is unchanged by location, circumstance or time. And le Grand K will become just a fancy paperweight.

*Correction: An earlier version of this post incorrectly reported the accuracy of the NIST-4 watt balance's measurement. It is the second most accurate watt measurement of Planck’s constant and the third most accurate measure overall. *

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