Since 1889, human civilization has depended upon a magic rock. Known as Le Grand K, it sits under protective glass in St. Cloud, France. Humanity’s magic rock is actually a chunk of metallic alloy that’s 90% platinum and 10% iridium. Its magic comes not because of a supernatural power, but because human scientists declared it to be exactly one kilogram in mass. Today, an international committee representing more than 60 nations has declared Le Grand K is magic no more.

The kilogram is part of a system of weights and measures known as the Système international d’unités (SI), commonly known as the metric system. The metric system is used across the world to determine the distance of a road (meters), the duration of a workday (seconds), and the mass of raw materials (kilogram). While we Americans pride themselves on using their own system of using our own system of miles and pounds, these units are also defined in terms of the metric system. A mile, for example, is defined as *exactly* 1,609.344 meters. Our lives depend upon these standard units of measurement.

Throughout most of history, units of measurement were determine by royal decree or local standards. This meant even the same unit could have different values depending on the region. In France, the foot or *pied du roi *was about 0.325 meters long, while English foot was about 0.305 meters. Since most trade was local, these differences weren’t a problem, but as trade became more global there was a need for an international standard.

So in the 1800s there began an effort to create a system based not on a king’s whim, but rather on the physical world. The meter was defined on the circumference of the Earth, so that 1 Earth circumference was exactly 40 million meters. A second was set by declaring the length of an average day to be exactly 86,400 seconds (24 hours). The kilogram was defined as the mass of 1 liter of water, which is a cube of water 10 centimeters on a side. This meant that the kilogram depended not only on a physical material (water), it also depended on the length of a meter. If the length of a meter changed, so would the mass of a kilogram.

This system was not without its problems. In the early 1800s, the circumference of the Earth wasn’t known with high precision. Also, the Earth is not a perfect sphere, so the length of a meter would depend upon which circumference you used. So in 1867, using the best measurements of Earth at the time, they made two marks on a bar of platinum-iridium alloy. When cooled to the temperature of melting ice, the two marks were declared to be *exactly* one meter apart. In other words, the meter was defined by a “magic stick.”

Time was measured by the apparent motion of the stars. As the Earth rotates, the stars rise and set in the night sky, and astronomers could measure their motion with great accuracy. Astronomers became so accurate that they could measure small changes in Earth’s rotation. The Earth’s rotation is gradually slowing down, and that meant the length of a second was gradually getting longer. So in the 1960s the General Conference on Weights and Measures** **redefined the second using a very precise atomic clocks. These atomic clocks are based upon an element known as Cesium Like all elements, Cesium 133 emits light at specific frequencies. Light is emitted from an atom when an electron moves from a higher energy quantum state to a lower one, and under the right conditions they are always the same. One particular emission from cesium 133 is known as the hyperfine ground state, and it is used to regulate an atomic clock the way the swing of a pendulum regulates a grandfather clock. In 1967 the frequency of light emitted by this hyperfine transition was defined to be 9,192,631,770 Hz. By measuring the frequency, you know the length of a second.

For the first time, a human unit of measurement was based upon a cosmic physical standard. You no longer had to be on Earth to determine the length of a second. No matter where you are in the universe, you can build an atomic clock, and know exactly how long a second is.

In 1983 the General Conference on Weights and Measures eliminated the metric system’s magic stick by defining the meter in terms of the speed of light. The speed of light in a vacuum is always the same, so the conference declared the speed of light to be *exactly* 299,792,458 meters per second. From anywhere in the universe, you can just measure the distance light travels in a second, and you know the exact length of a meter.

Over time, more measurements were defined upon a universal standard, but the kilogram was still defined by the magic rock. And it was becoming a serious problem. Since Le Grand K was always exactly a kilogram, if you shaved a bit of material off it, it would still be a kilogram. It would weigh less, but then so would a kilogram. For various reasons, Le Grand K does change mass slightly. When compared to official copies of the kilogram, it seems to have increased a bit in mass. So there has been a big push to define the kilogram in the way we define meters and seconds.

The obvious choice would be to define the kilogram in terms of the universal constant of gravity G, but gravity is a weak force. We have only been able to measure G to within 1 part in 10,000, which isn’t nearly accurate enough to define a new kilogram. So instead, scientists have sought to use a different physical constant known as the Planck constant.

The Planck constant is central to quantum mechanics. It’s famous for giving quantum theory its fuzzy uncertainty properties, but it also relates to the energy and momentum of quantum objects. As Einstein discovered, energy is related to mass, so mass and the Planck constant are related. One of the ways the Planck constant can be measured is through a scale know as the Kibble balance. It measures how much current an electromagnet needs to hold up a particular mass. By measuring the current needed to hold a kilogram, you can determine the Planck constant. But if you instead define the Planck constant to have a specific value, then the kilogram is defined in terms of the Planck constant. For it to work, the Kibble balance had to be more accurate than the variations in the standard kilogram. It’s taken decades to reach that accuracy, but we finally succeeded in the past couple years.

Which brings us to today. Today the General Conference on Weights and Measures declared that as of 20 March 2019, the Planck constant will be exactly h = 6.62607015 × 10^{−34} **J ⋅ s**. On that day, humanities magic rock will be no more. On that day, our measurement of the universe will truly be based upon the universe itself.

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