Hank:A meter is about... about this long. It is the official unit of length in the International System of Units, the standard used by scientists and most countries around the world.

The meter might seem kind of arbitrary, and of course it is, but its definition was carefully chosen and made more precise over the past few centuries. For a long time, countries used all kinds of different units, like the length of someone’s forearm – called the cubit – or the English yard. But around the 18th century, the French Academy of Sciences set out to standardize some measurements.

There were a couple ideas floating around about how to define the basic unit of length. One was the length of a pendulum that could swing from one side to the other in one second. But as elegant as that is, this idea didn’t stick, because the force of gravity can vary a bit depending on where you are on Earth, which changes how pendulums will swing.

Instead, they decided this unit would be a fraction of the circumference of the Earth 0:53– specifically, the distance from the Equator to the North Pole, passing through Paris, divided by 10 million. A group of surveyors ended up measuring just between Dunkirk, France and Barcelona, Spain. With a bit of geometry, they estimated the full distance. But... they didn’t correctly take into account how much the Earth squishes as it rotates. Despite that error, they used that distance to calculate what is now the standard metric unit length.

In 1799, France produced a platinum bar of that length called the Meter of the Archives. Platinum atoms don’t really shed subatomic particles or react very easily. That’s good, because they didn’t want the bar to shrink or change composition very much over time, like how an iron bar would rust. But platinum metal is a little bit soft, and, like most materials, it expands and contracts if the temperature changes. These problems weren’t super pressing, though, and the meter became more popular over the next century.

By 1875, 17 different countries agreed to sign the Treaty of the Meter at the International Metric Convention, establishing organizations to decide on all measurement-related things. Of course, everyone also wanted their very own copy of an official meter bar. So in 1889, each country received an International Prototype Meter, made out of a mix of platinum and iridium metals – for extra stability, and more hardness.

And to deal with any temperature effects, the official measurement was defined at zero degrees Celsius. All this made the bar very, very reliable, and this definition was good enough for most human activities. But good enough for human activities is not the same as good enough for science.

Scientists still weren’t satisfied because the meter wasn’t defined by standard, universal quantities. Like, say if you wanted to explain a meter to aliens, they would need to know the approximate distance from Earth’s Equator to the North Pole. Not super convenient. Over time, some people suggested using wavelengths of light instead.

They argued that light was much more fundamental, because it exists throughout the universe. But light emitted from different sources has different wavelengths. Every atom, for example, has electrons that can absorb energy and then re-emit that energy as photons of light when they transition back to their original position.

So scientists looked at the emitted light from different elements and tried to choose one that barely had any variation in wavelength. They ended up picking the sharp, orange-y light produced by a form of krypton called krypton-86, during one particular energy transition. And in 1960, at the Eleventh General Conference on Weights and Measures, the meter was redefined to be 1,659,763.73 times the wavelength of light emitted from krypton-86, in a vacuum.

The actual length of meter didn’t change at all, just the definition. But we didn’t stop there. As our understanding of light grew, and we could measure it more and more precisely, we realized there was an even more fundamental way to define the meter.

See, the speed of light in a vacuum is a constant, no matter your reference frame. Our current understanding is that the speed of light is one of the most fundamental quantities in the universe. So in 1983, the meter was redefined again as the distance that light in a vacuum travels in 1/299,792,458 of a second.

And we’re pretty sure that’s about as fundamental as you can get. Of course then, you have to find what a second is, which is a story for another time.

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