# The World Gets Ready to Change the Fundamentals of Measurement, Again

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## The organization in charge of measurement standards is considering a change to one of the most basic forms of measurement.

Measurements are, in both direct and indirect ways, a key to our society. They help us agree on a lot of important things that civilization is based upon. At a personal level, agreement on measurements makes it possible, for example, to buy a 10-pound bag of flour and know that you are getting the correct amount. Or, measurements can prove that you own exactly 100 meters of land, and where that property begins and ends. It can sometimes even be used against you, like when a police officer records your vehicle traveling above the posted speed limit.

On a national scale, measurements are the basis of trade agreements, peace treaties and even national borders. All of those things can be disputed, but the measurements that they are based on are almost always accepted as true.

Because of that, it’s always a big deal when measurements change. The last time this happened was only a few years ago, in 2018, when 60 countries voted to change how the world defined several key measurements including the kilogram, kelvin, ampere and mole. At the time, all of those measurements were changed so that they could be based on universal constants. For example, a meter is defined as the distance traveled by light in a vacuum—299,792,458 meters per second—over a given period of time. It’s a universal, and unchanging definition.

Changing most measurements so that they align with universal constants, like was done in 2018, basically involves adding the element of time, because it’s a constant property. For example, to properly define a meter, we need to be able to first record how far light can travel over time, so we first need an accurate definition of time. If our definition of the passage of time is somehow skewed, then almost every other measurement that relies on it will also be affected.

Perfect timing

The most basic measurement of time is the second. But defining how long a second lasts, which is now a key to many other measurements, is not an easy thing to do with perfect accuracy. Ancient civilizations like the Egyptians, Babylonians and Greeks did a pretty good job of it using the tools they had available. They mostly used the Earth’s spin to come up with their definitions. But what they could not know was that the Earth’s spin is actually decreasing over time. Over the past 2,000 years, measuring time based on the Earth’s rotation has resulted in the loss of over three hours, requiring corrections. A day in the life of a dinosaur living during their prime was, for example, a few hours shorter than the days we experience now, because the Earth was spinning faster back then.

Then in 1967, science had advanced to the point where we could start to use a new definition of time that was independent of the Earth’s rotation. Devices like the first NIST atomic clock were created to improve accuracy, with subsequent clocks getting even more precise. Because of devices like atomic clocks, we can now define a second as the time it takes a cesium atom to vibrate about 9 billion times—9,192,631,770 times to be precise. Cesium is a heavy metal, and one of the only elements whose atoms vibrate at a constant rate.

So, it would seem that we now finally have constant measurements, because a second is well defined, and almost every other measurement now relies on it. That was certainly the thought when the Bureau International des Poids et Mesures—BIPM— the French-based organization that spearheads all worldwide measurements standards, adopted the new definitions in 2019.

But wait...well, a second. Because it looks like we may need to change the definitions again.

A second take on seconds

It turns out that atomic clocks are not completely, universally accurate. There are three in Colorado, and each of them record marginally different measurements. It turns out that they are affected slightly by gravity, so their distance above sea level—being higher up means less gravity—affects their accuracy. Those that are higher up run faster. And because the clocks are so sensitive, even a two centimeter change in altitude can affect their measurements.

I guess Einstein’s Theory of General Relativity was pretty accurate, because he correctly said that time would run more slowly within the gravity of a planet. The difference between the measurements of the atomic clocks is almost infinitesimally minor, but precise measurements need to be just that, as precise and accurate as possible.

Most problems with current atomic clocks, minor as they might be, may have been solved by the introduction of comb lasers, which in turn enable a new generation of optical atomic clocks. Those are reportedly up to 1,000 times more accurate than current atomic clocks, and can look at the wavelengths of other atoms beyond just cesium. Because of that breakthrough, the BIPM is considering the rather unprecedented move of changing the definition of a second once again, basing it on the vibration of atoms as recorded by the optical atomic clocks. If that proposal is approved, it could mean that the world will get a new definition of a second as soon as 2030.

Any change to the definition of a second will surely be overlooked by most people. However, at larger scales or over long periods of time, even a tiny change can make a big difference. And now that many other measurement constants are at least partially based on the definition of a second, changing how a second is recorded could be a big deal that ripples out into other areas. Basically, if the definition of a second changes, so will the definition of a meter, along with any other key measurements that use time in their calculations. That is why the BIPM and its partners like NIST are moving so slowly with the proposal. If it passes, even though most of us probably won’t notice, our world will forever be changed.

John Breeden II is an award-winning journalist and reviewer with over 20 years of experience covering technology. He is the CEO of the Tech Writers Bureau, a group that creates technological thought leadership content for organizations of all sizes. Twitter: @LabGuys

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