Everything Everywhere Daily: History, Science, Geography & More - Base Units of Measurement

Episode Date: February 22, 2021

Every day we are constantly using measurements. We have ways of measuring distance, temperature, time, light, pressure, energy….everything. Yet, why do we measure everything the way we do? Why is a second, a second, and why is a meter, a meter? Learn more about why our units of measurement are the way they are on this episode of Everything Everywhere Daily. Learn more about your ad choices. Visit megaphone.fm/adchoices

Every day, we're constantly using measurements. We have ways of measuring distance, temperature, time, light, pressure, energy, basically everything. Yet, why do we measure everything the way we do? Why is a second a second? And why is a meter a meter? Learn more about why our units of measurement are the way they are on this episode of Everything Everywhere Daily. What if your perceptions about the past were wrong?

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Way back in the day, every locality might have had its own unit of measure. Some king or local official might have used their foot or arm as the basis for length, for example. and then everyone would just use that. I've been to a few old European cities where they still have a metal bar hanging in the town square, which was the local unit of measure. However, this system of local units of measure wasn't very effective as trade between places grew. There was a need to standardize measurements so everyone was on the same page. This was actually one of the big policy changes which came from the French Revolution.

Not only did they propose a universal standard of measurement, but also a decimalized system as well. Here I'll refer you to my previous episode on why the United States is not on the metric system, where I talk about some of the history of the metric system. Once they had a system in place for units such as kilograms, meters, and seconds, the question then became what exactly is a kilogram, a meter, or a second. As science advanced, there developed a need for greater precision. Even if measurement techniques got better, there was a need for a better definition of what you were measuring against.

Let's start by looking at the meter. The original definition of a meter was going to be the length of a pendulum with a half period of one second. A full period is the time it takes for a pendulum to go back and forth. So a half period is just the time it takes to go from one side to the other. This sounds like a pretty good definition, especially for the 18th century. However, they found a problem. The period of a pendulum can change depending where it is on the earth. So that idea got thrown at the window and they needed to replace it with something else.

The next definition, which was actually implemented, was that a meter was one 10 millionth, the distance from the North Pole to the equator going through the meridian which goes through Paris. However, at the time they chose that definition, no one had actually been to the North Pole. In fact, there hadn't even been a proper survey done to get a good measurement to the Earth. They didn't even know that the Earth wasn't a perfect sphere. It's slightly flatter at the poles than it is at the equator. This just wasn't very accurate, and it wasn't something that any way. could just figure out if they wanted to measure a meter themselves.

In 1875, the meter convention was held in Paris, and they created the International Bureau of Wights and Measures, which was responsible for the creation of the prototype meter bar. They literally created a metal bar made out of 90% platinum and 10% aridium, and that bar was the meter. And I can't stress this enough. That metal bar sitting in a vault in Paris wasn't a representation of a meter. By definition, that bar was the meter.

Technically, it was the meter when it was at zero degrees Celsius. Identical copies of that metal bar were given to various countries, and those bars became the basis of measurement in the countries that they were given to. But they were still all based on a single metal bar in Paris. This was better than using the earth as a measurement, but it still wasn't great. As measurements got ultra-precise, even a millionth of a difference between those various bars became significant. Moreover, objects, even metal objects, can lose atoms over time. If you've ever smelt something metallic, you can see how this can happen. The gold standard would be to create a

definition of a meter that wasn't dependent on any physical object. It would solely use universal constants that are the same everywhere in the universe. The current definition of a meter is the distance that light travels in one, 299,792,450 A,000. of a second in a vacuum. The speed of light is the exact same everywhere in the universe, as far as we know, so there isn't any ambiguity as to what a meter is anymore. However, it does raise the question, what is a second? The second has been around a long time, and it's based on the length of a day on Earth. In particular, it's one-60th of one-60th of one-one-24th of a day, or one-68,400th of a day. The problem with this definition is

that the Earth Day isn't constant. The Earth loses and gains seconds every year. Thankfully, defining a second is theoretically easier than other units. I've done an entire episode on the history of timekeeping, so I'll refer you to that. However, measuring the second is just a matter of counting. Counting really fast for sure, but counting nonetheless. Here is the official definition of the second. Quote, the second is defined to be equal to the time duration of 9 billion,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the fundamental, unperturbed ground state of the cesium-133 atom, unquote. So, if you want to measure a second, just count some 9 billion or so vibrations of a cesium atom.

It's nothing more than counting vibrations, which you can do with a laser. So we have the second and we have the meter, time and length. The real challenge was getting a definition for mass. Up until 2019, the definition of the kilogram was similar to the old definition of a meter. A kilogram was the mass of that thing over there, that thing being a weight, which, like the meter, sat in a vault in Paris. That was the kilogram. The kilogram proved to be far more challenging to define than either the meter or. the second. One idea was to define a kilogram as the mass of one cubic

deciliter of water at 4 degrees Celsius. Another approach was just to count atoms. In particular, the idea was to define a kilogram as the mass of a 93.6 millimeter diameter sphere of pure silicon. In the end, these would have had similar problems to the international prototype of the kilogram that sits in Paris. What they wanted to do was tie the definition to some universal constant. What they selected was Planckx constant, which is a constant with units of energy times time. Energy is defined as mass times length squared over time squared, so you have mass in it. As we already have definitions for time and length, it would then be a short step to having a definition for mass. Researchers can accurately determine mass

by using something called a kibble balance, which can determine mass based on the electrical current used to counterbalance the force of the object. So those are the three most important base units and how they're defined. However, in the international system, there are seven base units. One of the units is the ampere, which is the base unit for electrical current. This is defined from the value of a static electrical charge from a single proton or electron. Take a ton of single static electrical charges, and you get a unit called the coolum. An ampere is just the electrical current of one coolum of charge per second. The fifth unit is the mole, which is just counting a large number of atoms. One mole is Avogadro's number, which is over 10 to the 23rd, or one with

23 zeros after it. It's huge. One mole of sand grains, for example, would cover the entire Iberian Peninsula one meter deep. The six unit is Kelvin, which is the unit of temperature. This was redefined using the universal Boltzman constant, which relates the average relative kinetic energies of particles in a gas, with the thermodynamic temperature of the gas. Kelvin starts at absolute zero in temperature and then goes up in units that are the same size as the degrees in Celsius. FYI, there are no degrees Kelvin. You wouldn't say something is 100 degrees Kelvin. You would just say 100 Kelvin.

The seventh and final base unit is the calendar, which is the unit of luminous intensity. After trying for a long time to figure out how to explain it, I figure I would just be easier to leave it at that. It's a measure of light in a three-dimensional channel of light, and it can be calculated knowing the definition. of the kilogram, the meter, and the second. With these seven base units, you can figure out almost any other type of measurement. That includes pressure, area, volume, energy, power, work, voltage, oms, heat, and pretty much anything.

Moreover, with the definitions we now have based on physical constants of the universe, we could, in theory, communicate with aliens to tell them how our system of measurement works. They could calculate everything themselves, and we wouldn't even have to send them some hunker. of metal, which is currently sitting in a vault in Paris, France.

Executive producer of Everything Everywhere Daily is James Mackala. The associate producer is Thor Thompson. Remember to leave a five-star review to get your review read on the show. They can be left at Apple Podcasts, Podcasts, Podcast Republic, or wherever you listen to the show. Also, you can help support the show over at patreon.com. Patrons can get merchandise like t-shirts and hoodies, as well as having direct access to provide suggestions for future episodes.