Thread by @krishashok

A few days back, scientists changed the official definition of the kilogram, and it’s quite an amazing story. Here is a video that gets into the physics of how it’s now defined Since that video is rather sciency, here is my attempt at an #ELI5

So mankind has needed to measure stuff for a long while, particularly after the invention of agriculture, which resulted in the growth of cities & trade. The question “how many” necessitated a number system, & most neolithic civilizations had some form of numeracy

The question “how long” was also quite common. Land had to be measured, pyramids had to be built, and stone blocks had to be pretty precise in dimensions, so most civilizations had their own anthropic (human based) units of distance, like cubit, palm, span and foot.

And since ancient conmen would use a Sumerian Shaq's foot when buying land & a Harappan Peter Dinklage's foot when selling, the likes of Sargon of Akkad (the actual guy, not the YouTube alt-right polemicist) started issuing standard rods that defined distance units.

Fast forward to Industrial Age, countries realized that standardizing units of measurement was rather critical to the entire endeavor of making stuff. If a factory didn’t accurately know what a kilogram, meter & a second were, it could simply not function

So the Europeans got down to defining stuff for everyone else while. They came up with a standard rod that defined 1 meter. There was a cylinder of Platinum Iridium Alloy that defined 1 kilogram. 1 second was defined as 1⁄31,556,925.9747 of the year 1900 and so on

But here’s a problem. Cylinders and rods have a tendency to lose or gain mass over time and we reached a situation where standard kilogram bars in different countries started diverging in terms of mass.

That’s when scientists said “enough with this rod in a secure Paris underground vault business” and set about redefining basic units of measurements on the basis of things that never change in the universe - universal constants.

Because, if you could define the kilogram, second and meter on the basis of universal constants, countries don't have to rely on storing accurate copies of the meter rod and kg cylinder in France

Let’s take the meter. Rather than rely on rods that degrade over time, we now define the meter on the basis of the distance traveled by light in a fixed amount of time.

But there is a bit of circular logic here. The speed of light is 299,792,458 meters per second, so how can we define a unit on the basis of something else that relies on that very unit?

Here’s the trick. We first super accurately measure the speed of light with whatever is our current most accurate definition of the meter and set that number in stone. Then every country is free to then derive the exact length of the meter by doing a simple experiment with light.

In short, we put the onus of the definition of the meter on the speed of light, which is a universal constant, instead of an unreliable Platinum alloy cylinder chilling in a Paris vault smoking cigarettes and watching Krzysztof Kieślowski films

The next obvious question is - to accurately measure the speed of light to define the meter, we also need a better definition of time - what is a second?

It turns out that a precisely excited Cesium-133 atom vibrates with such an astonishingly consistent precision that we can define the second as the time taken for 9,192,631,770 such periods.

Which now brings us to the knotty question of the kilogram. How could you possibly define mass without using some tangible substance? Length can be measured using the speed of light and time can be measured using vibrations of atoms. How do we deal with mass?

This is where a certain Albert Einstein comes into play. Mass and energy are related. The best way to understand this is to realize that this relationship is the exact premise of a nuclear bomb. Mass can be converted to energy and his E = mc^2 equation describes this relationship

When a large atom like Plutonium disintegrates, it splits into smaller atoms but sheds a bit of mass in that process. This mass is converted into energy & since the speed of light (c) is a massive number, this is why a tiny amount of Plutonium generates an insane amount of energy

Well, energy is much simpler to measure in a very accurate way. So we need to find way to relate mechanical energy (which is what a mass has under Earth's gravity) and electrical energy (which is generally easier to measure)

So scientists use a device called the Kibble balance that is able to measure the equivalent electromagnetic force required to balance a weight. This is a way to relate Earth’s gravitational force (which is weight) & an electromagnetic force in a motor

And we can then measure the relationship between that electromagnetic force required to balance a specific weight thanks to a device called the Josephson Junction which allows you generate precise voltages using microwave radiation.

Radiation is typically measured in frequency (Hertz, the number of times the electromagnetic field changes direction every second). The more the frequency, the more the energy.

This is why radiation from Chernobyl caused cancer in its first responders (high frequency) while WhatsApp forwards about cellphone tower radiation do not (low frequency)

So how do we know how much energy radiation of a specific frequency has? Drum roll...Planck’s constant (h), which is a fundamental constant in the universe! The amount of energy radiation of a specific frequency has is E = h times f (frequency)

So now, bear with me. We took a physical object with mass, which under gravity exerts a force, which can be balanced by electromagnetic forces in a motor coil (Kibble balance) and we can use radiation of a precise frequency to control the voltage in that coil accurately.

In short, and I know this will seem bizarre, but if you go through the steps we just did, and measure energy of radiation accurately, we can define a kilogram accurately.

On May 20 2019, it became official! We have an accurate enough measurement of the Planck’s constant and therefore, using this circuitous, yet "doable in a decent national science lab" process, every country can now have a super accurate definition of the kilogram.

So this is a science news story that connects a one kg weight used by a vegetable seller to earth’s gravity, to current flowing through a motor, to voltage generated by radiation, to nuclear bombs & Einstein’s equation & finally quantum mechanics & Planck’s constant

If this isn't worth sitting back in absolute awe at humanity’s ability to work these things out, I don't know what is

If you like this rabbit hole, a few more videos that will give you more clarity
1. How does a Kibble balance (also called a Watt balance) work?
2. What is Planck's constant -

And it also turns out that a side effect of the physics required to redefine the kilogram is also turning out to be useful in realizing quantum computers -