Tag Archives: planck constant

Goodbye, My Sweet, Sweet Kilogram

So today is a historic day. Why? The kilogram has been officially re-defined to be based on a fundamental constant of nature rather than a physical object.

Since 1889, our bro the kilogram had been defined by “Le Grand K,” a small platinum alloy object that was kept as isolated as possible (to try to avoid any increase/decrease in its mass) and was THE definition of the kilogram. Whatever Le Grand K weighed, it equaled a kilogram.

And up until today, the kilogram was the last of the SI base units to be defined by a physical object rather than a fundamental constant. But that all changed today when the International System of Units officially changed the definition of the kilogram to be in terms of the Planck constant.

Y’all might be like who gives a poop, but this is seriously cool and historic news. We redefined a basic unit of measurement today. How often does that happen? The last time we did it was in 1960 with the meter. Now the kilogram is actually constant and is not in flux with the minute changes that take place with a physical object.


It’s also kinda sad. The uniqueness of the kilogram always made me happy and made it my favorite of the base SI units. It still is my favorite, though, ‘cause of its history.


(Yes, I’m fangirling over an SI unit, what’re you gonna do about it?)





It’s official! The definition of the kilogram (and a few other units) is being changed next May. Specifically, our buddy the kilogram will now be defined using the Planck constant as opposed to Le Grand K, the physical object that has been the kilogram since forever.

Very cool. Kinda sad, though. The kilogram will still be my favorite SI base unit, even though some of its uniqueness is being stripped away for more precision. Not that that’s a bad thing, but still…

TIL: The Planck Constant can be hilarious

Alternate title: GOD I’M OBNOXIOUS

Hokay. So Nate and I were playing Jeopardy this evening and some question* came up that made me think of the kilogram. This got me ranting and raving about said kilogram, as I am wont to do, so I looked it up on my phone because I knew that there have been recent attempts to redefine the kilogram based on a physical constant and I wanted to see exactly what that redefinition would be.

This eventually led to looking up the Planck constant, which led to viewing this equation:


Of course it’s the mobile version of Wiki so it scrolls right in order for you to view the rest of the equation, but I initially didn’t think of that and I thought it was beyond hilarious that the Planck constant equaled 4.1. 4.1 what? Who the hell knows, that’s why it was funny.


*I can’t recall the specifics of the question, because like any well-adjusted happy person, I gloss over large amounts of my existence so that it’ll feel like I reach death faster.


I’ve done a couple of posts about the kilogram, and if you’ve read any of them (or have done any reading about the SI units at all (‘cause that’s a common interest, right? (I mean, I can’t be the only one (…right?)))), you know that the kilogram is the only one of the basic seven measures that is still defined by a physical object rather than a calculation or constant.

Specifically, the mass of the kilogram is defined by an egg-sized alloy of platinum and iridium. This little dude sits beneath not one but three glass bell jars ion a climate-controlled, hermetically sealed room in Paris. Why? Because it’s the object that defines the kilogram, meaning that it is the benchmark against which all other kilograms are compared. So if it changes weight—due to dust or residue or moisture—the kilogram itself changes weight. In fact, it’s so important that the kilogram remains unchanged that it is only removed from its prison every 40 years in order to compare it to other similar replicas that are stored around the world.

These issues with the physical copy are the main reasons why scientists wish to define the kilogram with something that is an inherent standard in nature—like the speed of light or the wavelength of photons. For quite some time, physicists have been considering using the Planck constant as part of the definition of the kilogram. Specifically, the Planck constant could be used in conjunction with Einstein’s E = mc2 equation in a way that could determine mass solely through physical constants. However, no one has yet been able to actually measure the Planck constant to a level of precision that would surpass that of using the physical kilogram as the standard.

However, based on the current pace of progress, physicists suspect that they might be able to redefine the kilogram in terms of the Planck constant by as early as 2018, rendering Le Grand K, as the physical kilogram is known, obsolete.

Crazy, huh? Check out the article here!