I find it disturbing/puzzling that there is this fundamental physical behaviour like emission of light with wavelength of _exactly_ 21cm -- assuming one centimeter wasn't based on any such property but was just a "random" unit measure that stayed with us historically and through sheer volume of use (in U.S. inches filled the same niche; still do). I mean what are the odds that the wavelength is _exactly_ (the word used in the article) 21cm?
The article does say "precisely 21cm" in the subtitle, repeats it in the "key takeaways" section, and then close to the end of the article these's this:
>By measuring light of precisely the needed wavelength — peaking at precisely 21.106114053 centimeters
Which I assume is the actual measurement every time "21cm" is brought up in this article.
No more probable than any other value, whole or otherwise. In particular, its (per wikipedia) 21.106cm.
Its funny how our brains find nice whole numbers unsettling in the natural world. I was always sort of weirded out by the distance light travels in a nanosecond: just shy of 1 foot. How weird it is that it flops between systems!
isn't a cm now defined based on the distance light travels in a vacuum in a very small period of time?
so it's not arbitrary really, or rather it probably goes the other way around. a cm used to be based on an arbitrary physical distance but was I think redefined to avoid needing to keep a standard meter cube in Paris.
It started with the grandfather clock. Everyone's clock pendulum needed to be the same length to have the same length of a second. So a meter also happens to (approximately, this was before we could easily be precise to several decimal places) be the length of pendulum that cycles at 0.5 hz (each swing back and fourth is a second) in 9.8 m/s^2 gravity.
I think there were multiple competing suggestions at the time, the grandfather clock was one while the standard ended up being the French proposed one that you mention
Ah yes, you're right. Another nice coincidence that a seconds pendulum is less than 1% away from 1/10 millionth the distance between the equator and poles.
Was, they made the smoothest silicon sphere, Avogadro project. And now apparently they define it via physics as mentioned in ops article, "namely a specific transition frequency of the caesium-133 atom, the speed of light, and the Planck constant"
