[FateAmenableToChange]

I thought metric measurements were based upon physical constants — a meter being based upon one wavelength of a certain frequency of light, a kilogram based upon a litre of water at 4C, etc. I knew there were standards but figured they were outmoded already.

[Right Wing Assault]

a meter being based upon one wavelength of a certain frequency of light

The speed of light was defined in 1983 as exactly 299 792 458 meter/s. A meter is how far light travels in 1/299 792 458th of a second.

The platinum-iridium cylinder has been the standard for over 100 years, I believe. The mass of water at 4 C is too hard to do reliably.

A second is is the time it takes for 9 192 631 770 oscilations of a specific wavelength of light emitted by the cesium-133 atom.

[Starter]

The kilogram is defined to be approximately the weight of a liter of water, but in actuality, it’s the weight of that cylinder.

Proposals have been made to change it to something more concrete (I remember my chem teacher describing a kilo as a mole worth of an element) but the actual decision to do so hasn’t happened yet and is currently scheduled for a time 3-4 years from now.