Mass, Space & Time

Have you ever wondered how various measures are defined? Here are the biggie's from The International Bureua of Weights and Measures. Remember the arbitrariness of many things, but also the great utility the comes from consensus plus the metric system.

Kilogram (kg)
The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram (made of platinum-iridium). (They may be moving to a quantum based measure in the future)

Metre (m)
The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second. (This was an improvement upon a platinum-iridium bar. If you want some history)

Second (s)
The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.
(I know I always think of all ~9 billion periods when I count seconds)