We could start to measure everything in multiples of 21 cm, the “wavelength” of hydrogen, as depicted on the Golden Records on the Pioneer and Voyager spacecraft.
The spin-flip transition of a hydrogen atom’s electron has a frequency of about 1420.405 MHz, which corresponds to a period of 0.704 ns. Light at this frequency has a vacuum wavelength of 21.106 centimetres (8.309 in) (which is also the distance the light travels in that time period). Below the symbol, the small vertical line—representing the binary digit 1—specifies a unit of length (21 cm) as well as a unit of time (0.7 ns). Both units are used as measurements in the other symbols.
