There are many system of units used in physics. In the CGS, the units are,

  • length : centimetre

  • mass : gram

  • time : second

And in the MKS system the units are,

  • length : metre

  • mass : kilogram

  • time : second

Though other units change in different systems, but time's unit never changes. Why?

  • $\begingroup$ I don't think this can be answered (but I may be wrong). Time has always been special with regard to larger intervals than seconds (no kiloseconds etc.). $\endgroup$
    – Jasper
    Jan 13, 2019 at 12:24
  • $\begingroup$ This is not a physics question. The choice of units is just arbitrary. You can create a $MKM$ system if you want to.. $\endgroup$ Jan 13, 2019 at 13:11
  • 4
    $\begingroup$ I'm voting to close this question as off-topic because I think it belongs to hsm.stackexchange.com $\endgroup$
    – FGSUZ
    Jan 13, 2019 at 23:06
  • $\begingroup$ It is a mistake to think Giorgi's choice was just arbitrary, and not a physics question. On the other hand, the question may belong to HSM. $\endgroup$
    – jkien
    Jan 13, 2019 at 23:29
  • $\begingroup$ Making inferences off a sample of size 2 can be dangerous. Just take a gander at the Wikipedia entry on 'natural units' and see that the time units are not necessarily seconds. $\endgroup$
    – Kyle Kanos
    Jan 14, 2019 at 11:16

1 Answer 1


The problem of the CGS system was that the base units of CGS were not coherent with the practical electrical units ampere and volt. In CGS, the unit of energy is the erg (1 erg = 1 g cm2/s2), whereas the electrical units volt and ampere result in 1 joule = 1 V A s = 107 erg. The joule was introduced in 1889. Although the joule is called a derived unit in SI, historically MKS and SI are derived from the joule.

Giorgi noticed that a small change in the prefixes of the base units of CGS could achieve coherence with the joule. His new base units were decimal (sub)multiples of the old base units, and only one of the base units had a prefix, similar to CGS. So the unit of length lost its prefix, $cm \rightarrow m$, while either the unit of mass or the unit of time obtained a prefix.
Giorgi's solution was to change the unit of mass: $ {joule} = [M]\ m^2\ s^{-2} \rightarrow [M] = 1\ {kg}$

Trying to change the unit of time did not result in a decimal (sub)multiple of the second: $ {joule} = g\ m^2\ [t]^{-2} \rightarrow [t] = 10^{-1.5}\ second$


Not the answer you're looking for? Browse other questions tagged or ask your own question.