a) What is the modern definition of the time unit and the length unit? b) How people actually Materially realize these units ? (a) in the wiki, it reads: Second: Under the International System of Units, the second is currently defined as 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 caesium-133 atom. This definition refers to a caesium atom at rest at a temperature of 0 K (absolute zero). The ground state is defined at zero magnetic field. Meter: Today, it is defined as 1⁄299 792 458 of a light-second. Would anybody like to answer question (b) ?