4 ms·

By using the well defined meter.

• grecy 13y ago
  Remind me again what that's defined as?
  • nknighthb 13y ago
    Per http://www.bipm.org/en/CGPM/db/17/1/ http://www.bipm.org/en/CGPM/db/17/1/ "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."
  • rpearl 13y ago
    grecy, the meter is the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second. A 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 caesium 133 atom.