The modern method for determining the speed of light was developed by K.
M. Evenson et al. It relies on separate mesurements of wavelength and
frequency emitted by a stabilized laser. They achieved the value c=2 999
792 458 m/s, which has uncertainty of only 1 m/s.
Theoretically Maxwell predicted that self-supporting electromagnetic waves must propagate at speed
c=1μ0ϵ0−−−−√
for more info, check out:
http://nvl.nist.gov/pub/nistpubs/sp958-lide/191-193.pdf
