Speed of light less uncertain than the previously accepted value. The remaining uncertainty was mainly related to the definition of the metre. As similar experiments found comparable results for "c", the 15th General Conference on Weights and Measures in 1975 recommended using the value for the speed of light. In 1983 the 17th CGPM found that wavelengths from frequency measurements and a given value for the speed of light are more reproducible than the previous standard. They kept the 1967 definition of second, so the caesium hyperfine frequency would now determine both the second and the metre. To do this, they redefined the
Speed of light which all massless particles and changes of the associated fields travel in vacuum (including electromagnetic radiation and gravitational waves). Such particles and waves travel at regardless of the motion of the source or the inertial reference frame of the observer. In the special and general theories of relativity, interrelates space and time, and also appears in the famous equation of mass–energy equivalence . The speed at which light propagates through transparent materials, such as glass or air, is less than ; similarly, the speed of electromagnetic waves in wire cables is slower than . The ratio between and the speed