<P> The speed of light in vacuum , commonly denoted c , is a universal physical constant important in many areas of physics . Its exact value is 7008299792458000000 ♠ 299,792,458 metres per second ( approximately 7008300000000000000 ♠ 3.00 × 10 m / s , or 300,000 km / s ( 186,000 mi / s ) ) . It is exact because the unit of length , the metre , is defined from this constant and the international standard for time . According to special relativity , c is the maximum speed at which all conventional matter and hence all known forms of information in the universe can travel . Though this speed is most commonly associated with light , it is in fact the speed at 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 c regardless of the motion of the source or the inertial reference frame of the observer . In the theory of relativity , c interrelates space and time , and also appears in the famous equation of mass -- energy equivalence E = mc . </P>
<P> The speed of light in vacuum , commonly denoted c , is a universal physical constant important in many areas of physics . Its exact value is 7008299792458000000 ♠ 299,792,458 metres per second ( approximately 7008300000000000000 ♠ 3.00 × 10 m / s , or 300,000 km / s ( 186,000 mi / s ) ) . It is exact because the unit of length , the metre , is defined from this constant and the international standard for time . According to special relativity , c is the maximum speed at which all conventional matter and hence all known forms of information in the universe can travel . Though this speed is most commonly associated with light , it is in fact the speed at 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 c regardless of the motion of the source or the inertial reference frame of the observer . In the theory of relativity , c interrelates space and time , and also appears in the famous equation of mass -- energy equivalence E = mc . </P>
what is the speed of light in km
what is the speed of light in km
300,000 km / s
300,000 km / s