<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 300,000 km / s ( 186,000 mi / s ) ) . It is exact because by international agreement a metre is defined to be the length of the path travelled by light in vacuum during a time interval of 1 / 299792458 second . 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 special and general theories 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 300,000 km / s ( 186,000 mi / s ) ) . It is exact because by international agreement a metre is defined to be the length of the path travelled by light in vacuum during a time interval of 1 / 299792458 second . 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 special and general theories of relativity , c interrelates space and time , and also appears in the famous equation of mass -- energy equivalence E = mc . </P>
how fast is the speed of light in a vacuum
how fast is the speed of light in a vacuum
299,792,458 metres per second ( approximately 300,000 km / s ( 186,000 mi / s )
299,792,458 metres per second ( approximately 300,000 km / s ( 186,000 mi / s )