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Grace Hopper's "nanoseconds"
The speed of light in a vacuum (postulated to be constant for all inertial observers by the [[Theory of Relativity|special theory of relativity]]) is, to nine significant figures, exactly 299,792,458 meters per second (approximately 186,282.3 miles per second). As the speed of light is used to define the [[SI]] meter, this is the value ''by definition''.
The speed of light in a vacuum (postulated to be constant for all inertial observers by the [[Theory of Relativity|special theory of relativity]]) is, to nine significant figures, exactly 299,792,458 meters per second (approximately 186,282.3 miles per second). As the speed of light is now used to define the [[SI]] meter, this is now the value ''by definition''.
The speed of light is about one foot per nanosecond. The late computer pioneer Admiral Grace Hopper was fond of keeping foot-long lengths of wire in her purse; she used them as props for her talks, referring to them as "nanoseconds," and using them to explain how the speed of light set limitations on computing systems.<ref>Chiarella, Donald Joseph Gray (2002), ''Life in God's Management Corps,'' [http://books.google.com/books?vid=ISBN0595256430&id=VhM9jDwbywUC&pg=PA14&lpg=PA14&ots=ZXHirs9jQI&dq=grace+hopper+nanosecond&ie=ISO-8859-1&sig=XmbYtV2laxCMIKMLzR1eXHP1Eok p. 14]</ref>
The speed of light is slower in any medium which is not a vacuum, and varies from medium to medium. This variation gives rise to (as a result of [[quantum mechanics]], particularly the concept of a path of least action) the phenomenon of [[refraction]]. When a charged particle exceeds the speed of light in the medium in which it is travelling, it emits [[Cherenkov Radiation]].
The speed of light is slower in any medium which is not a vacuum, and varies from medium to medium. This variation gives rise to (as a result of [[quantum mechanics]], particularly the concept of a path of least action) the phenomenon of [[refraction]]. When a charged particle exceeds the speed of light in the medium in which it is travelling, it emits [[Cherenkov Radiation]].