We know that speed of light is 3x10^8 m/s (299 792 458 m/s). How was this value found?
Is it related to Einstein's Theory of Relativity, Maxwell's Theory of Electromagnetism, or any other theory?
This value is found from the unit of meter, a 40,000,000th of the circonference of the Earth, and the one of the second, a 86,400th or the revolution time of the Earth with respect to the Sun. With this units, the speed is measured as the distance over time travelled by the light. A historically important measure was done from the apparent irregularities of the orbit of one satellite of Jupiter.
Today, the speed of light is the standard, and the meter is defined from it.
In relativity, c can be set to 1, it is just the speed limit of information.
In Maxwell's theory, c is related to the permeability and the permitivity, mu and epsilon, of vacuum. It can thus be calculated from the electromagnetic interaction without being directly measured.
The present value of the physical speed of light is the result of a measure method which is improved in the course of centuries. Theories make use of that value.
This value is found from the unit of meter, a 40,000,000th of the circonference of the Earth, and the one of the second, a 86,400th or the revolution time of the Earth with respect to the Sun. With this units, the speed is measured as the distance over time travelled by the light. A historically important measure was done from the apparent irregularities of the orbit of one satellite of Jupiter.
Today, the speed of light is the standard, and the meter is defined from it.
In relativity, c can be set to 1, it is just the speed limit of information.
In Maxwell's theory, c is related to the permeability and the permitivity, mu and epsilon, of vacuum. It can thus be calculated from the electromagnetic interaction without being directly measured.
The first quantitative estimate of the speed of light was made in 1676 by Rømer. From the observation that the periods of Jupiter's innermost moon Io appeared to be shorter when the Earth was approaching Jupiter than when receding from it, he concluded that light travels at a finite speed, and estimated that it takes light 22 minutes to cross the diameter of Earth's orbit. Christiaan Huygens combined this estimate with an estimate for the diameter of the Earth's orbit to obtain an estimate of speed of light of 220,000 km/s, 26% lower than the actual value.
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