Question

What is the value of c?

Answer 1

The speed of light in vacuum, commonly denoted c, is a universal physical constant important in many areas of physics. Its exact valueis299,792,458 metres per second (approximately 300,000 km/s (186,000 mi/s)).

Answer 2

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.

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