The speed of light is the constant speed at which light and other electromagnetic radiation travel in a vacuum. The value of the speed of light in a vacuum is equal to 299,792,458 m/s (186,282.397 mi/s), as recommended by the Committee of Data for Science and Technology on July 1999 for international use in all fields of science and technology. A commonly used approximation for the speed of light is 3.0 x 108 m/s (186,000 mi/s). The electromagnetic constant c denotes the speed of light. Light travels slower in a medium (such as glass or water) than it does in a vacuum. Since space is not a perfect vacuum, light travels through space at a very slightly lower speed and decreases somewhat more when the radiation enters, for instance, a planet's atmosphere. Light would take about 1.2 seconds to travel from the Moon to Earth, around 8.5 minutes to travel from the Sun to Earth, and roughly 100,000 years to travel across the Milky Way galaxy.
The first demonstration that light travels at a finite speed was provided by observations from astronomer Ole Röemer in 1675. Röemer observed that the elapsed time between eclipses of Jupiter's satellites by Jupiter became shorter as Earth moved closer to Jupiter and became longer as Earth and Jupiter traveled further apart. He correctly concluded that this phenomenon was caused by the time needed for light to cross the increased distance between the two planets. Röemer's measurement of this velocity was the first reasonable approximation of the currently accepted value for the speed of light.
Physicist Armand-Hippolyte-Louis Fizeau performed the first laboratory measurement of the speed of light in 1849. His procedure sent a beam of light to a mirror by passing it through a gap between two teeth in a wheel. If the wheel were stationary, the mirror would reflect the beam back through the same gap. If the wheel was set in rapid motion, by the time the reflected beam reached the wheel a tooth would have moved into the location occupied by the gap when the light first passed the wheel. Knowing the wheel's rotational speed, Fizeau calculated the time required for a tooth to move into the position occupied by the adjacent gap. This was the same time that the light was sent traveling the distance from the wheel to the mirror and back. The result of the experiment was accurate to within four percent of the currently accepted speed of light.
In 1864, following the groundwork laid by Coulomb, Ampére, Gauss, and Faraday, James Clerk Maxwell created his classical theory of electromagnetism. This theory predicts that electromagnetic waves will propagate through a vacuum at a constant speed. Recognizing that this calculated speed was very similar to the speed that had been measured for light, Maxwell suggested that light must be one form of electromagnetic radiation.
There are distinctive features of the speed of light that set it apart from other speeds. One feature is that the speed of light is an absolute barrier, as predicted by Albert Einstein's special theory of relativity. Nothing that imparts energy or information can go faster than the speed of light, and no body containing mass can travel as fast as the speed of light.
Another feature of the speed of light is that it cannot depend on the motion of the source. Astronomer Willem de Sitter proved this when he found that two stars in a double-star system, while revolving about a common center of mass, both send light to Earth at a velocity "c" even though one star is sending light while traveling toward Earth at a velocity "c + v," while the other star is sending light while traveling away from Earth at a velocity "c - v." The speed of light from both stars, de Sitter proved, does not include the speeds of the stars themselves. The motion of the source does not change the speed of light.
A third feature of the speed of light is that its speed is independent of the motion of the observer. A famous experiment that proved this is the Michelson-Morley experiment performed in 1887. At that time Albert Michelson and Edward Williams Morley were attempting to measure the speed of the Earth through "ether" (the so-called medium for transmission of electromagnetic waves) by measuring the speed of light in two different directions. However, the pair did not observe any difference in the travel time along two light beams set at right angles to each other. With equipment precise enough to detect the small orbital speed of Earth compared to the speed of light, their result indicated that the Earth was stationary (with respect to the expected ether). Eventually Michelson and Morley concluded that the speed of light does not depend on the motion of the observer. The theory of special relativity, as laid out by Albert Einstein in 1905, assumed as a premise that the speed of light c is independent of the motion of both source and observer.
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