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Fundamental Properties of The Speed of Light

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The speed of light, which was defined in 1983 to be exactly 299,792,458 meters per second, is perhaps the most important physical constant there is.
Besides being the speed at which light propagates, it is also the universal speed limit - no matter or signal can exceed this speed.
As Einstein showed, the speed of light is also fundamentally connected with space and time and how they are intertwined in ways in which no one before him imagined and which are still counterintuitive to us today.
Since the 17th century, scientists have devised more and more accurate means of measuring the speed of light.
The first finite speed measurement came from the Danish astronomer Ole Roemer in 1676, who measured the speed of light by observing the transit times of Jupiter's moons with the earth on both the near side and the far side of the sun.
His estimate was off by 26%, but it was still quite impressive in that it was the first experiment which showed that its value was finite.
Throughout the 18th century, other astronomers devised similar methods to measure the speed of light.
The first terrestrial method came from a French physicist named Fizeau, who used a rotating toothed wheel and a distant beam of light to measure the speed.
Various modifications of this method were devised afterward, the most accurate being performed by Michelson in 1926, who used a rotating octagonal mirror to determine the value of c to within 1 part in 100,000.
By the late 20th century, lasers were used to measure the speed of light to better than one part in 10 billion.
This eventually led to the meter being defined in 1983 in terms of the second and the speed of light, whose value was now defined to be exactly 299,792,458 meters per second.
Since light is an electromagnetic wave, the speed of light is also the speed of other electromagnetic waves, such as radio waves, X-rays, and gamma rays.
A bizarre fact about the speed of light, which led to Einstein's development of the theory of relativity, is that light, or any other electromagnetic wave, is always observed to have the exact same speed, no matter how fast one is moving relative to it.
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