ARTSHOP MAGAZINE
July 2006

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How to Measure the Speed of Light

Early ideas about light propagation: Before the 17th century, scientists believed that there was no such thing as the "speed of light". They thought that light could travel any distance in no time at all. Later, several attempts were made to measure that speed.

1667 Galileo: at least 10 times faster than sound.
Galileo is often credited with being the first scientist to try to determine the speed of light in 1667. His method was quite simple. He and an assistant each had lamps which could be covered and uncovered at will. Galileo would uncover his lamp, and as soon as his assistant saw the light he would uncover his. By measuring the elapsed time until Galileo saw his assistant's light and knowing how far apart the lamps were, Galileo reasoned he should be able to determine the speed of the light. His conclusion: "If not instantaneous, it is extraordinarily rapid". Most likely he used a water clock, where the amount of water that empties from a container represents the amount of time that has passed. Galileo just deduced that light travels at least ten times faster than sound.

1675 Ole Roemer: 200,000 Km/sec
In 1675, the Danish astronomer Ole Roemer noticed, while observing Jupiter's moons, that the times of the eclipses of the moons of Jupiter seemed to depend on the relative positions of Jupiter and Earth. If Earth was close to Jupiter, the orbits of her moon's appeared to speed up. If Earth was far from Jupiter, they seemed to slow down. Reasoning that the moons orbital velocities should not be affected by their separation, he deduced that the apparent change must be due to the extra time for light to travel when Earth was more distant from Jupiter. Using the commonly accepted value for the diameter of the Earth's orbit, he came to the conclusion that light must have traveled at 200,000 kilometers per second.

1728 James Bradley: 301,000 Km/s
In 1728 James Bradley, an English physicist, estimated the speed of light in vacuum to be around 301,000 kilometers per second. He used stellar aberration to calculate the speed of light. Stellar aberration causes the apparent position of stars to change due to the motion of Earth around the sun.

Stellar aberration is approximately the ratio of the speed that the earth orbits the sun to the speed of light. He knew the speed of Earth around the sun and he could also measure this stellar aberration angle. These two facts enabled him to calculate the speed of light in vacuum.

1849 Hippolyte Louis Fizeau: 313,300 Km/s
A French physicist, Fizeau, shone a light between the teeth of a rapidly rotating toothed wheel. A mirror more than 5 miles away reflected the beam back through the same gap between the teeth of the wheel. There were over a hundred teeth in the wheel. The wheel rotated at hundreds of times a second; therefore, a fraction of a second was easy to measure.

By varying the speed of the wheel, it was possible to determine at what speed the wheel was spinning too fast for the light to pass through the gap between the teeth, to the remote mirror, and then back through the same gap. He knew how far the light traveled and the time it took. By dividing that distance by the time, he got the speed of light. Fizeau measured the speed of light to be 313,300 kilometers per second.

1926 Leon Foucault 299,796 Km/s
Another French physicist, Foucault, used a similar method to Fizeau. He shone a light to a rotating mirror, then it bounced back to a remote fixed mirror and then back to the first rotating mirror. But because the first mirror was rotating, the light from the rotating mirror finally bounced back at an angle slightly different from the angle it initially hit the mirror with. By measuring this angle, it was possible to measure the speed of the light.

Foucault continually increased the accuracy of this method over 50 years. His final measurement in 1926 determined that light traveled at 299,796 kilometers per second.

Today: 299,792.458 Km/s
Today, according to the US National Bureau of Standards, the speed of light is = 299792.4574 0.0011 km/s. According to the British National Physical Laboratory, the speed of light = 299792.4590 0.0008 km/s (making an average with the US standard = 299792.458 km/s).

299,792.458 kilometers per second is the speed of light in vacuum, that is, outside matter and gravitational fields. However the velocity of light varies with the intensity of the gravitational field, that is, this is not the speed of light inside gravitational fields.

1,400 years ago it was stated in the Koran (Quran, the book of Islam) that angels travel in one day the same distance that the moon travels in 1,000 lunar years, that is, 12,000 Lunar orbits / Earth day. Today we know that if we remove the Earth-moon system from the gravitational field of the sun all observers will see the speed of light outside gravitational fields to be equivalent to 12,000 Lunar orbits / Earth day. Read more at www.speed-light.info.

Raef Fanous lives in Beruit, Lebanon. His website, www.speed-light.info, provides a fascinating analysis of relativity in the Quran.