Clocking the Speed of Light
Why is the speed of light important?
-Having a accurate number for the speed of light allows for accurate measurements of the universe around us -Even small inaccuracies make large differences when measuring the space between stars, planets, and galaxies -The more precise our measurement of the speed of light the more precise our measurements are of the universe because light is the universal measurement used
Light as Infinite
-Most scholars agreed that light had an infinite speed until the 17th century. Light having a finite speed was a violation of Aristotelian Mechanics (later replaced by Newtonian mechanics) -There were however some notable dissenters. Aristotle condemned the ideas of Empedocles of Acragas who spoke of light as traveling, but the movement being unobservable to humans. -Around 1000 AD Islamic Scientists Avicenna and Alhazen separately stated that light must have a finite value. Alhazen argued light has a finite speed that differs depending on the medium it travels through.
Historical Measurements of Light
-Light has been measured many ways over the centuries. -As scientific tools have become more complex the accuracy of the measurement of the speed of light has increased
Early Attempts at Measuring the Speed of Light
- Galileo was the first scientist to attempt to determine the speed of light. He did this by placing lanterns a few miles apart and manual covering them and uncovering them. He recognized that his experiment was too simple to accurately measure the speed of light. - In 1676 Ole Roemer determined the speed of light based on observations of Jupiter’s moons and the relative positions of Jupiter and Earth. He used the commonly accepted value of Earth’s orbit to solve for the speed of light. Unfortunately, he assumed that other stellar bodies did not affect orbital speeds, so his conclusions were incorrect.
18th & 19th Century Attempts
-In 1726 James Bradley, an English Physicist, used stellar aberration to calculate the speed of light. Being caught in a rain storm can be a useful analogy for stellar aberration. When standing still rain hits vertically, but as you begin to move rain starts striking at an angle. By measuring this angle of starlight and knowing the speed of the Earth around the sun, Bradley found the value of the speed of light.
-In 1849 Armand Fizeau, a French Physicist, created an experiment using a light which shone through a toothed, spinning wheel. The light reflected on a mirror approximately 8km distant and returned through the same gap on the toothed wheel. He varied the speed of the wheel until it was too fast for the light to pass through the gap and return through the same gap. Fizeau computed the speed of light by the distance it traveled and the time it took.
As technology progressed many scientists were more successful calculating the speed of light. They were no longer reliant on stellar motion and man’s physical abilities. Maxwell’s theory of electromagnetism made it possible to calculate the light speed by measuring the magnetic permeability and electric permittivity of free space. First such results were published by Weber and Kohlrausch in 1857. In 1907 Rosa and Dorsey obtained the most accurate value at the time in this manner. A major impact was made by Leon Foucault in the late 19th century using rotating mirrors. Albert Michelson worked to improve on Foucault’s apparatus during the 1920’s using the most up-to-date electronics and optics with a vacuum tube for his light beam.
Late 20th Century
In 1958 K. D. Froome calculated light speed within .1% of today’s accepted value using kylstron oscillators, a microwave interferometer and a Kerr cell shutter. He analyzed the frequency and wavelength of millimeter waves. New improvements in laser technology during the 1970’s, including tungsten-nickel point contact diodes and caesium clocks, made better measurements possible. Evanson’s 1973...
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