Reality → Energy → Motion → Speed of light
Light travels about a million times faster than sound [1] , at a speed equivalent to 7 loops around the Earth in a second. Massless particles (like photons) travel instantaneously at this speed and always at this speed only. All electromagnetic radiation propagates at this speed through free space. Subatomic particles with mass (such as protons) can be brought close to this speed in large accelerators. Early attempts to directly measure the speed of light were undertaken by Fizeau and Michelson [2] . Today, laser interferometers allow highly accurate measurement of the speed of light. Accurate values can also be obtained from measuring wavelengths and associated frequencies of radio and microwaves. Maxwell’s equations allow the determination of the speed of light from the electromagnetic field constants.
Speed of light and sound can only be compared when traveling in a medium (no sound waves are possible in vacuum). Light slows down in air by only 0.03 %. However, in water, the speed of light decreases 25 %, while the speed of sound increases more than 4-fold.
In 1849, Fizeau and Foucault developed an apparatus that sent a light beam through the gap between the teeth of a rotating cogwheel to a distant mirror (about 5 miles away) and adjusted the speed of the cog such that the reflected beam would hit the back of the tooth next to the gap and therefore not be visible from the front side of the cog. From the light’s traveled distance and the short time to move the cog from gap to tooth, Fizeau could determine the speed of light with 5% accuracy. Later in the 19th century, Michelson invented a more accurate interferometer that splits a coherent light beam into two paths, lets them travel different distances, and then recombines them to show interference patterns. In the famous Michelson-Morley experiment, astronomical measurements with the interferometer showed that there is no ‘aether’ that carries light waves and that the speed of light in free space is always the same, independent from the movement of the observer.