# Speed of Light

The physical qualities of light have always been important, but its speed has been difficult to measure. Light is the fastest known physical phe­nomenon, so it took the development of modern timing equipment to be able to measure it. The early civilizations, such as that of the ancient Greeks, tried to quantify light but found its quali­ties elusive. It was only in 1973 that the speed of light in a vacuum was accepted at the current value of 299,792,458 meters per second (m/s).

One of the first observed facts about light was that it was faster than sound. When something produced both a loud sound and a bright light, such as a cannon firing, the light always reached the observer before the sound. The difficulty trying to quantify this difference was the huge difference in the speed of the two phenomena and the lack of good clocks. Sound has a speed of 331.3 m/s (at 00 C, 0% humidity, and 1 atmosphere of pressure) while light goes at 299,792,458 m/s. To early observers light seemed to travel instanta­neously. For many centuries the speed of light was a philosophical concept, and there were debates between those who believed it had an infinite speed and those who felt it had to be finite.

It was the invention of more accurate clocks that help generate the first realistic calculation of the speed of light. One of the first experiments to yield a rough estimate was made by Ole Christensen Roemer in 1676, who used the orbital motion of Jupiter’s moon Io to make his measurement. By recording the exact moment Io entered Jupiter’s shadow when Jupiter was closest to Earth and comparing that time to the time many months later when Jupiter was a known distance farther from Earth, he calculated a speed of 220,000,000 m/s for light. In 1728, this observed estimate was improved by James Bradley, who measured the apparent motion of stars at different times as com­pared to the speed of Earth in its orbit. His calcula­tion of 298,000,000 m/s was accepted as the more accurate value.

As clocks and clockwork mechanisms improved, it became possible to divide time into smaller and smaller segments. The first earthbound measure of the speed of light came in 1849, when Hippolyte Fizeau used a mechanical apparatus to measure the speed of light. He used a beam of light focused on a mirror several thousand meters away. By forcing the light to pass through a rotating cogwheel, he found that at a certain rotation, light would pass through the cogwheel on both its forward and return journeys. A calculation based on a combi­nation of distance of the light source from the cogwheel, the number of teeth on the cogwheel, and the rotational speed of the cogwheel yielded a speed of 313,000,000 m/s. This type of experiment was later refined using rotating mirrors and prisms to estimate a speed of 299,796,000 m/s in 1926. It took the development of oscilloscopes with time resolution in the subnanosecond range to further refine this number by measuring the delay of a light pulse from a laser or LED.

One of the interesting behaviors of light is that its observed speed lowers when it passes though media other than a vacuum. The refraction of light as it passes from air to water is a visible manifesta­tion of the slowing of light as it moves into a denser medium. This has led scientists to explore the concept of “slow light” by increasing the index of refraction of various media. It is possible to increase the path that photons must take through a medium by using specialized conditions such as a Bose-Einstein condensate. Scientists have suc­ceeded in slowing the measurable apparent speed of light to less than 1 m/s.

Since 1983 the speed of light has been treated as a defined constant, with refinement of its present value by further experimentation not needed. As a constant it functions as an absolute measure of distance (1 meter = the path traveled by light in 1/299,792,458 of a second), which has allowed for finer and finer descriptions of the velocity of objects as time measurement tools have improved.

John Sisson

See also Black Holes; Clocks, Atomic; Einstein, Albert;

Experiments, Thought; Newton, Isaac; Observatories; Planetariums; Time, Relativity of; Singularities; Space Travel; Stars, Evolution of; Time, Measurements of;