Kraig Akers, age 11, of Tempe, Arizona, for his question:
How is the speed of light measured?
Light sets the speed limit for the universe and so far as we know nothing, can exceed it. This is one reason why clocking it is a tricky task. Through the centuries, researchers have devised more refined methods to measure its velocity. Each one came a hair's breadth closer to the truth. In 1971 and 1972, a surprising new system was used to measure the speed of light. These latest findings are very¬very close to accurate.
In the 1600s, Galileo tried to measure the speed of light from a flashing lantern on a hilltop. His apparatus was too clumsy and he concluded that light gets from here to there instantaneously. In the 1800s, more refined techniques proved that this is false. But the problem was to find a method to clock its speed with some degree of accuracy.
Until very recently, it seemed logical to clock a light beam traveling a known distance. Researchers used various systems of spinning mirrors and slots to separate when its speed was clocked over longer distances. In most systems, distance was multiplied by letting the beam reflect back from a distant mirror. Until the late 1950s, the most accurate estimate was based on the speed of a light beam through a tunnel three miles long.
The answer was pretty close, but not perfect, and the search for a more accurate system went on. Light and radio, as we know, are both forms of electromagnetic energy. In the 1950s, physicists noted some new findings on the speed of radio waves. The speed of these energies is directly related to their wavelengths and frequencies. Perhaps this relationship could provide a more precise method to measure the speed of light. The idea was simple, but making it work was not.
The plan was to measure the exact frequencies and wavelengths of a control light
beam and use these factors to compute its velocity. The tantalizing work was undertaken by the United States Bureau of Standards. Two teams worked on the system to cross check the findings. The special light used was laser beams single strands of highly concentrated energy.
The big problems involved the stabilizing of groups of lasers to emit narrow bands of the same frequency. This was solved by directing the laser beams through methane gas. Helium neon lasers were best for the job and wavelength checks were made with krypton lamps.
The results were announced to the world on October 18, 1972. The newly estimated speed of light is 299,792.4562 kilometers per second plus or minus 1.1 meters. In everyday terms, this amounts to 186,282.3959 statute miles per second. The result is said to be accurate to within a yard or so.