A famous experiment with a null result

In 1878, the New York Times announced, “It would seem that the scientific world of America is destined to be adorned with a new and brilliant name,” predicting that light would soon be measured “with almost as much accuracy as the velocity of an ordinary projectile.” The new, brilliant name was Albert A Michelson, who went on to win the Nobel Prize in 1907, for his pioneering efforts in measuring and analyzing light, the first Nobel prize in science awarded to an American.

galileo Measuring the speed of light had been a concern of scientists ever since the time of Galileo. Kepler and Descartes had imagined light to have infinite speed. Galileo had suggested in Two New Sciences, standing on a hilltop, an experimenter would flash a bright light toward a distant hill, where an assistant would answer back by flashing. Of course, no hills on earth are far enough to measure the delay, if any.

The first scientist to prove to the world that indeed, light travelled at a finite speed was Ole Roemer in 1676. He got an idea about the speed of light by studying the apparent speed of Jupiter’s moon Io. According to his calculations, light had a speed of 140,000 miles or 225,000 Kilometers per second.

In 1849, a French physicist Louis Fizeau designed a self-contained terrestrial experiment for measuring the speed of light. From a house in the Western suburbs of Paris, he projected a light beam toward a mirror atop Montmartre, which reflected it back again. Interposed in the path was a rapidly spinning cog wheel with 720 precisely machined teeth. The rotational speed of this cog wheel could be accurately adjusted so that the light, going and coming, would pass through a gap between the teeth and appear in Fizeau’s eye piece as “a luminous point like a star”. Spin the wheel a little faster or slower, and the beam would be eclipsed. From the length of the light path and the speed of the wheel, Fizeau estimated the speed of light at about 196, 000 miles or 315, 400 Kilometers per second.

Michelson & Morley

This is an article for subscribers only. You may request the complete article by writing to us at editorial@teacherplus.org.