History Of Measurements Of The Speed Of Light

Most of physics is tied undoubtedly to the quest to determine the speed of light, c, of 299,792,458 m/s. Today, it’s regarded as the universal speed limit, interrelating space and time. But, to really understand the importance of the speed of light, we must start at the beginning. It was first thought that light traveled at infinite speed. Around the eleventh century, however, some people began to have their doubts, giving rise to the possibility that light had a finite speed, but that it was just too fast to be measured. It would be six more centuries before a number was finally pinpointed. In 1675, Danish astronomer Olaf Roemer made the first scientific determination of the speed of light based on observations of the eclipses of the innermost moon of Jupiter. Roemer noted that there were delays in the occurrence of eclipses depending on the distance between Earth and Jupiter. Although Roemer is not believed to have made the final calculations, his data would lead to a speed of 200,000,000 m/s. In 1728, British astronomer James Bradley calculated the speed of light by another method of astronomical observation. His experiment involved the observation of a star using a telescope with its axis set perpendicular to the plane of the earth's rotation. He found that he had to compensate for the speed of the incident light by tilting the axis through a small angle. From this angle, Bradley calculated the speed of light to be 301,000,000 m/s. The first non-astronomical measurement of the speed of light was recorded by the French physicist Armand Fizeau, in 1849. His experiment, shown in Figure 1, entails focusing a light source, a candle at the time, through a beam splitter onto an image plane where a spinning toothed wheel is located. The light passing between teeth of the wheel is then projected to a mirror at a distance of about 8 km, where it is then reflected back to the point of origin. The wheel was then spun with increasing speed until the returning light was entirely blocked by the teeth on the wheel. From the rate of spin, Fizeau was able to calculate the speed of light to be 315,000,000 m/s. This leads to my chosen work, Foucault’s determination of the speed of light, found at Musée des arts et métiers. In 1850, Jean Léon Foucault, a peer of Fizeau at the Paris Observatory, modified Fizeau’s experiment by replacing the toothed wheel with a rotating mirror, which was driven with a small steam turbine and clock mechanism. The trick was that with the rotating mirror, as the light would take time to reach the stationary mirror and bounce back, the dynamic mirror will have rotated and thus light will be deflected away from the original source by a small angle.

With this, Foucault could determine the speed of light more precisely and without need of such a long distance as in Fizeau’s experiment. However, Foucault, like his peers at the time, was initially more concerned in settling the particle-versus-wave debate than in determining an absolute value for the speed of light. So, by inserting a tube filled with water between the rotating mirror and the distant, stationary mirror, he showed that light traveled more slowly through water than through air, disproving Newton's corpuscle theory of light. His results would lead to the beginnings of wave theory regarding light. Eventually, Foucault, with the support of Fizeau, took out the water-filled tube and determined the absolute speed of light to be 298,000,000 m/s. Although Foucault’s method was further improved, his experiment is considered by most scientists as the basis of modern measurements. In combination with the theoretical backing of Maxwell’s equations, his discovery has become one of the most exciting moments in physics. Not only were the laws of electromagnetism united by this quantity c, but this moment paved the way for all of quantum mechanics, from Max Planck and the quantization of light to Albert Einstein and the theory of relativity to many others, who have all contributed to our current scientific theory in that all particles exhibit wave nature and vice versa. Furthermore, Foucault determined the accurate value of the Earth-Sun distance, namely the Astronomical Unit or A.U. Until the 1850’s, this distance was deduced only from parallax methods, which depended on the Earth’s radius. Foucault’s measurement yielded an independent determination of A.U. Returning to Bradley’s experiment, the stellar aberration he noticed is simply the ratio of the Earth’s orbital speed to the speed of light. If we reverse this process, by knowing independently the speed of light, we are able to find the Earth’s orbital speed, and thus A.U. In determining the value of A.U., Foucault performed a giant leap in our understanding of our own solar system, allowing us to make precise measurements from the Sun to other planetary objects and opening up the doors to spatial explorations. Growing up in Florida, an hour away from NASA’s Kennedy Space Center, I witnessed many launches of the space shuttle program. Throughout my childhood, this program introduced me to real-life science and mathematics, and although the program is now closed, Foucault’s experiment, the basis for all space flight, reminds me of these experiences and the reasons I chose to pursue a career in science. It also makes me extremely appreciative of the place where I grew up. Returning to the speed of light itself, today, it defines our universe and remains our universal speed limit, but in the future, we may have a “theory of everything”, in which there may be a few exceptions to this constant c. Until then, we will need more people like Foucault to challenge our current understanding.