Speed of Light

Speed of Light

The speed of light in vacuum is believed to be both universal in nature and constant in time. Its value is 299,792,458 metres per second and it is approximately 186,282 miles per second. It is the maximum speed at which all energy, matter and information in the universe can travel.

History of Speed of Light

In 1667, Galileo Galilei is often credited with being the first scientist to try to determine the speed of light.

His method: Galileo and his assistant each took a shuttered lantern, and they stood on hilltops one mile apart. Galileo flashed his lantern, and the assistant was supposed to open the shutter to his own lantern as soon as he saw Galileo's light. Galileo would then time how long it took before he saw the light from the other hilltop. However, his experiments took place over terrestrial distances and the timing methods available to him were far to crude to make a successful determination given such distances and the very great speed of light hence he was unable to measure the speed of light.

In 1676, Olaus Roemer, a Danish astronomer, first successfully measured the speed of light. Roemer's estimate for the speed of light was 220,000,000 metres per second.

His method: It was based on the observations of the eclipses of the moons of Jupiter. Roemer noticed, while observing Jupiter's moons, that the times of the eclipses of the moons of Jupiter seemed to depend on the relative positions of Jupiter and Earth. If Earth was close to Jupiter, the orbits of her moons appeared to speed up. If Earth was far from Jupiter, they seemed to slow down. Reasoning that the moons orbital velocities should not be affected by their separation, he deduced that the apparent difference must be due to the extra time for light to travel when Earth was more distant from Jupiter. Using the commonly accepted value for the diameter of the Earth's orbit, he came to the conclusion that light must have traveled at 220,000,000 metres per second.

In 1729, James Bradley used the aberration of light method to estimate the speed of light and he measured it to be 301,000,000 metres per second.

In 1849, Hippolyte Fizeau used the toothed wheel method to estimate the speed of light and he measured it to be 315,000,000 metres per second.

In 1862, Leon Foucault used the rotating mirror method to estimate the speed of light and he measured it to be 298,000,000 metres per second.

In 1907, Rosa and Dorsey used the electromagnetic constants method to estimate the speed of light and they measured it to be 299,710,000 metres per second.

In 1926, Albert Michelson used the rotating mirror method to estimated the speed of light and he measured it to be 299,796,000 metres per second.

In 1950, Essen and Gordon-Smith used the cavity resonator method to estimate the speed of light and they measured it to be 299,792,000 metres per second.

In 1958, K.D. Froome used the radio interferometry method to estimate the speed of light and he measured it to be 299,792,500 metres per second.

In 1972, Evenson et al. used the laser interferometry method to estimate the speed of light and he measured it to be 299,792,456.2 metres per second. The development of lasers with very high spectral stability and accurate caesium clocks made even better measurements possible.

In 1983, to further reduce uncertainty, the seventeenth CGPM replaced the definition of the metre with its current definition, thus fixing the length of the metre in terms of the second and the speed of light. This definition fixed the speed of light in a vacuum at precisely 299,792,458 metres per second.

Reflections:

I feel that all these people who tried to measure the speed of light is very creative as they were able to think of methods to measure it. I was also touched by their determination to try and measure the exact value of the speed of light. Many scientist in the past believed that light has a infinite speed and there is no way to measure it. However, it was Galileo who was willing to put in effort and try to measure the speed of light and even though he failed, it was a motivation for other scientists to try and measure the speed of light. The measurement of the speed of light has helped in many areas of physics and it is very important. This taught me that we should not give up but try to think of different methods to figure it out when sometimes it might seem to be impossible.