Man had tried to deduce, or had contemplated, for over two thousand years the following question:
How fast is light?
Where to start in this journey of discovery, let’s start in the middle with a medieval geek Galileo.
Most geeks are aware of Galileo’s experiments with falling bodies. No goofball, not dropping bodies from the leaning tower of Pisa 😛 But did you know that Galileo attempted to measure the speed of light as a part of this experiment with falling bodies?
Contrary to popular belief the experiment with ‘falling bodies’ were actually balls rolling down an inclined plane and unpublished working papers from as early as 1604 clearly showed that he had performed earlier experiments demonstrating the time-squared law.
In order to perform his experiments, Galileo had to set up standards of length and time, so that measurements made on different days could be compared in a reproducible fashion. For measurements of particularly short intervals of time, Galileo sang songs with whose timing he was familiar.
Galileo also attempted to measure the speed of light, wisely concluding that his measurement technique was too imprecise to accurately determine its value.
- He climbed one hill and had an assistant climb another hill; both had lanterns with shutters, initially closed.
- He then opened the shutter of his lantern. His assistant was instructed to open his own shutter upon seeing Galileo’s lantern. Galileo then measured the time interval for his assistant’s shutter to open.
- Knowing the time interval and the separation between the hills, he determined the apparent speed of light.
On repeating the experiment with more distant hills, Galileo obtained the same time lapse, concluding that the time for the light to travel was much less than the reaction time of the person, and therefore that the actual speed of light was beyond the sensitivity of his measurement technique.
But Galileo wasn’t the first, not by far, to make an attempt at the measurement, nor to consider whether light even had a speed.
Before Galileo, Isaac Beeckman proposed an experiment in 1629 in which a person would observe the flash of a cannon reflecting off a mirror about one mile away.
Probably the most bizzare attempt to classify the speed came in the 1300’s by Sayana. He was an important scholar and minister of his king’s court in India and had written many books on various subjects including religion, medicine and philosophy.
He is claimed to have mentioned the speed of light, in the following comment he wrote on verse 1.50 of the Rig Veda:
Thus it is remembered: O Sun you who traverse 2202 yojanas in half a nimesa.
These ancient units translate into a speed of 186,536 miles p/s, a value amazingly close to the modern value of c of 186,285 miles p/s and has been called the most astonishing “blind hit” in the history of science.
Astonishingly lucky indeed.
The history books on light go back as far as Empedocles (430 BC) and Aristotle (322 BC) and Heron (c 70).
Empedocles maintained that light was something in motion, and therefore there had to be some time elapsed in travelling.
Aristotle said that, on the contrary, “light is due to the presence of something, but it is not a movement”. Furthermore, if light had a finite speed, it would have to be very great; Aristotle asserted “the strain upon our powers of belief is too great” to believe this.
One of the ancient theories of vision is that light is emitted from the eye, instead of being reflected into the eye from another source. On this theory, Heron of Alexandria advanced the argument that the speed of light must be infinite, since distant objects such as stars appear immediately when one opens one’s eyes.
So where does the story end? We end it with 20th century scientific experiments answering the question once and for all.
The speed of light was ‘discovered’ (as in proven) in 1926 by Albert Michelson using a rotating prism to measure the time it took to make a round trip from Mount Wilson to Mount San Antonio in California. His precise measurements yielded a speed of 186,285 miles p/s (299,796 k p/s).