How is the speed of light calculated?

Light! Probably the most important thing for any living organism. Without light there would be no life. This light originates from Sun and travels a long distance to reach us and that means light must be travelling at some speed. We now know that light has a finite speed. But how was the speed of light calculated? Lets find out.

Often denoted by constant c, the speed of light is rounded off to 3×10^8 meters/second. This number is actually 299,792,458 m/s to be precise. This is defined to be the speed of light in vacuum.

But this was not always the case. Initially, it was assumed that light had infinite speed and that it could travel any distance instantaneously. This was supported by the observation that there is no noticeable lag in the position of the Earth’s shadow on the Moon during a lunar eclipse, which would otherwise be expected if c were finite. But soon scientists started challenging this thought. Ole Rømer first demonstrated in 1676 that light travels at a finite speed (as opposed to instantaneously) by studying the apparent motion of Jupiter’s moon Io. The first known scientist to have tried to define the speed of light was Galileo in the 17th century. Galileo doubted that light’s speed is infinite, and he devised an experiment to measure that speed by manually covering and uncovering lanterns that were spaced a few miles apart. We don’t know if he ever attempted the experiment, but again c is too high for such a method to give an even remotely accurate answer and hence he was not successful in his attempt.

In 1865, James Clerk Maxwell proposed that light was an electromagnetic wave, and therefore traveled at the speed c appearing in his theory of electromagnetism. In 1905, Albert Einstein postulated that the speed of light c with respect to any inertial frame is a constant and is independent of the motion of the light source. He explored the consequences of that postulate by deriving the theory of relativity (E=mc^2) and in doing so showed that the parameter c had relevance outside of the context of light and electromagnetism.

Now the interesting thing to notice here is that light is information that we see from other planetary bodies like galaxies and the planets & stars that reside within them. This means, what we observe happening on a far away galaxy now (i.e the present) is something that has already happened, but due to the information reaching us slowly in the form of light, we are able to see it as though it is happening now. This phenomenon has helped scientists to study the origin of the universe and the stage when the universe had just started to expand after the big bang.

But the question here is how did scientists arrive at the constant c? There has to be some method to find out the figure. The speed of light was derived from our definition of a meter. As weird as it might sound, it makes sense as speed is the distance traveled per unit of time. So, if our definition of a meter changed, it would change the speed of light altogether.

The closest figure that scientists ever derived until the 19th century ranged between 299000000 m/s to 301,000,000 m/s using various experiments.

It all changed in 1970 when we invented lasers and therefore it became possible for us to calculate the speed of light accurately. Using lasers, scientists found out that the speed of light in the vacuum was 299,792,456.2 m/s. Scientists didn’t want this number to be in decimal form. So in order to convert it to a whole number, they redefined the length of a meter in 1993 using the speed of light (c) as 299,792,458 m/s and the rest is history. Meter was now a little bit longer than it used to be (even though if it’s just by a tiny fraction).

This indeed was a good move as we now have a unit length of meter that is not dependent on any physical element and can be derived anytime using the known speed of light.