The speed of light is 299,792,458 meters per second, or as we generally quote it, the speed of light c is 3*(108) meters per second in vacuum.  We use c in every other formula in modern physics.  Even Einstein’s mass-energy conversion formula, E=mc2, has the square of the speed of light in it. Undoubtedly, the value of the speed of light has become an important cornerstone of modern physics.  But several questions occasionally keep popping into our minds.

Why does light travel with a speed of exactly  299,792,458 meters per second in vacuum, and why not any other number instead of 299,792,458. Like, why and how was the speed of light calculated and even measured in the first place? This article will explain it.

How did it all begin?

It’s strange to digest, but light wasn’t always believed to travel with a particular speed. For instance, at the beginning of the 17th century, a general belief existed according to which light didn’t have a speed at all. Rather it just appeared instantaneously. It was either present at a place or wasn’t there at all. It was never traveling with a finite speed to reach somewhere. However, things began to change as the 17th century progressed, and the notion regarding light not possessing any speed was seriously challenged.

Beeckman’s first trial to figure out the speed of light

In 1629,  Dutch scientist Isaac Beeckman became the first to delve deeper into the mystery surrounding light’s speed. He set up a series of mirrors around gunpowder explosions to see if the observers noticed any difference in the flashes of light appearing in different directions. Since experimental science hadn’t advanced much by that time, Beechman’s efforts couldn’t yield any impressive conclusion. But, they successfully set up a stage for further advancements to follow up in this regime.

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The connection between Io’s eclipses and the speed of light

In the late 1600s, Danish astronomer Ole Rømer was continuously observing Jupiter’s geologically active satellite, Io. He was baffled by Io’s strange motions around Jupiter. Every once in a while, the gas giant blocked our view of Io, thereby causing an eclipse. But there was something weird with the timing between those eclipses. The timing seemed to change over the course of the year.

Ole tried to figure out the reason behind this and realized that when we see Io getting eclipsed, we are in a certain position in our own orbit around the sun. However, when Jupiter eclipse Io the next time a few days later, we are in a slightly different position, either closer or farther away from Jupiter with respect to the last eclipse.

The wierd eclipse that led to the evaluation of speed of light. Image Credits: ABCC
The eclipse that led to the evaluation of the speed of light. Image Credits: ABCC

Looking at this, Ole thought that light probably took longer to travel from Jupiter when Earth was further away and a lesser time when it was closer. So, definitely, the light wasn’t appearing instantaneously but was traveling with a finite speed. Rømer made his rough calculations with all the rough planetary data he had and eventually put the speed of light at about 220,000 kilometers per second, which is definitely not a bad estimate because the planetary data available at that time wasn’t as accurate as it is today. This was the first time when the speed of light was actually calculated scientifically.

Over the years,  further experiments advanced, and measurements were made with beams of light on our own planet, which eventually paved the way closer to the right value.

Maxwell’s contribution

In the mid of 19th century, physicist James Clerk Maxwell introduced his famous Maxwell’s equations to the world. These equations served as a way of measuring electric and magnetic fields in a vacuum. Using his equations, Maxwell tried to calculate the speed of massless electromagnetic radiations propagating through space.

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Electromagnetic waves (

To do so, Maxwell used two constants, the electric permittivity and the magnetic permeability, to evaluate the speed of electromagnetic radiation. Maxwell’s value came per the speed of light measured experimentally, which further established that light itself is electromagnetic radiation.

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Maxwell’s equations (Image: maxwell’s

Einstein’s entry

In the early 1900s, Einstein came up with his own theory about the speed of light. Einstein’s realized that the speed of light had nothing to do with light at all. Rather it was a maximum speed limit allowed in our universe. In his special theory of relativity, Einstein tried to connect space with time, thereby establishing a unified fabric known as space-time.

Now, if space and time are connected, then there has to be a constant, a certain speed, that could tell us how much space was equivalent to how much time, and vice versa. To find this constant, Einstein applied his special relativity to Maxwell’s old equations and surprisingly found that this conversion rate is exactly equal to the speed of light.

The constant calculated by Einstein had nothing to do with the concept of electromagnetic waves. Rather it was just the maximum speed that a massless particle could attain in our universe, and nothing can move faster than that. Therefore, since light (photon) is massless, it can travel at that specific speed.

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Why is this number important?

As we know of it today, the speed of light has been calculated by considering vacuum to be an empty space. However, according to the newly emerging quantum field theory, it is not the case, and vacuum does have some quantum fluctuations associated with it. So if these considerations are also taken care of, the speed of light might change slightly, but not much.

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A light photon takes 8 minutes and 20 seconds to travel from the surface of the sun to earth (Image: Pinterest)

Still, the accepted value of light’s speed has garnered a lot of appreciation in modern physics. It is one of the most used constants, and c, along with the other two constants, the Planck’s constant and the gravitational constant, collectively represent relativity and quantum theory. Furthermore, c directly relates to the fine structure constant (α) that gives the ratio of the electron’s speed around a hydrogen nucleus to the speed of light in vacuum and has a numerical value equal to 1/137.

So if the speed of light is something else, it would change the fine structure constant as well. But that’s not possible. Our universe has chosen the fine structure constant to be 1/137(approximately 0.007) and nothing else. If its value is changed, it will interfere with numerous processes happening in our universe. In other words, if the speed of light in vacuum is anything other than a finite absolute speed limit we know of, then all the laws of physics and the whole universe would be surprisingly different from the one we are currently living in.

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Did you always want to learn how the universe works? Then, read our 30-article Basics of Astrophysics series absolutely free of cost. From the popular topics such as stars, galaxies, and black holes to the detailed concepts of the subject like the concept of magnitude, the Hertzsprung Russell diagram, redshift, etc., there is something for everyone in this series. All the articles are given here. Happy reading!

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