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Why Is The Speed Of Light In Vacuum A Constant Of Nature?


Dr. Alfredo Carpineti

Senior Staff Writer & Space Correspondent

clockApr 30 2021, 15:29 UTC
Image Credit: Zonda/

Why is it this value? Why is it constant? And why does it pop up everywhere? Image Credit: Zonda/

Light in a vacuum moves at a constant speed of 299,792,458 meters per second (about 186,000 miles per second). This peculiar value is quite arbitrary and it depends on the conventions that we adopted back in 1975 of what a second and what a meter is. Light itself, as far as we can tell, always moves in a vacuum at the same speed – here, in the Andromeda galaxy, and in the furthest reaches of the visible universe.

And while we think of it as the speed of light, it crops up in so many places in physics that it is reductive to just consider it related to electromagnetism. It is the speed at which massless particles move, and no information can be transmitted in our universe faster than this specific value.


So why is it this value? Why is it constant? And why does it pop up everywhere? We don’t have a conclusive answer just yet. It is one of those things we don’t know that hint at the complex machinery of the universe... hopefully just hiding behind a curtain we are seeing flapping.

The Formula Of The Speed Of Light

Historically, there are been many ideas about how quickly light moves – but the first serious experimental evidence that it was fast but limited came in the 1600s with Ole Rømer, who noticed that apparent delays in the orbit of Jupiter’s moon Io could be explained by the speed of light being finite.

Since that early estimate, the value got more and more refined, but the game-changing moment came thanks to Maxwell’s theory of electromagnetism. In this theory, light is understood as an electromagnetic wave and its speed can be defined in terms of two other constants.


In this formula, ε0 represents the electric constant and μ0 is the magnetic constant. Both of them depend on other constants of the universe, such as elementary charge and the fine structure constant.

A Tangled Web Of Constants

Not to sound too much like a conspiracy theorist, but it appears that “it’s all connected!” At a fundamental level, there are hints that link all the properties of the universe to one another, and this is why we see these quantities popping up in formulae about different physical things. Based on what we know so far, it could be possible one day to have our whole physical understanding in a single model: the theory of everything. 

We don’t know if the universe would be possible with slightly different values, or if these were the only possible values it could have had, but the search for an answer continues. And the constant c might literally light the way for humanity to actually find that out. 




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