The speed of light is defined as 3✕108 m/s where a meter is defined as the distance travelled by light in 1/299792458th of a second. In 1849, Hippolyte Fizeau calculated the speed of light by an experiment known as the Fizeau experiment. He projected a pulsed beam into one direction towards a distant mirror and passed it through a toothed wheel. Thus, knowing the distance to the mirror, he calculated the speed of light to be 315000 km/s.

This experiment, though viable, only gave us the knowledge of the time it takes for light to complete a two-way journey. This is because it is impossible to trace the speed of light, for if we take two clocks to time the distance light takes to travel 1 km, we would never be able to synchronize them. It is also impossible to start both the clocks at the same time as it requires utmost precision and relying on trial and error would take a one-in-a trillion cosmic fluke. If we try to connect the two clocks via a wire and rely on the pulse, there would be a time delay as the pulse travels at the speed of light and to find the time delay, we will need to find the exact speed of light. The last resort could be to synchronize the two clocks and carry the second clock to a place such that the distance would be 1 km. However, according to Einstein, moving clocks tick slower and in conclusion, one could say that it’s impossible to measure the speed of light in 1 direction.

Let’s take an example:

If one were to shoot a beam of light towards a mirror and calculate the time it takes for light to reflect it such that the time taken was 20 seconds, we could say that light took 10 seconds to reach the mirror and 10 seconds to travel back. Even if we took the most advanced and physics-defying type of camera, the fact of the matter is that the only reason the camera could see anything was because of light and therefore, this method would still result in us finding the two-way speed of light.

In conclusion, the speed of light in one direction is impossible to find and the convention that the speed of light in all directions is “c” is only an assumption.

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