Measuring The Speed Of Light: Have We Been Doing It Wrong?

We all have learned in school that the speed of light travels in a vacuum space at specifically 299, 792, 458 metres per second. We have all been taught in such a way that the universal unit of length used to measure the universe uses the speed of light, also known as light years.

However, what if, fundamentally, nobody knows if the value above is the exact speed at which light travels? What if we told you that, to this day, no one had measured the specific speed at which light travels from the starting point to its destination?

2-way speed of light

To understand the fundamentals of this topic, it is crucial that we first understand how the universe measures the speed of light.

French physicist Armand Fizeau first discovered the method to calculate the speed of light in 1849 using the teeth of moving gear and shining a light source through the teeth, which is then reflected back to him using a mirror. In modern days, scientists use high-precision timers and lasers.

We can repeat the experiment in the comfort of our homes. All we need is a light source, timer, and mirror. If we know the distance between the mirror and the light source, we can essentially measure the time taken for the light to travel from point A (light source) to point B (mirror) and then reflect back to us.

This round-trip measurement is also called the 2-way speed of light and has been the only way of measuring it till today.

1-way speed of light

This measurement essentially calculates the speed of light in a single direction, rather than measuring from point A to B and then back to us; a 1-way speed of light measures just from point A to B.

Fundamentally, we are not able to verify the 1-way speed of light. Unlike the 2-way speed of light, we know that there is a constant speed at which light travels. That being said, we do not know if that measure is fundamentally accurate. Of all we know, light could travel at a certain speed from point A to B and then at another speed back. We would not be able to know unless we could measure the 1-way speed of light.

Are we able to measure the 1-way speed of light?

The right question we should ask ourselves is why no one has ever measured the speed of light in such a manner. We can simply put it that any method that we have considered is flawed in itself.

If we put a timer both at points A and B, we could measure the speed of light in that direction by synchronising both timers. However, how can we ensure that? Any communication errors between both points will cause a delay. We might be able to factor in those errors, but we are not able to control them.

We could start both timers at the same time at point A, then take one timer to point B. However, we have to factor in time dilation, which mentions that stationary objects observe time faster than moving objects.

It is essentially impossible to sync two timers at both points unless we know the exact speed of light, which is what we are trying to find out.


What we now know as the speed of light is the agreed-upon understanding which is measured 2-way. While the 1-way speed of light will never be measured, the odd thing is that even if the theory that light travels at a different speed in different directions, it does not change the fact that as long as the 2-way speed of light remains constant, the world of physics and those that depend on the constant speed of light will remain satisfied.

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