The speed of light could be one of those things. There are also things in nature that are unexplainably asymmetrical. Nevertheless, it is not an experimentally verified fact, just a widely accepted convention. After all, why wouldn’t the speed of light be the same in all directions? Many things in the universe are symmetrical by nature. We consider the two-way speed of light to be the speed of light because we follow the convention that light travels with the same speed in all directions. It is not experimentally verified, but assumed, that the speed of light is the same in all directions. He defines that the time taken by light to travel from A to B equals the time taken by light to travel from B to A. In his paper On the Electrodynamics of Moving Bodies, Einstein establishes the convention for synchronizing clocks. The speed of light is taken as a constant in special relativity. Any other method you might think of will probably run into similar issues. In other words, by the time the timer gets to B, it would be out of sync with A. We could start the timers together at point A, and then take one timer to B, but what we know of time dilation tells us that moving clocks slow down relative to stationary observers. We cannot adjust for this delay, as it will be equal to the speed of light… which is what we’re trying to measure! We could measure the time it takes for light to get from A to B by synchronizing the timers, but how do we make sure the timers are synchronized? Any communication between the points will inherently experience a delay. Imagine we put a timer at point A and another at point B. But the fact is, any method you’re thinking of is bound to present a problem. Can we Measure the One-way Speed of Light?Ī fair question would be-why has no one ever measured the one-way speed of light? It seems simple enough. It could even be that light traveled from A to B with a speed of c/2, and then traveled instantaneously from B to A! We simply would not be able to tell the difference, unless we could measure that one-way speed of light. As long as the speed averages to c for a round trip, there are no other restrictions as to what the situation could be. That said, light could travel with one speed from A to B and then a different speed from B to A. We know that for a round trip, light travels at a constant speed, c. (Photo Credit : Pon圓000/Shutterstock)Īs long as we cannot experimentally verify the one-way speed of light, there is no telling what it could be. The one-way speed of light is the speed of light measured as it travels from one point to another. For this, we would have to measure the speed of light from point A to point B, rather than for the round trip. The one-way speed of light is the speed of light in one direction. This is the only speed of light that has been measured by anyone to date. The round trip speed of light is also called the two-way speed of light. The speed of light is taken as the average of the speed of light as it travels from A to B, and its consequent speed as it travels from B to A. That is, measurements are of the time taken for a round trip. We are measuring the time it takes to go from point A to point B and then reflect back to point A. In such a case, we are not measuring the time that light takes to go from point A to point B. If we know the distance between the source and the mirror, and we measure the time it takes for a beam or pulse of light to be reflected back, we can easily calculate the speed of light from there.

(Photo Credit : Wikimedia commons)Įssentially, we need a source of light, a mirror, and a timer. It is defined by taking the fixed numerical value of the speed of light in vacuum $c$ to be 299792458 when expressed in the unit m⋅s−1, where the second is defined in terms of the caesium frequency ΔνCs.Experimental setup employed by Fizeau to measure the speed of light. The meter is now defined as “The metre, symbol m, is the SI unit of length. Note, this is no longer the definition of the meter. The meter is the length of the path travelled by light in vacuum during a time interval of 1/299792458 of a second. Either way would lead to the same set of equations and the same size of all the units.

But it could easily have been done the other way around.

Currently the s, m, kg, A, K, cd, and mol are defined in terms of $c$, $h$, $N_A$, $e$, $k_B$, $K_$. However, the order of definition is not particularly important. I would use the word “defined” rather than “derived” since it is not a logical proof but rather an authoritative statement of the meaning of the meter. So in that sense the meter is derived from the speed of light. The meter is defined in terms of the speed of light.

Speed of light in meters persecond