Why is the current definition of metre what it is? Why not something else?
The metre is defined as the distance travelled by light in a vacuum in 1/299,792,458 seconds. Why was this definition of metre chosen? Why not something else?
Think back to history. Earlier people used some arbitrary length as their single unit by which they measured all other lengths. For example, a bar was used as a standard length of 1 meter all over the world from 1875 onwards till 1893. Then in 1893, one metre was defined as the number of wavelengths of a specific type of light. You can notice that the definition of one metre has changed many times and that is the nature of human studies. We try to improve on what we have.
Other arbitrary lengths
Think back to even earlier times. The human then must have used some arbitrary length of a stick to measure things. And that stick was then decided to be standard in that town or community. The current definition is also no different than that. It just has more advantages.
And so, in 1983 the new definition of one metre was adopted. the 299,792,458 is the speed of light in vacuum in one second. This precise definition gives even better global standard with which every scientist can study. Now, different people in different areas do not have to find some stick with which to measure things to study. We have discussed this issue with measurement in our blog post here.
Effect of relativity.
Einstein’s theory of special relativity changed the property of length of any object from being a fixed thing to being dependent on the frame of the observer. Thus, your scale of 1 metre and my scale of 1 metre will not be the same if we are travelling at different speeds and dependent on the frame of reference.
You will start noticing a pattern from all our blog posts that many of the things we use to study nature are human definitions and not some universal standard. This, we hope, gives you the confidence to grasp the concepts better.