Why not 1/300,000,000 or something different constant ( why only light ) ?
In: Physics
TL;DR TL;DR because I don’t know the concept of ELI5: the speed of light is one of the few things that doesn’t change anywhere, and we wanted to represent the metre as something about that, but 1/300,000,000 seconds isn’t quite close enough.
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TL;DR it was defined a long time before the speed of light was known, and recently scientists wanted to update the definitions of length measurements, time measurements etc, so they picked the number that closest matched the previous length of the metre so that they didn’t screw over previous data used (which happened to be close to 1/300,000,000, but not close enough to redefine it as such).
The speed of light is an unchanging speed, and using that was better than finding a similarly unchanging length.
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We had the metre defined as an arbitrary length (with a reference metre stick somewhere in France or something)
Again the second was an arbitrary length of time.
Scientists wanted more thorough and unchanging definitions for measurements, which included metres and seconds. It would be a massive pain to redo all data collection and calculations etc such that they marched the new, more precise definition of the second and the metre, so they pick some non changing time (something about the Caesium atom happens very sequentially at exactly the same intervals all the time), and defined the second to be 9,192,631,770 of those. That’s because the Caesium atom has 9,192,631,770 intervals occuring in a second. (That number probably goes a bit above or below a second as previously defined, but it’s so big that it doesn’t matter).
Now for distance, we don’t really have any arbitrary, unchanging and very precise distances, like the earth is not exactly spherical, for example, so we couldn’t just say it’s some fraction of the diameter/”width” of the earth.
However, there is an unchanging speed, the speed of light is 299,792,458 m/s which means over the course of one second (which now has its thorough definition), a photon will travel 299,792,458 m. (In a vacuum, it slows down a little bit when going through air, but speeds up again in a vacuum).
If in one second, light travels 299,792,458 m, then to travel 1 m, it should go for 1 / 299,792,458 s. It’s not 1 / 300,000,000 s, because our previous length of the metre would be changed too much by setting it to that.
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