# Eli5 : Why do we define metre as length of path of light in vaccum during a time interval of 1/299,792,458 of a second ?

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Why not 1/300,000,000 or something different constant ( why only light ) ?

In: Physics

i think the length was defined earlier than it’s scientific explanation so to not mess up like all of the world by changing a meter by a small amount, they just used the exact time to travel the OG meter

Light moves at a constant speed so it’s good for measuring things precisely. The reason we don’t use a round number is because the meter existed before we had computers and that could measure in millionths of a second and we don’t want the meter to be longer than it used to be.

The metre was originally defined in 1793 as one ten-millionth of the distance from the equator to the North Pole along a great circle, so the Earth’s circumference is approximately 40000 km.

However, that definition didn’t prove exact enough for some scientific work, so we redefined it to be the same length, just using a far more accurate way to measure it.

Because we defined the size of a metre before we knew what the speed of light is. Instead of changing the size of the metre, we used a more accurate way of measuring the same length.

We use the speed of light because it is constant, and we can measure time way more accurately than we can measure distance.

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.

Because you need a constant. Something that should never ever change. Which already make pretty much anything material bad. Because any material can get damage by erosion, corrosion, or any form of time related issue. Matter will always change over time. There is nothing we can use that is forever.

But one of the only constant we can rely on is the speed of light. So we use it as a benchmark.

As for why not to use a round number, it’s a matter of history and practicality. While I say anything material is bad, it doesn’t mean it wasn’t used. At some point we didn’t know the speed of light, nor that it had a speed. Yet we still needed a benchmark. So we used plenty of other benchmark, but none of them were “permanent”. Nonetheless, once we realized we could use the “somewhat perfect” speed of light, we decided NOT to change the actual size of the meter too much. Changing it might mean that every other previous size related thing would become incorrect. Suddenly the house you bought is considered slightly bigger, average changes, etc…

So instead they simply changed the scale. They adapted the scale to fit the previous assumptions. For most people, it meant that nothing changes. Your ol’ ruler is still correct enough, while we’re certain now that this scale is not at risk to change.

Adding on to what other people said; it used to be the case that the meter was defined as the length of a particular metal bar whose sole purpose was to be exactly 1 meter long.

If we were to use that same standard today, we would an interesting problem. Obviously, we would need to put error bars around the speed of light. By itself, this is not unreasonable, as the speed of light is an empirical physical constant. However what is weird is the source of those error bars. The biggest source of error in measuring the speed of light would be measuring the meter stick.