Anyone come up with a good measure of distance that makes the speed of light a nice round number? I like the metric system, but the meter feels pretty arbitrary. We could do better!
this post was submitted on 26 Oct 2025
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I would like to give a massive shout out to the fact that a foot is only 5mm off from being a light nanosecond. (Pure coincidence, but imagine if the next God emperor of America changed the foot definition by 5mm to make a truly science based unit of measurement.)
Not arbitrary.
Since 2019, the meter has been defined as the length of the path traveled by light in vacuum during a time interval of 1/299792458 of a second, where the second is defined by a hyper-fine transition frequency of caesium.
I mean, that is pretty arbitrary. The reason the divisor is that specific constant is because we already had meters before we knew the speed of light.
we already had meters before we knew the speed of light.
It's true.
Genesis 1:1 In the beginning God created the heavens and the earth and the metric system.
Genesis 1:3 And God said, "Let there be light," and there was light.
Then the devil created Britain.
Then it's lucky the numbers line up as well as they do, no?
299792458? That's amazing; I've got the same combination on my luggage!
One light year is 9.4607379e+15 meters, so there’s a power of 10 that could give us a unit of length close to 94 cm. That would not be as arbitrary.
But fuck me if we discover the speed of light in a vacuum has not been constant along the history of the universe, the c would be an awful base for cosmic distance, or very long term science.
But don’t worry, humanity doesn’t look like it will exist long enough to do very long term science.
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You are correctly trying to say it's well defined, but you are complaining about the wrong comment. You should check the meaning of "arbitrary" again.
Anyway, it's not entirely arbitrary because it was created to represent a "round" fraction of the Earth's circumference that is similar to the length of a person's arms. But it deviated from that too, so it's subjective how much that counts.
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c is pretty round (universal symbol for the speed of light)
aside from that, nothing. as science and maths are mere attempts at describing the universe all our units are arbitrary, decided to be the way they are purely because you just need to pick something to be your reference point.
at no point has a true non-artificial unit emerged, there is no constant size of anything that could aid in that (one contestant for that title could be the planck lenght but that'ss just incredibly inconvenient to use. "honey could you pelase move the couch 6,25 × 1034 planck lengths to the left? [1m])
Math isn't arbitrary. Otherwise there wouldn't be constant debate about whether it's a human creation or fundamental to any existence.
Proton masses, the distance light travels in a vacuum in a certain time, and cesium oscillation times are quite constant.
proton masses are rather small - inconvenient
the distance light travels at a certain time - then it'll just be based on our artificial units of time
cesium oscillation i don't know much about but from what i quickly read it's also about keeping time, 1s to be precise, which is still an arbitrary unit
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I think it's (1 Planck length / 1 Planck time). If you take the smallest distance that exists and divide it by the shortest amount of time that can pass, you have exactly c.
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We do, light travels 1 lightsecond per second.
Oh, and 1 lightpicosecond is around 2.998mm.
100 lightpicoseconds is also very close to 1'.
In many advanced physics fields, they use an arbitrary unit system in which c=1, making equations easier to write down. E=m
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The meter isn't really arbitrary, even when you ignore the redefinition posted by @jumperalex. It was originally defined as 1/10,000,000th the distance from Earth's pole to the equator, which is a pretty reasonable basis to use by 1791 standards.
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Speed of light in a true vacuum.
Speed of light through any non-vacuum decreases.
The speed of causality remains the same.
is the speed of causality tied to speed of light in a vacuum, or independent of it?
As I understand, the speed of light in vacuum is bound by the speed of causality. So, light would go at infinite speed, if it could (it being massless means any acceleration should result in infinite speed), but instead it goes as fast as the universe allows, which is the speed of causality.
Speed of Causality is the absolute maximum speed. It's the theoretical maximum that any cause could propagate an effect. Speed of Light in a (perfect) vacuum happens to be equal to the Speed of Causality.
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