this post was submitted on 26 Oct 2025
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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!
Not arbitrary.
https://en.wikipedia.org/wiki/Metre
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.
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?
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.
299792458? That's amazing; I've got the same combination on my luggage!
Fluff you that made me snort.
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.
Why use a ratio of the length of the Earth? Why not the Moon? Or the Sun? Or Mars?
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])
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
Time can be non arbitrarily defined as a round number value of times cesium oscillates between two hyperfine states, to allow time to be non arbitrary and still a useful size.
The round number would still be arbitrary, no? It’s roundness would be based on the base 10 counting system, which is also arbitrary.
That's still an arbitrary number to pick, and the choice of cesium oscillation seems pretty arbitrary in the grand scheme of things.
fwiw, engineers round Pi and are fine with it...
To be fair, literally everyone rounds pi, since it's transcendental.
In many advanced physics fields, they use an arbitrary unit system in which c=1, making equations easier to write down. E=m
That is the least arbitrary unit system. It's the only unit that actually matters. Meters are arbitrary, in that it's a number chosen to be useful to humans. The speed of light isn't. It's a measurement of a natural phenomenon, which we didn't decide. (arguably, the time measurement is arbitrary though.)
The choice to fixate on it is arbitrary, though. That the math is easier when C=1 doesn't really mean that C is 1 any more than it is 299,792,458. C is C. The 1 is convenient.
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.
That's pretty damn arbitrary on a universal scale
Everything is pretty arbitrary on a universal scale. Except the speed of light. Which is really fucking slow on a universal scale too.
But not arbitrarily.
True, but it was the 18th century. They could measure earthly things well enough, not so much photons.
It's a bit of a shame it wasn't redefined as 1/300,000,000th of the distance light travels in a second when it was redefined, but the redefinition was about 50 years too late for that to happen. A difference of 0.07% in the base unit of measurement used by all science would've been far too much for 2019, given all the precision measurements we do these days.
That's still arbitrary. The definition is just something that gave a result that was a useful scale for humans. There's no reason to pick that over, say, the average distance to the moon, or something else. That distance is just fairly easy to measure and reasonably consistent over time. There are other choices for it though. The 1/10,000,000 is just whatever number was needed to make it useful. Nature doesn't care about that distance, unlike the speed of light.
that is not arbitrary at all!
I have for my worldbuilding project, but it's not famous or anything.
In base 12, there are 2 000 000 000 cesium oscillations in a tik (about 1.12 seconds), and light travels 80 000 000 mata in a tik (a mata is about 0.85m)
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.
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.)
We do, light travels 1 lightsecond per second.
Oh, and 1 lightpicosecond is around 2.998mm.
100 lightpicoseconds is also very close to 1'.
The speed of light is one lightyear per year
Only problem – which year? They've got different lengths.
In radio electronics we abbreviate c to 300 000km/s (when working in kHz, different multipliers in other bands for easy maths). The number as it is is round enough when rounded to the whole hundred million for practical purposes with commodity hardware
We could redefine the metre to be 1.00069229...x it's current size (increase it by 0.69229...mm) to make the speed of light exactly 300 000 000ms^-1^. This would also change area and volume, and any other units that are derived from length
Just use the speed of light as base and measure the distance in time units (implying *c). 100 psc (lightpicoseconds) are a bit more than 1⅛ inch, 4 ~ 1 mm, 1 nsc (lightnanosecond) is 1 foot or 29.9 cm, 1 μsc (lightmicrosecond) ~ 299 m. Would be totally possible. Within city boundaries we should introduce a speedlimit of 1 pc (picolightspeed), pretty easy to implement.
Just use meters and round up to 300 million m/s for the speed of light.