[gorn]

Recently I have been thinking about if any of our units makes sense in cosmic perspective. Let's take speed of light for example. It's approximately 300000 km/s. But then what is a second? It's 1/60 of a minute which is 1/60 of an hour which is 1/24 of a day(and so it goes) and all those numbers are arbitrary. A day doesn't make any sense outside our planet anyway, I doubt that there is another celestial body in the universe that takes the same time to complete a rotation. Period of some natural phenomena (like atomic electron transition) sounds better as a unit but it's a really tiny period of time so we have to scale it to make it practical for us. We will use decimal numeral system to do that, another arbitrary choice. What if we had 12 fingers or 8? This can be extended to all kinds of measurements so I wonder if any of this would make sense to another civilization. What would a cosmic system of units would like? Any reading about this would be greatly appreciated.

[carljoseph]

The definition of a second isn't based on the Earth's motion, but some natural phenomena like you recommended. "The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom." [1]

[1] https://www.scientificamerican.com/article/how-does-one-arri...

[saagarjha]

Of course, that definition was made to make our current definition of the second continue to work.

[vlasev]

You might be thinking along the lines of natural units [1] or even Planck units [2], where you set some fundamental constants to 1 and take it from there.

[1]: https://en.wikipedia.org/wiki/Natural_units [2]: https://en.wikipedia.org/wiki/Planck_units

[Ono-Sendai]

Yes I think you're mostly correct, it's all pretty arbitrary. An alien civilisation is going to need units of length, time, speed, mass, energy etc..

Speed has an obvious unit which is the speed of light (in a vacuum), which seems to be the same everywhere.

All the others are a bit more arbitrary I guess.

[elefantastisch]

Sounds like you're looking for Planck units. https://en.wikipedia.org/wiki/Planck_units

I would suggest that Planck units scaled by powers of 2 is the closest we can get to a cosmic system of units. The choice of binary is non-arbitrary as it's the smallest base that can be chosen and still provide scaling.

[gnopgnip]

"The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom."