If two species with no previous knowledge of one another, and no knowledge of each other's environments (say they're on the other side of a wormhole through which they can shoot a radio signal), what system of measurement would they use to communicate?
Someone once suggested to me that the period length of a hydrogen atom, which they said was 21 cm, would make a good universal measurement standard for beings with our basic scale. Not knowing any better, I accept their assertion. Binary seems like a good numeric base, with useful "higher" numeric bases of base 8 and base 16 being easy to agree on.
How about mass, though? And time? What could they communicate those ideas with to establish them?
One answer suggests the International System of Units. Skimming through the Wikipedia article, it provides measurements such as "metre (current): the distance traveled by light in vacuum in 1/299792458 second."
How would we communicate the 299792458 denominator for a second? For that matter, how do we communicate a "second"? Its definition is even more obscure.
I'm looking for a more straightforward unit of measurement, such as the aforementioned length of the period of a hydrogen atom. Also, the units of measurement should be useful at scales that are useful to our physical form and attention span. I'm presuming that any alien race we'd have much in common with would have physical attributes (scale, time sense) similar to ours. We may learn a lot from sentient quasars, but we would seem horribly impatient to them given that our lifespans are so short.
Based on comments and answers that have been provided, I think I was unclear with my question. Perhaps I should re-frame the question this way: What units of measurement could we concoct that would be useful to us in our day-to-day lives that are based on things occurring in nature and are observable no matter where you are? For some reining in of possibilities, I'll limit the "where" to our galaxy and relative acceleration.
The suggested unit of length I posited (which turns out to be the hydrogen line) is useful at our scale. If every length measurement we used on Earth was based on this, we would be no worse off than we are using meters or feet. We could call it the hyl, craft a symbol for it, and start measuring things. If aliens ever call, and ask what unit of measurement we use, we could say, "Oh, it's this, which you should be able to measure on your end and understand what we mean when we say hyl".
The bit about communicating with aliens is really just a different way to say "how do you determine it no matter where you are"? At least that's the way I meant it; perhaps that's a different question.
So, again, what other measurements could we use that are useful to us at our scale? Things that are about the size of a pound or a kilogram, a minute or an hour... or...