Since this section discusses design, I thought I'd take a light-hearted excursion from the practical into the realm of "nice, but Thomas, you're never going to convince anybody." A bit of "fun" design.
Folks knowing me when I was growing up knew I was an avid trekker. Star Trek™, that is, and not the modern new-age-preachy Picard/Cisco/Janeway pablum: the old 60's classic, sometimes campy, always innovative Star Trek.
The thought of humans out in space was (and is) fascinating, and the show brought up some pretty interesting ideas.
But one thing that always bugged me: they measured things in kilometers, or more precisely, using the metric system. For its time, that was an innovative idea, but an entirely impractical one. The Federation would not use metric any more than the world now would use cubits.
Well, the Federation (as suggested by its full name—the United Federation of Planets) is supposed to be the United Nations of the Stars. And no one member is supposed to be any more "right" than any of the others.
Yeah, yeah, yeah, Thomas, so what's the metric system got to do with that?
Two simple things.
1. It's an entirely earth-based standard. A meter was originally defined as 1/10,000,000 of the distance from the pole to the equator. While the definition has subsequently been more rigorously defined in terms of natural phenomenon, it still traces its roots to a fraction of the Earth's circumference: 1/10,000,000 is a "pretty" number and 1/299,793,458 is not. (That fraction is the newer definition: the fraction of a second that light must travel to cover one meter of distance.)
2. It's base 10. That is to say, each progression is 10 times larger than the last: 1, 10, 100, and so on. Why? Because that's how many fingers we humans have. And that's a pretty stupid reason, considering that base 10 is lousy for dividing things into thirds or quarters. Yes, that's why things like hours, inches, and baked goods use numbers like 12. Plus, it's arrogant to assume that every race in the Federation has ten digits, either on their hands, or in their numbering systems.
(Okay, so they speak English, too—you have to cut the producers of the show some slack, taking certain artistic liberties—most 60's viewers weren't hip to subtitles, and you'd need a lot of subtitles otherwise.)
But forget about Star Trek. Let's say we've just made first contact, and we're ready to exchange ideas over the vast distances of space with our new-found breatheren. Curiosity about each other is natural (at least for us humans) so we're inclined to ask (or be asked) "how big is your world?" "How tall are you?" "What's the optimal range of your plasma rifles?" (Okay, hopefully neither side gets to that question ;-)
At first, it might be acceptable to say "40,000,000 meters," "a little less than 2 meters," and "more meters than you can count," but we'd have to send along the definition of a meter, too, (plus some hints of how many fingers we use before we run out.) And then the other side would have to do some math and figure it out in terms of their units.
In time, though, we'd want to do away with all that conversion math, and agree to use some mutually useful system.
Wouldn't it be cool, though, if we could plan for that day, and be ready with units of measure that weren't just global, but truly universal?
It's possible. But I have to confess, it's impractical: look how hard it is to get the United States to adopt the Metric system... so just imagine how hard it would be to get the world to adopt the Universal Metric system.
But just for fun, what might such units be?
Well, as for numbering systems, base 12 has its benefits: no more .33333333333 (etc) or .25; You can divide stuff in halves, quarters, and thirds without effort. Scientists like logrithmic numbers, so science would probably be expressed in that, but us real-world integer types would be more comfortable in base 12.
And as for units of distance, there just happens to be one universal unit of measure. Hydrogen, the most abundant element in the universe, happens to emit at 1420 MHz. Odd number, that, but what that means is the wavelength for this radiation is approximately 21 centimeters (or about 8 and a quarter inches.) That's about an average human hand-span. Convenient: in addition to being universal, it's a distance we can relate to. For lack of a better name, I've called this a Metrical Wave, (or just "Wave") over the years.
Why Hydrogen? Well, aside from that claim of "most abundant element"—indeed, because of it—a lot of astronomical research is being done with (radio) telescopes tuned to this frequency. Knowing this, the clever ET's that want to be heard need only emit an organized broadcast at this frequency where the rest of us are likely to be tuned in, and voila. Not exactly phoning homeThis is likely on the fringes of still-remembered pop-cultural references. I don't know how many remember the plight of the 1982 movie ET's eponymous character to contact home-world to get a lift back from Earth... but if the 1977 hit Close Encounters's 40th anniversary re-release is any indication, maybe a whole new generation may (re)experience (and understand) the meme "E.T. phone home!"..., but reaching out and touching someoneAnother pop-culture reference that may be lost on post-20th Century denizens: around the same time, AT&T (then a long-distance telephone carrier: no, there were no cell phones in widespread use, and you had to get separate long-distance service to call beyond your local service area) had slogan which became its own pre-internet meme none the less.
So, instead of answering those eternal questions in silly old meters ("yeah, a meter is 1/299,793,458th of a light-second... what's a second? well it's ...") we could answer in waves: "we stand about 8-9 waves tall on average, where 1 wave is the wavelength we originally made contact with."
Naturally, other measurements—like mass, volume and so on—would be expressed in terms of a Wave, just like units of mass and volume are expressed in terms of meters nowadays: one kilogram was originally defined to be the mass of one liter of water, which is defined as a 10cm cube. Whether water (important for our form of life, and hopefully most life) or, say, liquid hydrogen, was the basis for mass remains to be seen, but the first hurdle has been overcome: we have a common "yard stick." And, if you think about it, we have a common clock, too.