Everyone needs a little light relief sometimes, including the Nobel winning economist and writer/blogger extraordinaire Paul Krugman. A few months back he reminded the world of a short paper he'd written some years ago on the rather unexpected topic of interstellar finance. You can teleport a copy to your automated reading device by clicking here.
It's a lot of fun, with a satirical slant lurking between the lines. But it's also quite serious, because the specifics of sub-light-speed interstellar travel do pose some rather unfamiliar and intriguing conundrums: Krugman points out that transit times for goods are of course extremely large, and the passage of time itself is a function of inertial frames and the acceleration of frames. Relativity says that you can keep your roses fresh for Alpha Centauri, but your first customers may dead by the time you get there. Electromagnetic communication is naturally far faster than the transmission of material goods (although that's a situation we already encounter here on Earth), which allows for trade to be set up. And having one-way transport, rather than round trips, of cargo vessels or merchants is okay for stable economic function on an interstellar scale.
He also shows that, in terms of capital, interstellar arbitrage and competition will tend to equalize interest rates between two locations (planets, ring-worlds, Dyson spheres, whatever) as long as they're in the same inertial frame. This, as far as I can see, is astrophysically tenable. Typical relative motions of stars in our bit of the Milky Way are of the order of 10-40 kilometers a second. Although large by terrestrial standards, these velocities introduce a time dilation factor between systems of no more than 1.0000000089, or barely a millionth of a percent.
This is all lovely stuff, but it leaves me asking one simple question. What would sentient beings actually want or need to trade across interstellar space?
Despite the lore of so much science fiction, I don't think anyone (erm... anything) is going to be hauling metal ore, minerals, water, beryllium spheres, dilithium crystals, helium-3, or for that matter any kind of raw material. The fact being that almost any cargo along these lines (made of the elements produced across the universe by stellar nucleosynthesis and supernovae) is going to be a) most likely available in any system already, b) definitely available for the taking from billions of unoccupied regions of space.
The better options are 'native' products. I suspect these would fall into two broad categories. The first is biological. A few billion years of natural selection and evolution on any given planet will produce an array of wonderful and useful lifeforms, as well as biologically formed structures (I'm thinking seashells and honeycombs here), that could be unique enough to be of interest elsewhere. No matter how clever a species is at genetic manipulation or creation it will be hard to muster enough imagination to match what springs out of nature's vast playground. Bunny rabbits and butterflies could be very, very cool for young aliens from all corners of the galaxy.
The second product would, I think, clearly be a combination of technology, knowledge, and artistry. Again, native goods are what will have value. Interstellar Intel Corporation could have a glittering future ahead of it, as would Google Galactic, selling on some kind of informational essence, the intellectual scent of a species. But pure art could work too; maybe some lovely Picasso's for GL581, or perhaps a couple of Caravaggio's?
In fact here might be a hint of the biggest market, those items that can be transmitted rather than transported. Ideas, software, mathematics, scientific theories, literature (all those original Klingon versions of Shakespeare), music, and images. A vast trove from any civilization, assuming there's nothing dangerous about ideas themselves (a notion I explored way back here).
But wait! What are we doing!? We've been unwittingly giving away our most valuable assets for free during the past century! Radio shows, TV shows, all manner of communications have been spewing outwards to interstellar space - at least for a few decades, before we started adopting low-power digital transmissions and dimmed the signal.
Have we messed up our cosmic economic future? Perhaps not. We may have unwittingly invented the freemium sales model long before we used the term. There could be billions of sentient beings out there hanging on our every word, every second of Happy Days re-runs and knife-set infomercials. Now we just have to figure out how to get them to pay for upgraded service.
Luckily we already have a theory of interstellar trade to tell us what we should charge.