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Paul Krugman's 1978 Theory of Interstellar Trade
jerryasher recommends Paul Krugman's blog at the NYTimes, where he introduces a paper he wrote, The Theory of Interstellar Trade, with tongue very much in cheek. Some packrat academician was kind enough to send him a scan, because "back then academics did their work with typewriters, abacuses, and stone axes." Abstract: This paper extends interplanetary trade theory to an interstellar setting. It is chiefly concerned with the following question: how should interest rates on goods in transit be computed when the goods travel at close to the speed of light? This is a problem because the time taken in transit will appear less to an observer traveling with the goods than to a stationary observer. A solution is derived from economic theory, and two useless but true theorems are proved... This paper, then, is a serious analysis of a ridiculous subject, which is of course the opposite of what is usual in economics."
First -- if the player is traveling near the speed of light _with_ the cargo, then he won't notice that the people who are not moving have aged. If the cargo is something useful and rare (fusion fuel, perhaps) or from a far-advanced culture (fusion reactors, perhaps) then he's reasonably assured of finding a market upon arrival even if he didn't get a chance to set up a deal ahead of time.
Second -- I'm not sure that a transaction that takes longer to complete than the life of any trader is necessarily impossible. People make investments they know will outlast them, largely because they hope to sell to somebody younger when they need the money. If I set cargo in motion that will return a $100 million payment in 100 years, I know that I'm not going to live long enough to ever see the return. But, in 50 years I can probably sell the right to the value of the cargo for $5 to $20 million, depending on interest rates (see present discounted value). Some young whippersnapper (or an institution with a long horizon, or somebody else who hopes to trade again) would be happy to take the deal. It doesn't matter that I'll personally never see the dividends; if the payment down the line is certain enough then somebody will be happy to buy it from me.