These solutions will be based on a few assumptions. First, interplanetary trade may involve journey times in the realms of hundreds of years. Consequently, launching a cargo will necessarily be a very long-term investment project and, as Krugman points out, will hardly be conceivable lest there are very extensive futures markets. Hence, we will assume that investors are able to make accurate forecasts of prices over indefinite periods.
Second, we will assume that the trading planets in consideration lie in the same inertial reference frame and that the spacecraft travel at uniform velocity. This simplification permits us to keep our analysis within the realms of special relativity. We can ignore things like gravitational time dilation as it would be negligible. In this case, the planets Earth and Trantor — Krugman’s chosen destination — are assumed to lie in the same inertial reference frame.