Alice and Bob wish to associate in some way, but they live on separate planets. The spaceship tech required to span the distance is either prohibitively expensi

Alice and Bob wish to associate in some way, but they live on separate planets. The spaceship tech required to span the distance is either prohibitively expensive or hasn't been invented yet. This is a natural barrier. Alice and Bob wish to associate in some way, but Eve forbids it and she'll beat them up if they disobey. This is an artificial barrier.