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I'm trying to remember the title of a book I read years ago.
I think it was "the travelers" or "the journeyers" or something like that. It was about a group of human scientists that go on a long trip at relativistic speeds to visit a planet believed to have alien life.
There's a parallel story about the humans left behind. As they colonize space, a culture of spacefarers who run the routes between colonized planets materializes. They have their own culture separate from any of the planets they visit, because the time effects prevent them from getting to know anyone other than other spacefarers.
The ones on the trip to the inhabited planet have various relationships form and evaporate. They eventually get there, and meet the aliens, who are intelligent. A physicist on board figures out how to map their sciences to one another. The aliens use a different set of basic measurements (as opposed to time, length, and mass).
The aliens have a different method of reproduction, and ask the humans to demonstrate sexual reproduction with two genders.
Any help would be appreciated, thanks.