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I am looking for a book I read in the late 80's about twin brothers, one jumped ship and remained behind on Earth, the other on a slow-ship to another star. Hyperspace travel is considered a myth, starship crew are alienated from the rest of humanity due to the relativistic effects of near light-speed travel, returning after each cargo run to an Earth that has advanced by 20-100 (?) years or so, depending on which star they traveled to.
The starship brother jumps ship in order to find his twin, effectively kidnap him in order to return him to the starship, while he remains on earth to search for clues to a mysterious eccentric who claimed to have cracked the secret of hyperspace travel, yet disappeared without a trace and is presumed dead, idolized by the brother.
Over-oxygenation features as a danger later on in the story as the one twin brother is tracking down the clues to his scientific hero.