Forever Free by Joe Haldeman
William Mandela is a genetic throwback, one of the small group of humans who fought and survived the Forever War. They returned to find humanity has evolved into a group mind called Man. Surrounded by a society that is too autocratic and intrusive, living a dull existence which cannot compare to the certainties of combat and feeling increasingly alienated, the veterans plan an escape to the future by means of space travel and relativity. But when their ship starts to fail, their journey becomes a search for the Unknown, the elusive entity responsible.