A Robert Heinlein book about Pat and Tom, twins who turn out to be telepathic, to their great surprise. Tom is sent on a ship travelling near the speed of light, while Patrick remains on earth - along with other telepathic twin pairs, they are to serve as instantaneous communication while the ship is out of radio range and experiencing the relative time distortions of near-c travel. A neat concept, a neat book. The characters aren't lovable, but they do seem genuine.