Time For The Stars by Robert A. Heinlein

In this science fiction novel, a pair of identical twins, Tom and Pat, possess a unique telepathic connection that remains instantaneous regardless of distance. This ability makes them ideal candidates for a space exploration mission aimed at discovering habitable planets for an overpopulated Earth. As Tom embarks on a journey aboard a spaceship traveling at relativistic speeds, he communicates with Pat, who remains on Earth. The story explores the effects of time dilation, as Tom ages slowly compared to his twin, and delves into themes of family, sacrifice, and the relentless march of time as the twins navigate their changing relationship across the vastness of space.