One of Many Earths

This is a story of Earth recovering from nuclear war, and the technological progress toward faster-than-light travel, and the offshoot of the technology which led them to investigate alternate realities, and the cost of doing so. Young Roirdon Daley is a mathematical genius who solves equations that have been holding back science for decades and discovers the entry to alternate realities. He finds reason to doubt the wisdom of doing so when he finds the cost may be the elimination of his own reality.