Nuclear physics investigates the properties and behavior of the central portion, or nucleus, of the atom. This branch of physics had perhaps the biggest impact on the world in the 20th century because it evolved into the knowledge used to build the most destructive weapons people have ever known. The atomic bombs that ended World War II, and the weapons race that followed, changed the course of history. But applications of nuclear physics have also provided enormous energy for useful purposes, generating about 16 percent of the world's electricity. "Particles and the Universe" documents how modern physics formed and how it continues to evolve. Each chapter of this book delves into the observations, theories, and tests of a particular topic, including nuclear physics, quantum mechanics, particle physics, relativity, and cosmology.