Fewer than eight hundred Americans earn a Ph.D. in physics each year. Worldwide, the number is probably in the thousands. And yet from this small pool comes the discovery and innovation that shapes the way we live and think. From X-rays, lasers, radio waves, transistors, atomic energy—and atomic weapons—to our view of space and time, and the nature of the universe, all this has arisen from this dedicated pool of individuals. To be a physicist is to have an enormous potential to change the world.