Quantum physicist Etienne Klein and astrophysicist Marc Lachieze-Rey present and comment on the successive unifications that have marked the fundamental advances in physics. A good deal of modern theoretical physics is lightly and gracefully explained, rooted in a philosophical examination of the motivations that drive physicists and physics. The book is a discourse on the nature of elegance and beauty in physics, seeking to explain how and why beauty is a reliable guide in the ongoing search for Truth in physics.