This book offers a fresh approach towards introducing three major topics in physics: quantum mechanics, classical mechanics and special relativity. These topics are then unified in terms of their joint mathematical framework: Lie algebras and their theory. This way, the material flows smoothly from algebraic tools only, avoiding heavy analysis. Each chapter concludes with a substantial set of engaging exercises, complete with solutions, designed to introduce new material step by step. The only prerequisites are linear algebra and calculus. The required background is covered throughout the book, and especially in the appendix, which may serve as a point of reference. This new edition features major revisions throughout the book (notably in Parts II, IV-V, VIII, and more), along with two new chapters on Feynman diagrams, the Killing form, spin, and polarization.