What exactly did Einstein do that's so important in physics? We provide an introduction to his physics at a level accessible to an undergraduate student. All equations are worked out in detail from the beginning. Although the book is written with primarily a physics readership in mind (it can also function as a textbook), enough pedagogical support material is provided that anyone with a solid background in introductory physics (say, an engineer) can, with some
effort, understand a good part this presentation.We show why Einstein's papers were decisive to our understanding of matter as composed of molecules and atoms; why is he regarded as a founding
father of quantum theory; how did his relativity theory bring about the new understanding that time, just like space, is relative; and how did his general relativity extend Newton's theory to new physical realm, allowing us to study black holes and cosmology.