The 6 lectures in the present book have been carefully selected to be slightly more advanced than the 6 that constituted Feynman's previous lecture book titled "Six Easy Pieces." Furthermore, they interconnect harmoniously to form a unique and unparalleled introduction to one of the most significant general areas of modern theoretical physics. I am referring to the field of relativity, the awareness of which humanity gained in the early 20th century. The name of Einstein is prominently featured in the public's perception of this field.

Indeed, Albert Einstein was the first to articulate, in 1905, the key principles that form the basis of this new field of research in the physical sciences. In all his lectures, Feynman made a particular effort to be precise in his descriptions and almost always provided the necessary clarifications when there was a risk that his simplifications or analogies might mislead or lead to incorrect conclusions. I believe, however, that the simplified way in which he presented needed some further explanations, which he did not provide.

For in Einstein's theory, the "active" mass that is the source of the gravitational field is not simply equivalent to energy (according to Einstein's equation E=mc²); the source of gravitational attraction is the energy density plus the sum of the material pressures. With this explanation, Feynman's description becomes unique and serves as an excellent introduction to the most beautiful and self-sufficient of all physical theories.

Although Feynman's lectures are primarily aimed at those aspiring to become physicists—either professionally or as amateurs—they are undoubtedly accessible to those without such ambitions. Feynman deeply believed (and shared his conviction) in the great importance of conveying an understanding of our universe to a much broader audience than that which can be covered by teaching in mathematical physics circles.

[Excerpt from the text of the preface]