This book offers a modern and accessible introduction to the quantum–relativistic world of fundamental interactions. It presents advanced concepts in a clear and structured way, combining detailed calculations with intuitive explanations of core ideas. Written in a concise and contemporary style, the book adopts interactive teaching approaches to support active learning. A wide range of exercises, including standard problems, open-ended questions, and discussion topics, helps readers consolidate and deepen their understanding.

The material is organized into twelve chapters divided into three conceptual parts.
The first part introduces classical field theory, covering relativistic frameworks, symmetries, spin, and the basic elements of group theory.
The second part focuses on quantum field theories and the relationship between fields and particles, including bosons and fermions, together with their relativistic quantum description. It also addresses several open challenges in modern theoretical physics, such as symmetry breaking, topological phenomena, the role of quasiparticles, and the nature of interactions, with particular attention to the properties of the S‑matrix in quantum mechanics and quantum field theory.
The final part examines particles and interactions in greater detail, presenting electromagnetism as a prototype theory and discussing its extensions and modern developments. Connections to the Standard Model and gravity are explored as frontier topics.

Taken together, the chapters provide a clear and compact overview of the fundamental interactions, bridging classical and quantum perspectives.

This book is intended for students who have completed introductory courses in classical mechanics, relativity, and quantum mechanics, typically during the first three years of a bachelor’s degree. It is well suited for readers seeking a coherent introduction to relativistic quantum theory and quantum field theory.

Orlando Luongo is a theoretical physicist and Associate Professor of Theoretical Physics at the University of Camerino, where he teaches master’s-level courses in Theoretical Physics, Quantum Field Theory, Advanced Quantum Field Theory, and Cosmology. He is the head of the Cosmology and Theoretical Physics Group (cosmology.unicam.it) and serves as Principal Investigator for several national research projects.

Previously, he was an assistant professor at the National Institute of Nuclear Physics (INFN) in Frascati and held postdoctoral positions at the University of Pisa, as well as international research appointments. He received his Ph.D. in Relativistic Astrophysics from Sapienza University of Rome in 2012. During and after his doctoral studies, he undertook multiple research visits abroad, including at UNAM University (Mexico), the University of Cape Town (South Africa), and Al‑Farabi Kazakh National University.

He serves as Chief Editor and referee for several international scientific journals. His research has been recognized with numerous national and international awards. He is the author of approximately 200 peer‑reviewed publications, which have received over 7,000 citations, and he has an h‑index of around 50.