This book offers a rigorous and systematic introduction to the principles underlying the quantum behavior of particles at relativistic speed. Designed for advanced undergraduate and graduate students of physics and mathematics, it bridges the gap between quantum mechanics and special relativity, providing the mathematical tools and physical insights needed to explore the relativistic domain. The book begins with foundational chapters on coordinate systems, tensors and covariant differentiation, laying the groundwork for understanding physics in four-dimensional spacetime. It then explores the Klein–Gordon and Dirac equations—cornerstones of relativistic quantum theory—before discussing their physical implications and mathematical structure. Topics such as bilinear covariants, bispinor transformations and the transformation between wavefunctions in different frames are treated with clarity and depth. A dedicated chapter on the Hole theory provides historical context and physical interpretation. Comprehensive and conceptually driven, the book is ideal for students and researchers seeking a deeper understanding of quantum fields and particle dynamics in relativistic settings.

Mohammad Masroor Ahmed is former Associate Professor in the Department of Physics at Mirza Ghalib College, Gaya, under Magadh University, Bodh Gaya, Bihar, India. A PhD from Magadh University and master's from Aligarh Muslim University, Uttar Pradesh, India, his research interests are in quantum mechanics, theoretical physics and classical mechanics. He delivered popular lectures on "The God's Particle", "Gravity: The Engine of the Universe" and "Time". He also attended several conferences including on "Physics and Astrophysics of Quark-Gluon Plasma (ICPAQGP-2010), in Goa, India, and "Heavy Flavour Meet 2016" at the Saha Institute of Nuclear Physics, Kolkata, India.