What is the void in modern physics? Fields, Gravitation, and emergent structure

This work provides a rigorous and conceptually disciplined analysis of the notion of the vacuum in modern physics. Rather than treating the vacuum as a trivial absence or a metaphysical entity, the book examines it as a well-defined theoretical construct whose meaning depends critically on the physical framework in which it is formulated. The exposition follows the historical and conceptual evolution of the vacuum from classical physics to quantum field theory and modern cosmology, emphasizing the profound ontological shift introduced by quantum mechanics. Core phenomena such as zero-point energy, vacuum fluctuations, the Casimir effect, and the Lamb shift are analyzed with strict attention to their formal definitions and experimental status. Special emphasis is placed on the tension between quantum field theory and gravity, particularly in relation to vacuum energy and the cosmological constant problem. The book carefully distinguishes between consolidated results, open problems, and speculative or heuristic approaches, including frontier frameworks such as string theory, loop quantum gravity, semiclassical gravity, and informational perspectives like the Quantum Information Theory of the Vacuum (QITV). Throughout, the work adopts an epistemologically cautious stance, maintaining a clear separation between mathematical formalism, physical interpretation, and ontological extrapolation. The vacuum is presented not as an ultimate explanation, but as a critical diagnostic concept that exposes both the strengths and the limits of contemporary physical theories. This volume is intended for advanced students, researchers, and readers seeking a deep, critical, and non-sensationalist understanding of the vacuum in fundamental physics.