Why does the speed of light remain the same for all observers, no matter how fast they move? This deceptively simple question sits at the foundation of modern physics, yet its implications reach far beyond equations and experiments. In The Speed of Light , Jim Berg, MD, shows that this constancy is not merely a curious fact about light, but a structural feature of spacetime itself—one that determines how causality, time, energy, gravity, and motion are possible at all. Beginning with established results from special and general relativity, the book traces the consequences of an invariant causal speed outward with careful discipline. It explains why massive objects can never reach light speed, why light requires no rest frame, how spacetime geometry replaces force as the explanation of gravity, and why time does not flow globally even though experience unfolds locally. Along the way, Berg clarifies common confusions about time dilation, energy, simultaneity, and the nature of physical limits, showing how they arise not from technological constraints but from the deep geometry of reality. The final chapters move beyond physics into philosophy, without speculation or mysticism. They explore how finite causal speed makes agency, responsibility, memory, and meaning possible in a world governed by law. Rather than treating limits as obstacles, the book reveals them as enabling conditions for structure and understanding. Written for scientifically literate readers, philosophers of science, and anyone interested in the foundations of reality, The Speed of Light offers a clear, rigorous, and quietly profound account of why one constant matters so deeply—and why it could not be otherwise.