This chapter explores Einstein’s theory of relativity as a unifying framework that reshaped our understanding of space, time, and physical forces. Beginning with the failure of the Michelson–Morley experiment and the abandonment of the luminiferous aether, it introduces the core principles of special relativity: the constancy of the speed of light and the equivalence of inertial reference frames. Through thought experiments rather than heavy mathematics, the chapter explains counterintuitive consequences such as time dilation, length contraction, and the relativity of simultaneity, illustrating how these effects are experimentally confirmed and technologically relevant. The chapter then reveals a deeper conceptual insight: Electricity and magnetism are not separate forces but different manifestations of a single electromagnetic interaction viewed from different states of motion. Finally, the discussion extends to general relativity, introducing the equivalence principle and the idea that gravity arises from the curvature of spacetime rather than a conventional force. Together, these ideas expose a hidden unity beneath seemingly disparate phenomena and exemplify how modern physics achieves simplicity and coherence by revising fundamental assumptions about reality.