Reimagining Gravity: Generalized Symmetries, Double Copy, and Spinning Black Holes
General relativity is a century-old subject. Yet modern explorations through generalized symmetries, scattering amplitudes, and effective field theory have raised open problems, motivating a reassessment of conventional views on gravitation, spacetime, and spin. First, do generalized symmetries exist in dynamical gravity as a low-energy EFT? Second, is there a field-theoretic explanation for the tree-level double copy relationship between general relativity and Yang-Mills theory? Third, what are the exact equations of motion or Lagrangians of four-dimensional spinning black holes in external fields, in point-particle effective theory? In this dissertation, three different perspectives on gravity are developed to shed light on each of these puzzles. In Part I, we view gravity as a gauge theory of Lorentz group to establish a one-form symmetry of dynamical gravity. In Part II, we investigate how far gravity can be viewed as a gauge theory of diffeomorphisms, in connection with color-kinematics duality and double copy. In Part III, we view four-dimensional gravity as a nonlinear interaction between self-dual and anti-self-dual parts. We apply this view to the derivation of four-dimensional spinning black hole solutions as well as their dynamics in point-particle effective theory. First, we derive a new self-dual black hole metric from classical double copy and show that the Kerr metric represents a pair of self-dual and anti-self-dual Taub-NUT solutions, elevating the Newman-Janis algorithm to a rigorous derivation. Second, we propose a dynamical probe counterpart of Newman-Janis algorithm. We give a Lagrangian derivation of spinning black hole Compton amplitudes that exhibit correct factorizations and are free of spurious poles for all helicity configurations while developing a chiral formalism that manifests and maximally utilizes the simplicity of the self-dual sector.