Gravity: WEP, Gauge Theory, Quantization, Unification

A gauge theory of gravity with an internal symmetry U(1), denoted as Gravito-dynamics, is established, which is dual to the Electrodynamics and complies with Special Relativity. The Gravito-dynamics is quantized and renormalized, denoted as QGD. The Gravito-dynamics is unified with Electrodynamics at classical level, and QGD is unified with QED at quantum level, denoted as Electro-gravity interaction. Following the line of generalizing the U(1) Electrodynamics to Yang-Mills theory, we generalize the U(1) gravity to SU(2) gravity that indicates short-range gravity. Two thought-experiments are proposed to test the underlying physics of the U(1) gravity and to detect the particle nature of gravitational wave that leads to wave-particle duality of gravitational radiation.