Quantum gravity is a theoretical framework that seeks to unify two fundamental theories in physics: quantum mechanics, which describes the behavior of matter and energy on very small scales, and general relativity, which describes the force of gravity and the structure of spacetime on large scales.
In terms of its significance, quantum gravity is certainly a significant area of research and a major goal in theoretical physics. If successful, it would provide a more complete understanding of the fundamental forces of nature and the nature of spacetime itself.
However, it's important to note that quantum gravity is not considered "half of physics" in the sense that it encompasses all of physics. Physics as a whole covers a broad range of phenomena, from the behavior of subatomic particles to the properties of macroscopic objects, the study of light and electromagnetism, thermodynamics, fluid dynamics, and much more. Quantum gravity is just one area of study within the larger field of physics, albeit an important and challenging one.