In the Physics World article "Loop Quantum Gravity" by Carlo Rovelli, he mentioned that: "General relativity is not about physics on curved spacetimes, asymptotic space–times, or connections between theories defined over different backgrounds. It is the discovery that there is no background; no space–time.The challenge for the physicists of the 21st century is to complete the scientific revolution that was started by general relativity and quantum theory. For this we must understand quantum field theory in the absence of a background space–time. Loop quantum is the most resolute attempt to address this problem." I thought General Relativity had Background and mass/stress/energy just curve the spacetime background. But Rovelli mentioned above there was no background, no space-time. What is he talking about?