Yuan K. Ha
A new direction to understand gravity has recently been explored by considering classical gravity to be a derived interaction from an underlying theory. This underlying theory would involve new degrees of freedom at a deeper level and it would be structurally different from classical gravitation. It may conceivably be a quantum theory or a non-quantum theory. The relation between this underlying theory and Einstein’s gravity is similar to the connection between statistical mechanics and thermodynamics. We discuss the apparent lack of evidence of any quantum nature of gravity in this context.
…A list of conceivable quantum effects of gravity as logical outcomes of quantum idea applied to gravity is the following:
1. Existence of graviton.
2. Spacetime fluctuations and spacetime foam.
3. Discreteness of spacetime.
4. Wave-particle duality for gravity.
5. Linear superposition of spacetimes.
6. Interference of spacetimes.
7. Uncertainty relations for spacetime metric.
8. Tunneling in gravity.
9. Entanglement in gravity.
So far none of these phenomena has been observed and the existence of any one of the above effects would be a strong indication of the validity of quantum gravity…