Famous mathematical physicist Freeman Dyson writes:
A simple calculation, based on the known laws of gravitation and quantum mechanics, leads to a striking result. To detect a single graviton with a LIGO apparatus, the mirrors must be exactly so heavy that they will attract each other with irresistable force and collapse into a black hole. In other words, nature herself forbids us to observe a single graviton with this kind of apparatus.There is no problem quantizing weak gravitational fields, or linear approximations. That is good enough for all conceivable observations. But the theoretical research puzzles have to do with gravitons and absurd black hole scenarios that will never be observed.
I propose as a hypothesis, based on this single thought-experiment, that single gravitons may be unobservable by any conceivable apparatus.
If this hypothesis were true, it would imply that theories of quantum gravity are untestable and scientifically meaningless. The classical universe and the quantum universe could then live together in peaceful coexistence. No incompatibility between the two pictures could ever be demonstrated. Both pictures of the universe could be true, and the search for a unified theory could turn out to be an illusion.
Raphael Bousso argues in his explanation:
As Quantum Mechanics surely spells trouble for General Relativity, the existence of singularities suggests that General Relativity may also spell trouble for Quantum Mechanics. It will be fascinating to watch this battle play out.It has played out. There is no trouble.
As I explain in my book, How Einstein Ruined Physics, theoretical physics has gone down a bad path of arguing about things that can never be observed. Most physicists will say that quantum gravity is one of the biggest problems in physics, but it is not a scientific problem at all.