In a new arXiv paper entitled “What are we missing in our search for quantum gravity?,” Lee Smolin of the Perimeter Institute for Theoretical Physics
summarizes various approaches to a theory of quantum gravity, lamenting that
Despite
enormous effort from thousands of dedicated researchers over a century, the
search for the quantum theory of gravity has not yet arrived at a satisfactory
conclusion.
At the end of the paper he seems to give up on
all of the current approaches.