Albert Einstein said:The theorist's method involves his using as his foundation general postulates or "principles" from which he can deduce conclusions...
But as long as no principles are found on which to base the deduction, the individual experimental fact is of no use to the theorist; indeed he cannot even do anything with isolated general laws abstracted from experiments. He will remain helpless in the face of separate results of experimental research, until principles which he can make the basis of deductive reasoning have revealed themselves to him
Leonard Susskind said:It seems unlikely that the usual incremental increase of knowledge from a combination of theory and experiment will ever get us where we want to go, that is, to the Plank scale. Under this circumstance our best hope is an examination of fundamental principles, paradoxes and contradictions, and the study of thought experiments.
Edward Witten said:The inconsistency between general relativity and quantum field theory emerged clearly as the limitation of quantum field theory. This problem is a theorists' problem par excellence. Experiment provides little guide except for the bare fact that quantum field theory and general relativity both play a role in the description of natural law.
As I have indicated, experiment is not likely to provide detailed guidance about the reconciliation of general relativity with quantum field theory. One might therefore believe that the only hope is to emulate the history of general relativity, inventing by sheer thought a new mathematical framework which will generalize Riemannian geometry and will be capable of encompassing quantum field theory.
Situations which in classical and semiclassical GR can be represented only as singularities.In what kind of a physical situations do our current models (QFT,standard model, GR) fail to predict with accuracy?