hi quantum gravity has many problems, I find up to now. I think this problems are conceptual and mathematical is it true? it's conceptual problems are: 1- problem of time 2- quantization and smooth space-time 3- uncertainly in space and the concept of continuous space-time that means you can close to every point you want. 4- measurement in quantum and relativity, that means in quantum mechanics primary and recency of event is important but in GR you can't determine which event is first and which is after. is it true? is there any thing else? and about mathematical problems,I don't any information.!!! i read a paper about five problems of quantum gravity by Andrew Strominger ( 0906.1313v1 [hep-th])and actually quantum gravity in 2+1 dimensions by steven carlip, but unfortunately i couldn't understand many of these problems. can you help me?