Correct me if I am wrong. i think string theory is searched not just because of its beauty, but also because of existing problems in general relativity. For example, the center of black hole is predicted to be `infinite` in terms of many metrics . But the center is also part of the spacetime, it should bear some measurable values. The answer might require quantum physics since it is microscopic. Therefore, the unifying of general relativity and quantum mechanics is asked for.