We don't really know what string theory is
I learn from Not Even Wrong that Steven Weinberg has said:
The critics are right. We have no single prediction of string theory that is verified by observation. Even worse, we don’t know how to use string theory to make predictions. Even worse than that, we don’t really know what string theory is.
I have deep respect for Steven Weinberg's views on physics, not merely because he is a Nobel laureate but because he is deeply thoughtful about what he says, so his damning comment on string theory carries a lot of weight with me.
The situation with string theory always reminds me of the brilliant quotation by Richard Feynman:
Physics is to math what sex is to masturbation.
It seems that I had a narrow escape when I left physics for even sexier research back in the early 1980's, because I suspect that had I stayed I would have been enticed into working on string theory, and would have thus unwittingly tossed away my entire research career.