String Theory, the idea that all the forces in nature can be explained in one unified theory, is something I have trouble accepting. I believe in loose ends, dead ends and split ends. But I could be very, very wrong. From a Browser Five Books Q&A with Steven Gubser on the topic, a passage in which he addresses what’s probably the main criticism of the field: 

“Question:

One problem with string theory that I’ve heard is that there is not just one string theory, there are a number that coexist, rendering the predictive power of string theory, its ability to explain physical phenomena, void. Is that a valid criticism?

Steven Gubser:

Yes and no. It’s certainly oft-repeated. One quick comeback would be to say quantum field theory is like that too, but nobody complains about it. This is the theory that Richard Feynman won his Nobel Prize for, where you are describing the quantum mechanics of relativistic particles. And if you just start with that as your goal you get a wonderfully broad and inclusive structure, which can deal with all sorts of things – it can deal with electrons, protons, neutrons and so on and so forth. But by itself, it only has so much information and you have to supplement quantum field theory with a lot of specific knowledge of physics before you’re going to get anything out of it. The quick comeback would be to say, it’s always like that – whenever you have a theoretical framework it has always been the case that you have to include facts about the world. It’s true that historically, in the 1980s, people did suggest the idea that string theory might be different. That maybe in string theory, you wouldn’t have to add in facts about the world before you could get something out of the theory; you could just sit down and calculate everything. I never said that. I wasn’t working in string theory at the time. I wouldn’t have expected it, and it didn’t happen, but what else is new? It’s true of all theories that we know – so string theory is no better and no worse in that regard.

Where I really do worry is the extent to which string theory can be connected to modern experiment. It’s one thing to say that you have to put in facts about the world before you can get anything out, but a far greater worry is, once you put in facts about the world, what do you get? So what I’m working on right now is that very question. What can you get out about modern physics, once you are willing to use string theory as a calculational tool rather than saying it’s going to be just a theory which predicts everything from scratch? Instead you say, I’m going to use this set of ideas to understand experiments. In fact there have been a number of calculations in the past five to seven years, where some strikingly successful numerical predictions have come out of string theory.”