Please forgive a string theory novice asking a basic question.

Over at this question Luboš Motl gave an excellent answer, but he made a side comment that I've heard before and really would want to know more about:

Quantum field theory is the class of minimal theories that obey both sets of principles [Ed: *SR and QM*]; string theory is a bit more general one (and the only other known, aside from QFT, that does solve the constraints consistently).

What are the arguments that string theory is more general than QFT? I get that you can derive many different QFTs as low energy effective theories on different string backgrounds. But from my limited exposures to worldsheet perturbation theory and string field theory I can also see string theory as a very *special* kind of QFT. I also realize these approaches don't fully characterise string theory, but I don't know enough about their limitations to understand why the full definition of string theory (M theory?) surpasses QFT.

My naive guess would be that *no*, string theory can't be more general than QFT because surely there are many more QFTs which are asymptotically free/safe than could possibly come from string theory. Think ridiculously large gauge groups $SU(10)^{800}$ etc.. Since these theories don't need a UV completion into something else string theory can't be a more general framework than QFT. Logically you could also have theories which UV complete into something other than string theory (even if no such completion is presently known).

Alternately you could turn this around and say that string theory limits the kind of QFTs you can get in the low energy limit. This is saying that string theory is more predictive than QFT, i.e. *less* general. I always thought this was the goal the whole time! If it is the other way around and string theory really is *more* general than QFT, doesn't this mean that string theory is *less* predictive than, for instance, old school GUT model building?

So is the relationship between string theory and quantum field theory a strict inclusion $\mathrm{QFT} \subset \mathrm{ST}$ or more like a duality/equivalence $\mathrm{QFT} \simeq \mathrm{ST}$, or a more complicated Venn diagram?

Note that I am *not* asking about AdS/CFT as this only deals with special string backgrounds and their QFT duals. I'm asking about the general relationship between string theory and QFT.

This post imported from StackExchange Physics at 2014-04-21 15:06 (UCT), posted by SE-user Michael Brown