String Theory Might Merge with Loop Quantum Gravity

The standard model isn't the same thing as quantum theory. It's possible to make a quantum theory of gravity that for one reason or another isn't compatible with the standard model.

More specifically, the SM is a theory of a bunch of fields plus a bunch of parameters describing how they interact with each other. Quantum mechanics is much more basic. There is an analogy (from Scott Aaronson I think) that quantum mechanics and classical mechanics are like two different operating systems, and the dynamical theories of physics (e.g. field theories) are programs that run on those operating systems. There are rules of thumb for porting programs from one OS to the other, but they aren't guaranteed to work.

>Other than unifying the standard model with general relativity, what else is left?

i wonder where the dark matter&energy fits here :)

Quantum field theory is a framework for building models. You can choose a largely arbitrary symmetry group structure and a largely arbitrary collection of matter fields in representations of that group (subject to a handful of mutual-consistency conditions), and wham: a possible universe. The Standard Model in particular is based on a not-very-elegant collection of symmetries and particles: SU(3)xSU(2)xU(1), with a weird (but miraculously self-consistent) grab bag of matter particles that inexplicably shows up copied three times.

Extending the underlying quantum mechanical formalism of QFT to be consistent with general relativity would be fantastic, no matter how it's done. But it's entirely possible that you'd still be left with an equally broad framework, and that the Standard Model would look just as weird and arbitrary as it does today. My impression is that something like this is the goal of most loop quantum gravity researchers, and proudly so: they're trying to solve one incredibly hard and incredibly important problem, so why borrow trouble by muddying the waters with more complications?

The ideal "theory of everything", if such a thing turns out to exist for our universe, would not just be a framework, but would also tell you the unique set of specific details that describes our universe: something that looks like our familiar Standard Model under ordinary conditions. As I said, I don't have the impression that most LQG researchers have ever considered that to be a likely outcome of their work: it's just not the business that they're in (perhaps by deliberate choice).

String theory has at various times in its history aspired to be precisely such a "theory of everything." (I'm not sure what fraction of folks in the field today would still claim that as a realistic expectation.) Because so many details of string theory are dynamical variables rather than God-given constants of nature, there's at least the possibility that there is a unique "lowest energy state" of all those variables that might turn out to be equivalent to the Standard Model. (Even almost philosophical questions like "How many dimensions of space do we perceive?" could in principle have a unique stable answer within string theory.)

Now, in practice, the way we generally analyze string theory today does not point us toward a unique answer, but rather something like 10^500 equally valid answers. Maybe there's some deeper principle that will select a unique one of those as "truth" (which we'd then eagerly compare against the Standard Model as a test of the theory). I've done a bit of work looking at such possibilities, and mostly my conclusion is "this is ridiculously complicated". Or maybe string theory is also merely a "framework" in the same sense that quantum field theory is. (But a very nice framework: one that does unify quantum mechanics and gravity in an elegant way, along with whatever approved collection of symmetries and matter you happen to stick in as its background.)

Thanks for the clarification. I didn't mean to imply that LQG researchers had no interest in proving their framework to be compatible with Yang-Mills theory: that's obviously an essential test of any realistic theory of gravity! My intent was to share my impression that they generally weren't expecting Yang-Mills to appear "automatically" as a necessary component of the LQG formalism (much less for a specific group structure and matter content to be required by it). That's in contrast to string theory, which does seem to fairly inevitably generate not just quantum gravity but also gauge theory by one mechanism or another. (And for a while, many or most string theorists were optimistic that a unique preferred background would turn up within a few years, too.)

I guess I'd say that string theorists I've known just don't think about LQG folks much. I've seen skepticism and some level of hostility, too, when there was an LQG talk in the department. But even then it usually came with a baseline level of professional respect. (Maybe I wouldn't say that so readily if I'd ever met Lubos Motl. There are always a few trolls, right?)

As you point out, quite a few LQG proponents talk about string theory rather a lot, and some even publish articles in which LQG proponent characters argue circles around straw man string theorist caricatures. (I read this one when it came out, as I recall. It made some good points, but it didn't go out of its way to portray the string theorist character as a decent human being, and the tone overall was just so damned smug.)