[zachf]

Well my main point is that AdS/CFT duality is asserting an exact equivalence between quantum field theories and string theories. So for example N=4 SYM, a quantum field theory viewed from one perspective, is exactly equal to a string theory from another perspective. That’s why I say it would be a spectacularly weird thing if string theory turned out to be unphysical but SYM was, because it would turn out then that they weren’t equal, and we really strongly believe they are.

In other words I’m not saying that string theory is (only) a generalization of known correct physics, but that many string theories are equal to (and therefore by definition as physically valid as) physically important quantum field theories. Then there are other string theories that are truly interesting and novel, too.

[tsimionescu]

Looking into this a little, AdS/CFT correspondence is only a conjecture, not a proven fact. More importantly, there are no AdS/CFT solutions that actually work with 4-dimensional curved space time, the working models are all in known non-physical geometries.

[zachf]

It’s a very strongly supported conjecture, but it has not been proven rigorously. As I said it would be extremely surprising if It were not true because of a frankly absurd amount of evidence. The second claim you make is false though— 3D holographic CFTs would be dual to a 4D quantum gravity theory.

Additionally, this conjecture is actually experimentally verifiable—you can set up strongly coupled systems in the lab and study the string theory description, and compare the predictions.