# Could string theory be an effective theory?

I know that many quantum field theories could be low-energy effective theories in String Theory (ST), but I've also read and heard that ST cannot itself be an effective theory.

I suppose this has something to do with the UV behaviour of scattering amplitudes in ST and its conformal properties. Is it known to be impossible, though, for ST to be a (e.g. low energy) limit of another theory? If so, why? Is there merely consensus amongst experts or is there a rigorous proof?

I can't see of any motivation for considering ST to be an effective theory, since it's supposed to explain everything without any adjustable parameters. I'm just curious about this point.

• +1 on the remark "I can't see of any motivation, I am just putting my finger here into your eyeball just for curiosity" :-D – arivero Sep 28 '15 at 23:55
• You could treat a string theory as an effective theory, an effective theory of the QCD flux tube for example. See this paper: arxiv.org/abs/1405.6197. – user2309840 Mar 8 '16 at 2:32
• what about higher spin gauge theory? – John Doe May 19 '16 at 18:23

However the full non-perturbative picture is still unclear. For instance the IIA superstring theory - and using $S$, $T$ and $U$ dualities all other superstring theories - can be seen as a $g_s\lll1$ limit ($g_s$ is the closed string coupling constant) of the eleven dimensional M-Theory.
Indeed a bunch of $n$ D0-branes in IIA can be seen as a bound state at threshold (the branes are BPS objects, so repulsion equals attraction), with mass:
$$M=\frac{n}{g_s \alpha'}$$
This is the typical mass spectrum $M=n/R$ of Kaluza-Klein with radius $R$. So $R_{11}=g_s \alpha'$ is another dimension that opens up in the $g_s\ggg1$ limit, and the D0 branes can be seen as Kaluza-Klein particles of M-theory.
• Actually the $g_s>>>1$ limit is different from the UV regime (it's more like the QCD at low energies). What I wanted to stress is that even though we claim string theory to be UV finite, there is a broader picture that we still not understand. In some sense string theory emerge as a sub-theory of M-theory and it's not excluded that we will discover more fundamental theories. Considering this "string-M-F-ecc." theory, there is no motivation to see it as an effective theory, as I've already said. – Rexcirus Sep 29 '15 at 8:21