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It is part of every introductory course on string theory (e.g. Tong) that the open bosonic string in $D = 26$ dimensions with Neumann-Neumann boundary conditions for directions $a \in \{0,\dots,p\}$ and Dirichlet boundary conditions in directions $I \in \{p+1,\dots,D-1\}$ implies the existence of a non-perturbative object, the $D_p$ brane, a $p$-dimensional submanifold of the total space. The idea is of course that the $D_p$ brane is the subspace on which the open string with Neumann-Neumann boundary conditions ends. It is also generally known, even though non-trivial, that branes are as important as strings and that therefore general string theory is not only a theory of strings but also of branes.

My question is now, if there is a similar indication of strings or higher-dimensional objects that are as important as particles in QFT? It comes of course to mind that string theory was at first introduced as a candidate for the strong force to model flux tubes, later replaced by QCD. Furthermore, I know that there one can model flux tubes in superconductors as strings. So I guess my question is about further emergent or fundamental "stringy" or "brane-y" systems that cannot be modeled by theories that only work with "points". I find the idea that fundamental physics needs objects of different dimensions to work interesting, but am not sure if this is only something implied by string theory.

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When we take a good hard look at QFT, we don't see any particles (dimensionless points traveling on world lines). All we see are localized interactions. When we do our experiments to confirm these theories we also see localized interactions. So, the concept of a particle is a bit philosophical. It is not a requirement in the successful theoretical formulation of the standard model.

So when string theory assumes the existence of strings, it is not so much to replace particles with strings. Instead, the reason is rather to introduce a cut-off scale given by the hypothetical length of these strings, which is assumed to be the Planck scale. Thus it avoids the infinities.

It is therefore unlikely that particles will give rise to the strings on which string theory is based. There are however topological defects that can appear QFT. They includes vortices, monopoles and instantons, each having their own number of dimensions associated with them.

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  • $\begingroup$ N=4 super Yang Mills "implies" strings by being holographically dual to them... $\endgroup$ Aug 5, 2022 at 2:45
  • $\begingroup$ @MitchellPorter: Does it have anything to do with the physical universe or is it just an interesting mathematical toy model? $\endgroup$ Aug 5, 2022 at 2:52
  • $\begingroup$ It has some features in common with QCD... But it is unlike reality in being a conformal field theory. It wouldn't surprise me if CFTs are the only QFTs literally equivalent to string theory (via AdS/CFT). But for now that's a guess. $\endgroup$ Aug 9, 2022 at 2:59

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