The fundamental postulate of string theory is that matter is composed of tiny vibrating loops of string, and each vibrational mode of the string corresponds to a different fundamental particle. Now, since there exists infinitely many possible vibrational modes of the string, it follows that there exists infinitely many matter particles (fermions). Since string theory requires supersymmetry to be consistent, it follows that there would exist infinitely many bosons, hence infinitely many forces. Now, would a phenominologically consistent theory of ''everything'' be possible if there exists infinitely many forces ?
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1$\begingroup$ Well, with a finite energy at hand, we can only access/generate a finite number (out of the infinite tower) of massive string modes. $\endgroup$– Qmechanic ♦Commented Aug 26, 2018 at 14:49
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$\begingroup$ related: Does String Theory predict a particle with twice the mass of the electron?. $\endgroup$– AccidentalFourierTransformCommented Aug 26, 2018 at 15:49
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1$\begingroup$ This question seems to be acting from wrong/ill-defined premises: 1. There is no equivalency between bosons and forces. What is true is that massless, spin-1 bosons are always gauge bosons and hence associated with a force, but not every boson needs to be associated with a force. 2. It is not clear what you mean by a "phenomenologically consistent theory" for infinitely many forces. "Force" means something very different e.g. in classical and quantum mechanics, a "QFT with N forces" is certainly something very different than a classical theory with N forces. $\endgroup$– ACuriousMind ♦Commented Aug 26, 2018 at 16:25
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QCD is already a little like this. Mesons and baryons form families of increasing spin and mass ("Regge trajectories "). Nucleons are bound together by exchange of spin-0 pions, but heavier spin-1 vector mesons also play a role, and even tensor mesons of spin-2 and higher. The attempt to develop a theoretical framework which could describe the interactions of such families of particles is exactly how string theory was discovered. In fact, the mathematical structure of interaction (Veneziano amplitude) was found first, then people realized that this is how the excitations of a relativistic quantum string interact.
Also: the higher modes are increasingly heavy (because of the energy they contain), and exchange of superheavy strings corresponds to "forces" felt only over ultrashort distances (before they decay to lighter states). Nonetheless, they are part of what makes string theory different from other forms of quantum gravity at short distances, an aspect of what makes it work. So they're not a bug, they're a feature.
answered Aug 26, 2018 at 17:18