# SUSY's Critical role in String Theory

In many popsci articles it is claimed that String Theory (ST) birthed SUSY. Yet ST was originally invented as a bosons-only theory, that later on brought fermions into the fold. This was only possible After SUSY was incorporated. Hence the old moniker, Superstrings. My understanding is that string theory (ST) is a house of cards' i.e., collapses & dies if SUSY is falsified. Is this true?

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Essentially a duplicate of physics.stackexchange.com/q/6438 . Related: physics.stackexchange.com/q/9337 – Qmechanic Feb 14 '12 at 15:26
You could have a look at this summer student lecture: indico.cern.ch/getFile.py/… . I do not think that ST was a bosons only theory. – anna v Feb 14 '12 at 15:33
Anna thank U for that beautiful PDF ! In 1971, Ramond, Neveu, & Schwarz were the first to incorporate fermions into purely bosonic string theory via SUSY, rendering it Superstring theory'. SUSY had been discovered by Gelfand & Likhtman the same yr, but not applied to bosonic string theory. – Jimbo Feb 17 '12 at 18:50
Hi Jimbo, be careful not to confuse "low-scale" SUSY (superpartners at the TeV scale that we might see at particle colliders) with the role of SUSY in the different string theories. You cannot exclude the possibility of string theory at the Planck scale if we just have the Standard Model at the LHC - the latter is consistent with the former. – Vibert May 12 at 14:48

"Falsifying supersymmetry" is a phrase that has to be properly qualified. Our ability to falsify with experiment is limited. We can rule out the existence of supersymmetry only at accessible energy/distance/density scales. LHC, for example, is not able to resolve physics at distance scales much smaller than $\frac{\hbar c}{7\mbox{ TeV}} \simeq 300000000000000000 \mbox{ Planck lengths}$. It isn't seeing any supersymmetry, but that doesn't prove that physics isn't supersymmetric at about $1$ Planck length. (It does prove that some theoretical physicists made wrong predictions. Too bad for them. But that's actually what theorists normally do. You're doing quite well if you're right once.)

The existing versions of string theory require supersymmetry at roughly 1 Planck Length. We are not able to do the kind of experiment that rules out supersymmetry at this scale. (I personally find somewhat unconvincing the arguments that the string theories we know of are the only possible kind.)

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Actually, no.

The supersymmetric transformations are elegant and simple ways of extending the Bosonic String theory to fermions, but if supersymmetry is falsified somehow, then maybe all of the discovered superstring theory would have to be discarded, but a new one may emerge... It would just use different supersymmetric transformations with different results.

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This seems like sheer speculation. What "different supersymmetric transformations" would these be? – Ben Crowell May 12 at 19:17
Superstrings aren't just a possible extension of the bosonic strings, but they exhaust the list of consistent string theories. So unless we're missing some deep ingredient and the classification is wrong (unlikely), no "new type of SUSY" can exist. – Vibert May 12 at 21:37
It comes from restrictions you get upon quantification of strings, but it's a very long story. To see the full story you'd need to read Ch. 2-6 in Green-Schwarz-Witten. In the Polchinski textbooks I think you'd need to read Ch. 1-11. – Vibert May 12 at 22:34
@Vibert We could easily be missing some non-perturbative part of the picture. I wouldn't be completely surprised if there were other classes of higher spin gauge theories or matrix models out there that generalized string theory, but didn't require supersymmetry in the same way. – user1504 May 12 at 22:50
That's a more difficult problem, but again it seems that M-theory should know about this (through this web of dualities that link the different theories). – Vibert May 12 at 23:06