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In a podcast with Cumrun Vafa (https://www.youtube.com/watch?v=yppqz12ngbM&t=654s with the relevant part from 35:30 to 35:50), Vafa mentioned that there exists string theories that can describe the real world without having supersymmetry. My questions are:

  • What are examples of such string theories?
  • Given that almost all string theorists who want to describe the world use superstring theories, why don't physicists pay more attention to the string theories without supersymmetry?
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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – Chris
    Aug 2 at 20:29
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There is a non-supersymmetric heterotic string theory with O(16) x O(16) gauge group. This theory has been known since the eighties - read "An O(16) x O(16) Heterotic String" by Alvarez-Gaume, Ginsparg, Moore and Vafa (https://inspirehep.net/literature/22725) and "String Theories in Ten-Dimensions Without Space-Time Supersymmetry" by Dixon and Harvey (https://inspirehep.net/literature/227374).

I think one reason this theory is not too well-known is just that (compared to superstring theories) it is harder to compute anything. I myself don't know much about this theory, so I don't know for sure, but I don't know of any simple way in which one can build semirealistic models from the O(16) x O(16) theory as a hobby. Furthermore, supersymmetry, while not yet directly observed in nature, does have some phenomenologically nice properties, so there are good reasons not to throw it away.

However, Vafa seems to have done some more work on this theory, and there seem to be other people working on it too.

There is also type 0 string theory, which however doesn't contain fermions and doesn't really count as a realistic theory.

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  • $\begingroup$ Is type 0 string theory the bosonic string in 26 dimensions? $\endgroup$
    – Andrew
    Aug 3 at 15:57
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    $\begingroup$ No, it's a funny type of superstring theory, which does have worldsheet supersymmetry, but no spacetime fermions. $\endgroup$
    – Stijn B.
    Aug 3 at 17:34
  • $\begingroup$ Very interesting, thank you. $\endgroup$
    – Andrew
    Aug 3 at 17:55

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