Why is Supersymmetry Necessary in String Theory?

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SUMMARY

Supersymmetry is essential in string theory as it resolves the instability issues present in bosonic string theory, which features an unstable vacuum due to the existence of tachyons. In contrast, superstring theory, which incorporates supersymmetry, eliminates these negative mass squared states, resulting in a stable vacuum. The GSO projection, introduced by Gliozzi, Scherk, and Olive in the mid-seventies, plays a crucial role in achieving this stability by truncating the string spectrum in ten spacetime dimensions, leading to a massless ground state.

PREREQUISITES
  • Basic understanding of string theory concepts
  • Familiarity with quantum field theory (QFT)
  • Knowledge of tachyons and their implications in theoretical physics
  • Understanding of the GSO projection in string theory
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  • Research the implications of supersymmetry in superstring theory
  • Study the GSO projection and its impact on string theory stability
  • Explore the differences between bosonic string theory and superstring theory
  • Investigate the role of tachyons in theoretical physics and their significance
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The discussion is beneficial for theoretical physicists, string theorists, and students seeking to deepen their understanding of the foundational principles of string theory and the necessity of supersymmetry.

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I was recently watching a talk by Witten and he mentioned that one of the magical things about string theory is that it forces us to accept certain symmetries of nature, as opposed to choosing them as we do in QFT. Can anyone give an enlightening explanation of this? I do have very basic knowledge of string theory but not enough over-arching knowledge to appreciate that claim.
 
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Perhaps he is referring to supersymmetry. I don't know if there is a proof that it is absolutely necessary, but bosonic string theory has an unstable vacuum, whereas superstring theory does not.

http://www.superstringtheory.com/experm/exper4a1.html
"The biggest problem with bosonic string theory (aside from the lack of fermions) is that the lowest energy state was a tachyon, or a particle mode with negative mass squared. This means the vacuum state of the theory is unstable.

In the mid-seventies Gliozzi, Scherk and Olive realized that they could implement a rule to consistently discard certain states from the RNS model, and after this truncation, known as the GSO projection, was made on the string spectrum in ten spacetime dimensions, the ground state was massless, and the theory was tachyon free."
 

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