The Five Superstring Interactions
There are five ways open and closed strings can interact. An interaction in superstring theory is a topology changing event. Since superstring theory has to be a local theory to obey causality the topology change must only occur at a single point. If C represents a closed string and O an open string, then the five interactions are OOO, CCC, OOC, OCO and COO.
All open superstring theories also contain closed superstrings since closed superstrings can be seen from the fifth interaction, and they are unavoidable. Although all these interactions are possible, in practice the most used superstring model is the closed heterotic E8xE8 superstring which only has closed strings and so only the second interaction (CCC) is needed.
Read more about this topic: Superstring Theory
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