In string theory, there are several types of fundamental strings that form the basis of the theory. These types of strings are distinguished by their vibrational modes and the properties they possess. The five main types of strings in string theory are:
Type I String: Type I strings are closed strings, meaning they form closed loops. They have both left-moving and right-moving vibrational modes. Type I string theory incorporates both open strings (which have two endpoints) and closed strings in a single theory.
Type IIA String: Type IIA strings are also closed strings and possess both left-moving and right-moving vibrational modes. Type IIA string theory predicts the existence of certain particles called D-branes, which are extended objects where open strings can end.
Type IIB String: Type IIB strings are closed strings with left-moving and right-moving vibrational modes. Similar to Type IIA strings, Type IIB string theory predicts the existence of D-branes.
Heterotic SO(32) String: The heterotic SO(32) string is a closed string theory that combines the features of closed strings with left-moving vibrational modes only (left-moving sector) and closed strings with right-moving vibrational modes only (right-moving sector). It is named after the specific symmetry group it possesses, which is SO(32).
Heterotic E8 × E8 String: The heterotic E8 × E8 string is another closed string theory that combines the features of the left-moving and right-moving sectors. It possesses a larger symmetry group, E8 × E8, which contributes to its mathematical properties.
These five types of strings correspond to different consistent string theories. However, it is important to note that string theory has undergone various developments and refinements over the years, leading to the understanding that these distinct types of strings may be connected through a framework called M-theory, which encompasses them all. M-theory is believed to unify the different types of strings and incorporate additional objects called membranes or branes.