In perturbative string theory, the string length scale is a unit of length which sets the scale for the extension of strings described by sigma-model worldsheet field theories. Specifically, the Nambu-Goto action for the string is nothing but the relativistic volume-functional, and the string length determines in which units the volume is measured.
The square of the string length is known as the Regge slope and traditionally denoted by :
The inverse of the string length squared/Regge slope is called the string tension, traditionally denoted
This way the Nambu-Goto action for the string with proper units attached is
where is the (induced) volume form on the worldsheet .
For discussion of relation to Planck length and string coupling constant see at non-perturbative effect the section Worldsheet and brane instantons
In the limit , of vanishing string tension, string field theory is supposed to become Vasiliev’s higher spin gauge theory. See there for more.
fundamental scales (fundamental physical units)
For instance:
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