In "Black Holes, Information and the String Theory Revolution" by Leonard Susskind and James Lindesay, the authors give, (Ch. 15: Entropy of Strings and Black Holes, pg. 170): "The string and Planck length scales are related by g^2* (l_s)^D-2 = (l_p)^D-2 (15.0.23) " They then find the consequences of varying g while keeping string length l_s fixed, saying, "This implies that the Planck length varies." [D is dimensionality of the system, g is the dimensionless string coupling constant, and l_p is the Planck length.] Variation of g and l_s seems pretty standard in such studies, but the implication of variation of l_p, the Planck length, struck me as a surprising statement in mainstream physics, given that l_p = (hbar*G / c^3)^.5 IF the authors are actually suggesting that the Planck length might vary under certain conditions in our universe, they are also suggesting changes in one or more of the 'constants' on the right hand side of the above equation. Unfortunately, they do not further develop this statement in the book, as far as I can see. Is my interpretation correct? Have some authors posited actual variation in l_p?