As I understand it, string theory only has one coupling constant, but there is a whole discrete landscape of false vacua, each of which gives different values to the quantities, such as coupling constants, that are taken to be fixed inputs to the standard model. Since the false vacua are metastable, presumably the theory allows transitions from one false vacuum to another, but the half-life is extremely long -- what fixes this time-scale? If such a transition occurred, would we get a bubble of spacetime with different properties that would spread outward from some point at c? Would the new vacuum's properties differ dramatically from the old one's, or would you be more likely to get a relatively small change? The motivation for this question is that if you believe (which I don't) Webb et al.'s claim that the fine structure constant varies over spacetime, then it would be interesting to understand whether string theory could accomodate such a change. Is the choice of a false vacuum in string theory essentially synonymous with the choice of a background?