We discuss an aspect of string theory which has been tackled from many
different perspectives, but incompletely: the role of nonlocality in the theory
and its relation to the geometric shape of the string. In particular, we will
describe in quantitative terms how one can zoom out from an extended object
such as the string in such a way that, at sufficiently large scales, it appears
structureless. Since there are no free parameters in free-string theory, the
notion of large scales will be unambiguously determined. In other words, we
will be able to answer the question: How and at which scale can the string be
seen as a particle? In doing so, we will employ the concept of spectral
dimension in a new way with respect to its usual applications in quantum
gravity. The operational notions of worldsheet and target spacetime dimension
in string theory are also clarified and found to be in mutual agreement.Comment: 13 pages, 2 figures. v2: derivation of the nonlocal model
considerably expanded and clarified, derivation of the spacetime spectral
dimension expanded in full with one figure added, typos corrected, main
results and conclusions unchange
