  Using a duality between the space of particles and the space of fields, we
show how one can compute form factors directly in the space of fields. This
introduces the notion of vertex operators, and form factors are vacuum
expectation values of such vertex operators in the space of fields. The vertex
operators can be constructed explicitly in radial quantization. Furthermore,
these vertex operators can be exactly bosonized in momentum space. We develop
these ideas by studying the free-fermion point of the sine-Gordon theory, and
use this scheme to compute some form-factors of some non-free fields in the
sine-Gordon theory. This work further clarifies earlier work of one of the
authors, and extends it to include the periodic sector.
