A second-quantized analysis is performed to examine many-body phenomena in closed bosonic strings. The covariant nonpolynomial closed-string field theory is developed in terms of particle fields and shown to contain interactions triggering a nonperturbative condensation of the tachyon field. We study the possibility that the higher-dimensional Lorentz symmetry spontaneously breaks. We show that the theory has asymptotic freedom due to a tree-level running coupling. The spectrum of states in the nonperturbative ground state is radically changed relative to the free case; in particular, there is no massless graviton. Similar effects are anticipated in any nonperturbative vacuum.