Strings from tachyons: The c = 1 matrix reloaded

We propose a new interpretation of the c = 1 matrix model as the world-line theory of N unstable D-particles, in which the hermitian matrix is provided by the non-abelian open string tachyon. For D-branes in 1+1-d string theory, we find a direct quantitative match between the closed string emission due to a rolling tachyon and that due to a rolling eigenvalue in the matrix model. We explain the origin of the double-scaling limit, and interpret it as an extreme representative of a large equivalence class of dual theories. Finally, we define a concrete decoupling limit of unstable D-particles in IIB string theory that reduces to the c = 1 matrix model, suggesting that 1+1-d string theory represents the near-horizon limit of an ultra-dense gas of IIB D-particles.