Level Truncation and Rolling the Tachyon in the Lightcone Basis for Open   String Field Theory
  A recent paper by Gross and Erler (hep-th/0406199) showed that by making a certain well-defined, unitary transformation on the mode basis for the open bosonic string--one that identifies the lightcone component of position with the string midpoint--it is possible to render the action for cubic string field theory local in lightcone time. In this basis, then, cubic string field theory possesses a well-defined initial value formulation and a conserved Hamiltonian. With this new understanding it seems natural to study time dependent solutions representing the the decay of an unstable D-branes. In this paper we study such solutions using level truncation of mode oscillators in the lightcone basis, finding both homogenous solutions by perturbatively expanding the string field in modes $e^{nt}$, and inhomogenous solutions by integrating the equations of motion on a lattice. Truncating the theory to level $(\tilde{2},\tilde{4})$ in $\alpha^+$ oscillators, we find time dependent solutions whose behavior seems to converge to that of earlier solutions constructed in the center of mass basis, where the cubic action contains an infinite number of time derivatives. We further construct time-dependent inhomogeneous solutions including all fields up to level $(\tilde{2},\tilde{4})$. These solutions at the outset display rather erratic behavior due to an unphysical instability introduced by truncating the theory at the linear level. However upon truncating away the field responsible for the instability, we find more reasonable solutions which may possibly represent an approximation to tachyon matter. We conclude with some discussion of future directions. 
Theodore G. Erler