The paper by Ashoke Sen discusses the construction of classical time-dependent solutions in open string theory, specifically focusing on the motion of the tachyon on unstable D-branes. Despite the infinite number of time derivatives in the string field theory action, which initially suggests a lack of well-defined initial value problems, the theory admits a family of time-dependent solutions characterized by the initial position and velocity of the tachyon field. The author constructs the world-sheet action of the boundary conformal field theories associated with these solutions and studies the corresponding boundary states. For D-branes in bosonic string theory, the energy momentum tensor of the system evolves asymptotically towards a finite limit if the tachyon is pushed towards the minimum of the potential, but hits a singularity if the tachyon is pushed towards the unbounded region of the potential. This qualitative difference is attributed to the absence of conventional bulk modes in open string field theory, leading to the energy density remaining confined to the plane of the original brane during the decay process. The paper also explores the conformal field theory description of the rolling tachyon solution and analyzes the boundary state associated with it, determining the source for closed string tachyon fields. The effective field theory analysis further supports the conclusion that the decay products reside in the plane of the original brane, rather than being carried away by radiation. The author concludes by highlighting the importance of further exploration, including detailed studies of boundary states in superstring theories and the construction of time-dependent solutions in vacuum string field theory.The paper by Ashoke Sen discusses the construction of classical time-dependent solutions in open string theory, specifically focusing on the motion of the tachyon on unstable D-branes. Despite the infinite number of time derivatives in the string field theory action, which initially suggests a lack of well-defined initial value problems, the theory admits a family of time-dependent solutions characterized by the initial position and velocity of the tachyon field. The author constructs the world-sheet action of the boundary conformal field theories associated with these solutions and studies the corresponding boundary states. For D-branes in bosonic string theory, the energy momentum tensor of the system evolves asymptotically towards a finite limit if the tachyon is pushed towards the minimum of the potential, but hits a singularity if the tachyon is pushed towards the unbounded region of the potential. This qualitative difference is attributed to the absence of conventional bulk modes in open string field theory, leading to the energy density remaining confined to the plane of the original brane during the decay process. The paper also explores the conformal field theory description of the rolling tachyon solution and analyzes the boundary state associated with it, determining the source for closed string tachyon fields. The effective field theory analysis further supports the conclusion that the decay products reside in the plane of the original brane, rather than being carried away by radiation. The author concludes by highlighting the importance of further exploration, including detailed studies of boundary states in superstring theories and the construction of time-dependent solutions in vacuum string field theory.