The tachyon does matter

We review the concept of S‐branes introduced by Gutperle and Strominger [1]. Using the effective spacetime description of the rolling tachyon worldsheets discussed by Sen, we analyze the possibility that the gravitational backreaction of tachyon matter is important in the time‐dependent process. We show that this is indeed the case in the example of the S0‐brane in 4‐dimensional Einstein‐Maxwell theory. This talk is based on [21].