Time Machines and the Breakdown of Unitarity

We present a generic way of thinking about time machines from the view of a far away observer. In this model the universe consists of three (or more) regions: One containing the entrance of the time machine, another the exit and the remaining one(s) the rest of the universe. In the latter we know ordinary quantum mechanics to be valid and thus are able to write down a Hamiltonian describing this generic time machine. We prove the time-evolution operator to be non-symmetric. Various interpretations of this irreversibility are given.