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The mechanics of open systems and applications in effective field theory

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Recent years have seen the paradigm of effective field theory (EFT) successfully applied to an increasing number of classical systems that range from the gravitational inspiral of compact binaries to hydrodynamics. Many of these systems exhibit dissipation in one form or another, such as radiation reaction or viscous fluid flow, that naturally results from the system being open. This "openness" can manifest as energy leaving the dynamical variables of interest via radiation or heat transfer, for example. As the EFT approach typically utilizes the action, and hence Hamilton's Principle of Extremal Action, it is crucial to determine how generally and consistently to accommodate dissipative effects in a variational principle. In this talk, I discuss why Hamilton's Principle fails to incorporate dissipation. I then provide a reformulation that has been used successfully to confirm well-established results as well as to provide new predictions regarding dissipative systems. I show specific examples drawn from EFT applications. Finally, I show how this reformulation of Hamilton's Principle turns out to correspond to the classical limit of quantum theories based on the so-called "in-in" or "closed-time-path" approaches.