BEGIN:VCALENDAR VERSION:2.0 PRODID:Data::ICal 0.22 BEGIN:VEVENT DESCRIPTION:Gautam Satishchandran\, Princeton University\n\n
A long-standing proble m in QFT and quantum gravity is the construction of an "\;IR-finite&qu ot\; S-matrix. In the gravitational case\, the existence of these "\;i nfrared divergences"\; is intimately tied to the "\;memory effect& quot\; (i.e. the permanent displacement of test masses due to the passage of a gravitational wave) and the existence of an infinite number of conser ved charges at spatial infinity. In this talk\, I shall explain the origin of these connections and illustrate that the construction of an IR-finite S-matrix requires the inclusion of states with memory (which do not lie i n the standard Fock space).  \;In massive QED an elegant solution to t his problem was provided by Faddeev and Kulish who constructed an incoming /outgoing Hilbert space of charged particles "\;dressed"\; with me mory. However\, we show that this construction fails in the case of massle ss QED\, Yang-Mills theories\, linearized quantum gravity with massless/ma ssive sources\, and in full quantum gravity. In the case of quantum gravit y\, we prove that the only "\;Faddeev-Kulish"\; state is the vacuu m state. We also show that "\;non-Faddeev-Kulish"\; representation s are also unsatisfactory. Therefore\, in full quantum gravity\, it seems that there does not appear to be any (separable) Hilbert space of incoming /outgoing states that can accommodate all scattering states. Therefore we argue that\, if one wants to treat scattering theory at a fundamental leve l\, one must take an "\;algebraic approach"\; which does not requi re an a priori choice of Hilbert space. We outline the framework of such a manifestly IR-finite scattering theory.
\n DTSTART:20221024T173000Z LOCATION:Physics\, Room 313 SUMMARY:Infrared Finite Scattering Theory in QFT and Quantum Gravity END:VEVENT END:VCALENDAR