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SUMMARY:How to Count States In Gravity - Tom Yildirim\, Oxford University
DTSTART:20251113T130000Z
DTEND:20251113T140000Z
UID:TALK236050@talks.cam.ac.uk
CONTACT:122734
DESCRIPTION:In this talk\, we will construct a family of complete bases fo
 r the non-perturbative Hilbert space of quantum gravity and use this to ad
 dress two puzzles regarding the Hilbert space of quantum gravity. Firstly\
 , Gibbons and Hawking proposed that the Euclidean gravity path integral wi
 th periodic boundary conditions in time computes the thermal partition sum
  of gravity. As a corollary\, the derivative of the associated free energy
  with respect to the Euclidean time period results in the celebrated black
  hole entropy formula S=A/4G. Why is this interpretation correct? That is\
 , why does this path integral compute a trace over the gravity Hilbert spa
 ce? We show that the quantity computed by the Gibbons-Hawking path integra
 l is equal to an a priori different object -- an explicit thermal trace ov
 er the Hilbert space spanned by states produced by the Euclidean gravity p
 ath integral. Secondly\, Holography suggests that the Hilbert space of qua
 ntum gravity with two asymptotic boundaries should factorise into two copi
 es of the single-boundary gravity Hilbert space.  However\, the existence 
 of spacetimes with Einstein-Rosen bridges connecting the two boundaries se
 ems to contradict this. We will show that the non-perturbative\, two-bound
 ary Hilbert space factorises nonetheless. One implication of our results i
 s that universes containing a horizon can sometimes be understood as super
 positions of horizonless geometries entangled with a closed universe. 
LOCATION:Potter room
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