Lecture 20: Gravity in the Bath
In this lecture, we completed our study of an example of a phase transition between RT surfaces that leads to the emergence of a Page curve in holographic systems coupled to nongravitational baths.
We then discussed how the physics changes when we turn on gravity in the bath. Within the framework of braneworlds, one example is obtained by introducing two branes in a black string geometry. The only extremal surface that extends between the two branes, and is also extremal with respect to its endpoints is the horizon.
This implies that when the bath is gravitating (a) one is unable to demarcate a "radiation region"; (b) the entanglement wedge of the defect at the intersection of the two branes is always the entire exterior; (c) there is no Page curve.
We explained how these features could be understood in the context of our earlier discussions of the holography of information.
Even in gravitational systems, it may be possible to find nongravitational questions whose answer is the Page curve. We concluded with a brief discussion of possible questions that could yield a Page curve for black holes in asymptotically flat space.
