Dynamics and Observer-Dependence of Holographic Screens

We study the evolution of holographic screens, both generally and in explicit examples, including cosmology and gravitational collapse. A screen $H$ consists of a one-parameter sequence of maximal surfaces called leaves. Its causal structure is nonrelativistic. Each leaf can store all of the quantum information on a corresponding null slice holographically, at no more than one bit per Planck area. Therefore, we expect the screen geometry to reflect certain coarse-grained quantities in the quantum gravity theory. In a given spacetime, there are many different screens, which are naturally associated to different observers. We find that this ambiguity corresponds precisely to the free choice of a single function on $H$. We also consider the background-free construction of $H$, where the spacetime is not given. The evolution equations then constrain aspects of the full spacetime and the screen's embedding in it.