I want to gather some links about this. If you know of some good articles and can add some links to the stash, please do. The gravitational field is the geometry of spacetime and can be roughly equated with spacetime itself, or a general idea of the vacuum----the ideas tend to overlap because in some sense spacetime is nothing besides its quantum geometry and because the other fields are defined on spacetime. I became aware from a Turbo thread that in 1920 Einstein was recklessly speaking of the gravitational field as a sort of "Machian aether" which he defined different from the old aether in that it agreed with relativity and had no preferred rest-frame. But even if it is given a special definition and used in a logically OK manner, merely using this word runs a risk of precipitating much confusion and argument. So probably we should just say (quantum) "gravitational field" among ourselves and not use the AE word. R. Loll and B. Dittrich recently posted an article that is very much about the paths of light through the CDT spacetime "counting a black hole"--- this paper is looking at the optical behavior of different simplex-assemblages, how beams are spread and concentrated Also Loll and Westra were using optical criteria to determine how much and what kind of topology change they could allow. http://arxiv.org/abs/hep-th/0507012 "Taming the cosmological constant in 2D causal quantum gravity with topology change" http://arxiv.org/abs/gr-qc/0506035 "Counting a black hole in Lorentzian product triangulations" So in these papers which are the most recent, save one, from Loll, light is playing an important and explicit role in quantum gravity. Maybe you think this is naive but I take it as a possible hint that I should learn about different interactions of light with the gravitational field including the flat-case vacuum so I am prepared in case some of these things come up in the CDT quantum spacetime. So I want some links. If you have some good ones please volunteer them. I will mine Turbo's references first.