why would bob see anything? I understood that the event horizon is a threshold, not a shell that you cross and suddenly can see inside.
to see something photons have to bounce on something and reach our eyes, we stop seeing stuff inside the horizon because those photons don't bounce back and they are pulled into the singularity.
my logic said that if light can't escape the horizon, then it can't escape alice to reach bob, even if he's inside the horizon, photons can't suddenly go backwards from alice until bob, they are being pulled further inside.
Let's keep it fully classical and drop "photon": we're interested in gravitational effects rather than quantum ones (uncertainty, fluctuations, tunnelling, details about scattering and more). Really what we want is something to illuminate (pardon the pun) interesting null geodesics, so a thin collimated beam -- a pencil of light -- will do.
Note the region inside the shell and the downward-pointing wedge in Fig 1. of Visser 2014 is flat Minkowski spacetime. Everything in that region will work like Special Relativity, as one would expect from the shell theorem.
In particular, a pencil of light directed outwards through the apparent horizon in Fig. 1. will ride the AH down to the singularity, but a receiver intercepting the pencil of light just inside the shell would notice nothing unusual: spacetime is flat there.
Eventually the collapsing shell collides with observers floating weightlessly inside it, and they have a bad time. But they can direct a pencil of light inwards just before the shell hits them.
I mean, we can say "electromagnetic wave" instead of a "photon". It doesn't really change much in this case.
I don't quite understand their diagram and their point. Are they looking at a region inside the collapsing shell of material? Then there's no contradiction here, the observer won't see anything until the reach the singularity (where the space-time stops being locally flat).
Even if we consider an observer in the center of the shell, they'll only encounter the singularity after they get hit by the infalling matter.
why would bob see anything? I understood that the event horizon is a threshold, not a shell that you cross and suddenly can see inside.
to see something photons have to bounce on something and reach our eyes, we stop seeing stuff inside the horizon because those photons don't bounce back and they are pulled into the singularity.
my logic said that if light can't escape the horizon, then it can't escape alice to reach bob, even if he's inside the horizon, photons can't suddenly go backwards from alice until bob, they are being pulled further inside.