Here's a hypothesis, which is very likely wrong. As the material falls in towards the black hole, it begins to radiate as it loses gravitational potential energy. We see this as visible light. As it continues, it falls faster and faster, so that it radiates much more intensely. The radiation quickly goes up into the x-ray range. As the material approaches the event horizon, however, gravitational time dilation (i.e., the gravitational redshift) starts to dominate, and the x-ray radiation gets redshifted back down into the visible range.
What's wrong with this is that material doesn't fall straight in to the black hole, but whips around in a tight orbit--the accretion disk--so that the radiation from the infalling matter is successively redshifted or blueshifted by the Doppler effect, depending whether it is travelling away from you or towards you.
What might save my hypothesis is the optical distortion that exists around a black hole. If you look at the accretion disk around a black hole almost edge-on, it doesn't look like the disk of a galaxy. Light takes a curved path around the black hole, so that while the near side of the accretion disk is seen edge-on, the far side of the accretion disk is actually seen face-on! (Look at the simulated black hole image on this page, and you'll get an idea of what it would look like.)
If you look at the accretion disk face-on, of course, there is no Doppler wobble as the material whips around. Since it is possible that the light output could be dominated by the face-on part of the accretion disk, regardless of the orientation of the disk with respect to the observer--thanks to the distortion--my hypothesis might still be correct.
What's wrong with this is that material doesn't fall straight in to the black hole, but whips around in a tight orbit--the accretion disk--so that the radiation from the infalling matter is successively redshifted or blueshifted by the Doppler effect, depending whether it is travelling away from you or towards you.
pings for good FReepers!
According to Gott, you know him probably, black holes can allow time travel by warping spacetime excessively. The constraints are such that it wouldn't be any practical use, but it wouldn't be a surprise to observe some time differences.