Stump the scientist: Black holes

We’re sorry Jim!! Somehow Jim’s question slipped through the cracks and we had him on the edge of his seat since November waiting for an answer.   We’re sorry Jim!  We’ve got your question answered now, however.  Hope this quenches your curiosity.   As always, everyone please submit your Stump the Scientist questions in the comments section or on Twitter.

Stump the Scientist question submitted by Jim Lindelien:

From the point of view of an outside observer, it takes forever to observe something reach the event horizon of a black hole. Also, the light carrying such information red shifts and dims.

How is it possible, then, for scientists to refer to collisions of black holes as having occurred on the past (or of a hole consuming matter in some form, such as when they say a star’s mass is being consumed by the hole), and offer this as an explanation for relatively rapid and energetic phenomena? Are all these phenomena merely instabilities in the accretion disk (i.e., well outside the horizon boundary)?

In some sense is there a way to observe the merger of a hole and another object and detect its completion in finite time?

Explanation from Chief Scientist Jim Bray:

This question involves details about black holes which are not certain at present. The questioner refers to a black hole’s event horizon, which is the small sphere about its center where time “stops” on its surface (to an outside observer). At the center of this event-horizon sphere lies a naked singularity of General Relativity theory. The existence of such a singularity is questioned by some physicists because such singularities often signal the breakdown of a theory. Hence, it may be necessary to develop a true theory of quantum gravity (a theory which combines General Relativity and quantum theory), which does not now exist, before we can really understand this singularity or the event horizon. Hence, some physicists have made other proposals and theories in physics for explanation of the observations associated with black holes (see, for example, C. Barcelo et al. Scientific American, p. 38, Oct. 2009).

Therefore, within the limits of our present understanding, we should regard collisions and “merging” of black holes as collisions and interactions of their accretion disks (as the questioner notes) and their other highly energetic immediate surroundings. Explaining exactly what happens to their singularities (if any) and event horizons should probably await further successful theories in physics, such as one for quantum gravity.