# Four Black Hole Questions

Mileman10
I've got a few novice questions, and I'd appreciate answers which are conceptual, rather than equation, based. Thanks in advance.

Regarding black holes:

1.) For a rotating black hole, are there two event horizons, one on each 'pole' as it were? If so, is this a function of the rotation?

2.) Depending on the answer to the previous questions, would this imply that a non-rotating black hole would have infinite event horizons, i.e., an event horizon exists at any angle of approach?

3.) I often see references to a black hole's size or mass such as "a super massive black hole equivalent to one million solar masses". This must mean the amount of mass necessary for the black hole to have the specific gravitational effect it appears to have. It does mean the amount of matter the black hole has ingested, which is impossible to know, correct?

4.) How has the "discovery" of dark matter (and dark energy) affected our understanding of black holes? Has it changed anything radically?

## Answers and Replies

Science Advisor
1.) For a rotating black hole, are there two event horizons, one on each 'pole' as it were? If so, is this a function of the rotation?

Rotating black holes only have one true event horizon. However, there are two surfaces of interest, that is, the EH and the ergosphere. See: http://en.wikipedia.org/wiki/Kerr_metric#Important_surfaces

2.) Depending on the answer to the previous questions, would this imply that a non-rotating black hole would have infinite event horizons, i.e., an event horizon exists at any angle of approach?
From 1, no. Note, an event horizon is not a point, it's a surface.

3.) I often see references to a black hole's size or mass such as "a super massive black hole equivalent to one million solar masses". This must mean the amount of mass necessary for the black hole to have the specific gravitational effect it appears to have. It does mean the amount of matter the black hole has ingested, which is impossible to know, correct?

The amount of matter a black hole has "ingested", as you say, is impossible to know since one cannot know at what mass it was formed. But the mass measured will be what ever initial mass was created by stellar collapse, plus the mass of whatever matter has fallen in, plus other tiny contributions that will pale in comparison to the above two.

4.) How has the "discovery" of dark matter (and dark energy) affected our understanding of black holes? Has it changed anything radically?
The two phenomenon are basically completely unrelated.