Q&A: Black Holes

Q:
Does the event horizon in a black hole face only 1 or all directions? In a three dimensional universe, a black hole would seem to form in a slightly round form. It seems hard to imagine a 3-d tear in space and time.

A:
We agree with you, it is hard to imagine a 3-dimensional tear in space and time. But this is different from imagining the event horizon of a black hole, because the tear in space and time occurs only at the very center of the black hole, at the "singularity" which means point of infinite density. The event horizon is easier for us to understand.

Imagine a star getting more and more massive, perhaps by a companion star dumping matter on it, until it has so much mass that it collapses down into a black hole. The star is a sphere, and you can easily imagine the surface of the star as a sphere. The event horizon is also a sphere, surrounding the former star just as its surface used to do. So you are right, it is round (or roundish).

There is a web site that explains the event horizon of a black hole:
http://archive.ncsa.uiuc.edu/Cyberia/NumRel/BlackHoleAnat.html
In particular, check out the link on that page that asks if you'd like to take a one way trip to a black hole.

We also have information in the Chandra web pages on black holes. See
http://chandra.harvard.edu/xray_sources/blackholes.html
which includes links to images of the matter surrounding black holes, questions and answers on black holes, and descriptions of different types of black holes.