What is a Black Hole, Really?
Slide 7 of 22
In the context of the membrane paradigm in the earlier slide, clearly the more matter
that is put in the center of the sheet, the deeper the well that is created, and
consequently the harder it is for matter to "climb out" . According to Einstein's theory,
if enough matter is packed into a small enough volume, the well will get so deep that the matter inside can never escape. A circle of no return forms. Any matter that passes the point of no return can no longer escape to the outside world. It necessarily keeps collapsing, moving towards the center. The well gets deeper and deeper until
finally a hole is literally torn in the fabric of spacetime: the density of matter at the
center becomes essentially inifinite, at least to the extent that Einstein's theory of
gravity is still valid. Thus, what I mean by " a hole in the fabric of spacetime" is:
a tiny region of space where the known laws of physics break down.
A black hole is then a region of space so tightly packed with matter, that nothing, not even light can escape. Hidden at its (crunchy?) center is a tear in the fabric of spacetime. Anything that falls into this region of space is irrevocably lost to the rest of the universe. No light can emerge or pass through this region, so it appears totally black. In some sense therefore, a black hole marks a boundary to spacetime: a horizon beyond which no one can see without travelling through it. This radius of no return is called the event horizon of the black hole.
