Page 53 - A Brief History of Time - Stephen Hawking

A Brief History of Time - Stephen Hawking... Chapter 7
CHAPTER 7

BLACK HOLES AIN’T SO BLACK

Before 1970, my research on general relativity had concentrated mainly on the question of whether or not there had
been a big bang singularity. However, one evening in November that year, shortly after the birth of my daughter, Lucy,
I started to think about black holes as I was getting into bed. My disability makes this rather a slow process, so I had
plenty of time. At that date there was no precise definition of which points in space-time lay inside a black hole and
which lay outside. I had already discussed with Roger Penrose the idea of defining a black hole as the set of events
from which it was not possible to escape to a large distance, which is now the generally accepted definition. It means
that the boundary of the black hole, the event horizon, is formed by the light rays that just fail to escape from the black
hole, hovering forever just on the edge Figure 7:1. It is a bit like running away from the police and just managing to
keep one step ahead but not being able to get clear away!

Figure 7:1

Suddenly I realized that the paths of these light rays could never approach one another. If they did they must
eventually run into one another. It would be like meeting someone else running away from the police in the opposite
direction – you would both be caught! (Or, in this case, fall into a black hole.) But if these light rays were swallowed up
by the black hole, then they could not have been on the boundary of the black hole. So the paths of light rays in the

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