Page 46 - A Brief History of Time - Stephen Hawking

A Brief History of Time - Stephen Hawking... Chapter 6

Figure 6:1

According to the theory of relativity, nothing can travel faster than light. Thus if light cannot escape, neither can anything
else; everything is dragged back by the gravitational field. So one has a set of events, a region of space-time, from
which it is not possible to escape to reach a distant observer. This region is what we now call a black hole. Its boundary
is called the event horizon and it coincides with the paths of light rays that just fail to escape from the black hole.

In order to understand what you would see if you were watching a star collapse to form a black hole, one has to
remember that in the theory of relativity there is no absolute time. Each observer has his own measure of time. The time
for someone on a star will be different from that for someone at a distance, because of the gravitational field of the star.

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