Page 54 - A Brief History of Time - Stephen Hawking

A Brief History of Time - Stephen Hawking... Chapter 7
event horizon had always to be moving parallel to, or away from, each other. Another way of seeing this is that the
event horizon, the boundary of the black hole, is like the edge of a shadow – the shadow of impending doom. If you
look at the shadow cast by a source at a great distance, such as the sun, you will see that the rays of light in the edge
are not approaching each other.

If the rays of light that form the event horizon, the boundary of the black hole, can never approach each other, the area
of the event horizon might stay the same or increase with time, but it could never decrease because that would mean
that at least some of the rays of light in the boundary would have to be approaching each other. In fact, the area would
increase whenever matter or radiation fell into the black hole Figure 7:2.

Figures 7:2 & 7:3

Or if two black holes collided and merged together to form a single black hole, the area of the event horizon of the final
black hole would be greater than or equal to the sum of the areas of the event horizons of the original black holes
Figure 7:3. This nondecreasing property of the event horizon’s area placed an important restriction on the possible
behavior of black holes. I was so excited with my discovery that I did not get much sleep that night. The next day I rang
up Roger Penrose. He agreed with me. I think, in fact, that he had been aware of this property of the area. However,
he had been using a slightly different definition of a black hole. He had not realized that the boundaries of the black

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