Page 21 - A Brief History of Time - Stephen Hawking

A Brief History of Time - Stephen Hawking... Chapter 2
of the twins went for a long trip in a spaceship at nearly the speed of light. When he returned, he would be
much younger than the one who stayed on earth. This is known as the twins paradox, but it is a paradox only if
one has the idea of absolute time at the back of one’s mind. In the theory of relativity there is no unique
absolute time, but instead each individual has his own personal measure of time that depends on where he is
and how he is moving.

Before 1915, space and time were thought of as a fixed arena in which events took place, but which was not
affected by what happened in it. This was true even of the special theory of relativity. Bodies moved, forces
attracted and repelled, but time and space simply continued, unaffected. It was natural to think that space and
time went on forever.

The situation, however, is quite different in the general theory of relativity. Space and time are now dynamic
quantities: when a body moves, or a force acts, it affects the curvature of space and time – and in turn the
structure of space-time affects the way in which bodies move and forces act. Space and time not only affect but
also are affected by everything that happens in the universe. Just as one cannot talk about events in the
universe without the notions of space and time, so in general relativity it became meaningless to talk about
space and time outside the limits of the universe.

In the following decades this new understanding of space and time was to revolutionize our view of the
universe. The old idea of an essentially unchanging universe that could have existed, and could continue to
exist, forever was replaced by the notion of a dynamic, expanding universe that seemed to have begun a finite
time ago, and that might end at a finite time in the future. That revolution forms the subject of the next chapter.
And years later, it was also to be the starting point for my work in theoretical physics. Roger Penrose and I
showed that Einstein’s general theory of relativity implied that the universe must have a beginning and,
possibly, an end.

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