Page 19 - A Brief History of Time - Stephen Hawking

A Brief History of Time - Stephen Hawking... Chapter 2

Figure 2:6

We would know about it only after eight minutes, the time it takes light to reach us from the sun. Only then
would events on earth lie in the future light cone of the event at which the sun went out. Similarly, we do not
know what is happening at the moment farther away in the universe: the light that we see from distant galaxies
left them millions of years ago, and in the case of the most distant object that we have seen, the light left some
eight thousand million years ago. Thus, when we look at the universe, we are seeing it as it was in the past.

If one neglects gravitational effects, as Einstein and Poincare did in 1905, one has what is called the special
theory of relativity. For every event in space-time we may construct a light cone (the set of all possible paths of
light in space-time emitted at that event), and since the speed of light is the same at every event and in every
direction, all the light cones will be identical and will all point in the same direction. The theory also tells us that
nothing can travel faster than light. This means that the path of any object through space and time must be
represented by a line that lies within the light cone at each event on it (Fig. 2.7). The special theory of relativity
was very successful in explaining that the speed of light appears the same to all observers (as shown by the
Michelson-Morley experiment) and in describing what happens when things move at speeds close to the speed
of light. However, it was inconsistent with the Newtonian theory of gravity, which said that objects attracted
each other with a force that depended on the distance between them. This meant that if one moved one of the
objects, the force on the other one would change instantaneously. Or in other gravitational effects should travel
with infinite velocity, instead of at or below the speed of light, as the special theory of relativity required.
Einstein made a number of unsuccessful attempts between 1908 and 1914 to find a theory of gravity that was
consistent with special relativity. Finally, in 1915, he proposed what we now call the general theory of relativity.

Einstein made the revolutionary suggestion that gravity is not a force like other forces, but is a consequence of
the fact that space-time is not flat, as had been previously assumed: it is curved, or “warped,” by the distribution

file:///C|/WINDOWS/Desktop/blahh/Stephen Hawking - A brief history of time/a.html (10 of 12) [2/20/2001 3:14:16 AM]