Page 13 - A Brief History of Time - Stephen Hawking

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

Figure 2:1

Using this procedure, observers who are moving relative to each other will assign different times and positions
to the same event. No particular observer’s measurements are any more correct than any other observer’s, but
all the measurements are related. Any observer can work out precisely what time and position any other
observer will assign to an event, provided he knows the other observer’s relative velocity.

Nowadays we use just this method to measure distances precisely, because we can measure time more
accurately than length. In effect, the meter is defined to be the distance traveled by light in
0.000000003335640952 second, as measured by a cesium clock. (The reason for that particular number is that
it corresponds to the historical definition of the meter – in terms of two marks on a particular platinum bar kept
in Paris.) Equally, we can use a more convenient, new unit of length called a light-second. This is simply
defined as the distance that light travels in one second. In the theory of relativity, we now define distance in

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