Page 84 - A Brief History of Time - Stephen Hawking

A Brief History of Time - Stephen Hawking... Chapter 11

Figures 11:1 & 11:2

A particle occupies one point of space at each instant of time. Thus its history can be represented by a line in
space-time (the “world-line”). A string, on the other hand, occupies a line in space at each moment of time. So
its history in space-time is a two-dimensional surface called the world-sheet. (Any point on such a world-sheet
can be described by two numbers, one specifying the time and the other the position of the point on the string.)
The world-sheet of an open string is a strip: its edges represent the paths through space-time of the ends of the
string Figure 11:1. The world-sheet of a closed string is a cylinder or tube Figure 11:2: a slice through the tube
is a circle, which represents the position of the string at one particular time.

Two pieces of string can join together to form a single string; in the case of open strings they simply join at the
ends Figure 11:3, while in the case of closed strings it is like the two legs joining on a pair of trousers Figure
11:4.

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