A brief History of Time pdfdrive com

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 (
Fig.
11.1
). The world-sheet of a closed string is a cylinder or tube (
Fig. 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 (
Fig. 11.3
), while in the
case of closed strings it is like the two legs joining on a pair of trousers
(
Fig. 11.4
). Similarly, a single piece of string can divide into two strings.
In string theories, what were previously thought of as particles are now
pictured as waves traveling down the string, like waves on a vibrating
kite string. The emission or absorption of one particle by another
corresponds to the dividing or joining together of strings. For example,
the gravitational force of the sun on the earth was pictured in particle
theories as being caused by the emission of a graviton by a particle in
the sun and its absorption by a particle in the earth (
Fig. 11.5
). In string
theory, this process corresponds to an H-shaped tube or pipe (
Fig. 11.6
)
(string theory is rather like plumbing, in a way). The two vertical sides
of the H correspond to the particles in the sun and the earth, and the
horizontal crossbar corresponds to the graviton that travels between
them.

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