Perhaps everyone has asked themselves whether there is anything in the universe that is perfectly isolated, separate and complete within itself. Some will answer that the universe itself is the only thing, because the universe alone is unique. However, the physics of elementary particles and cosmology suggests the possibility that many universes similar to our universe exist and interact with each other through quantum gravity.

The appearance of a discussion of the universe or universes in a book on economic theory may seem abrupt and even odd, but if we recast the point in terms that are more familiar it will not seem so peculiar. When strangers meet by chance, at a party or on a train for example, it is not uncommon for them to find that they have shared acquaintances. ‘It's a small world!’, they will exclaim, in genuine surprise. In fact, this happens all the time, so it is not so much the frequency that stimulates our wonder, but the reasons which underlie it, and these are the subject of long-standing and unsolved questions.

In an attempt to shed light on this matter, sociologists have conducted many kinds of experiments, and needless to say this attempt requires an understanding of human networks. However, it is just as important to extend our outlook and to consider the more abstract and far-reaching question: ‘What is a network?’