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The Philosophy of Vacuum Part 18. Physicists will find it extremely interesting, covering, as it does, technical subjects in an accessible way. For those with the necessary expertise, this book will provide an illuminating and authoritative exposition of a many-sided subject." -John D. Barrow, Times Literary Supplement. | The Vacuum and Unification 163 Kaluza-Klein picture but now the vacuum configuration in the compactified dimensions had to be that appropriate to the group structure and to the particle multiplet structure of the phenomenologically well established standard model for elementary particles. Then in 1984 came the discovery by Green and Schwartz of a consistent quantum theory of all forces including gravity technically the first anomaly-free theory based on a framework going beyond the local quantum field concept. This was a swper string theory in which the fundamental object has a finite extension of order the Planck length 10 33 cm . In such theories the whole of our mundane physics at distances much greater than this tiny size geometry symmetries and matter content is viewed as merely the ground state of the superstring system in short as its vacuum. Figure 2 shows part of the title page of a paper that generated much additional enthusiasm for the superstring idea. These considerations which have brought us to the end of Fig. 1 will be briefly described in the concluding section. 2 The Mechanical Vacuum of Maxwell s Electromagnetic Theory As indicated in Fig. 1 to the classical physicist matter and force were clearly separated. The nature of matter was intuitive based on everyday macroscopic experience force on the other hand was more problematical. Contact forces between extended bodies were easy to understand but forces that seemed capable of acting at a distance caused difficulties. As Maxwell reminded his audience in a Royal Institution lecture entitled On Action at a Distance so far was Newton from asserting that bodies really do act on one another at a distance independently of anything between them that in a letter to Bentley which has been quoted by Faraday in this place he says - It is inconceivable that inanimate brute matter should without the mediation of something else which is not material operate upon and affect other matter without mutual contact as
