20 - Taking stock

Summary

It is hoped that we have learned rather a lot about string theory in this book, and that the role of D-branes and other extended objects has been fascinating, entertaining, and instructive. It was with great pain that we had to sacrifice a tremendous amount of material in order to keep this book close to a sensible length, while retaining enough to succeed in telling a coherent story.

It is tempting to sit and reflect upon what great lessons we have learned, although it is not clear that this is a useful exercise at this stage, so we will be brief in our remarks. The main and most unambiguous lesson is that extended objects are vitally important to our understanding of string theory, and possibly (probably) whatever the final form of the fundamental quantum theory of space and time turns out to be. While extended objects are universally accepted as important, it is still (at a stretch) a matter of taste whether someone wants to go further and accept that it is unambiguously true that ‘string theory is not a theory of strings’. The author believes it to be so, but will not insist that the reader take a position, since it seems that nobody can yet say what string or M-theory actually are theories of.

Whatever the final theory turns out to be, and whether or not once it is found it turns out be directly relevant to nature at all, it is clear that we have many new tools to work with which should keep us busy for some time to come in various areas.