This book grew out of a conference entitled 'Universe or Multiverse?' which was held at Stanford University in March 2003 and initiated by Charles Harper of the John Templeton Foundation, which sponsored the event. Paul Davies and Andrei Linde were in charge of the scientific programme, while Mary Ann Meyers of the Templeton Foundation played the major administrative role. The meeting came at a critical point in the development of the subject and included contributions from some of the key players in the field, so I was very pleased to be invited to edit the resulting proceedings. All of the talks given at the Stanford meeting are represented in this volume and they comprise about half of the contents. These are the chapters by James Bjorken, Nick Bostrum, Robin Collins, Paul Davies, Savas Dimopoulos and Scott Thomas, Renata Kallosh, Andrei Linde, Viatschelav Mukhanov, Martin Rees, Leonard Susskind, Max Tegmark, Alex Vilenkin, and my own second contribution.

Several years earlier, in August 2001, a meeting on a related theme -entitled 'Anthropic Arguments in Fundamental Physics and Cosmology' -had been held in Cambridge (UK) at the home of Martin Rees. This was also associated with the Templeton Foundation, since it was partly funded out of a grant awarded to myself, Robert Crittenden, Martin Rees and Neil Turok for a project entitled 'Fundamental Physics and the Problem of Our Existence'. This was one of a number of awards made by the Templeton Foundation in 2000 as part of their 'Cosmology & Fine-Tuning' research programme. In our case, we decided to use the funds to host a series of workshops, and the 2001 meeting was the first of these.

The theme of the Cambridge meeting was somewhat broader than that of the Stanford one - it focused on the anthropic principle rather than the multiverse proposal (which might be regarded as a particular interpretation of the anthropic principle). Nevertheless, about half the talks were on the multiverse theme, so I was keen to have these represented in the current volume. Although I had published a review of the Cambridge meeting in Physics World in October 2001, there had been no formal publication of the talks. In 2003 I therefore invited some of the Cambridge participants to write up their talks, albeit in updated form. I was delighted when almost everybody accepted this invitation, and their contributions represent most of the rest of the volume. These are the chapters by John Barrow, Brandon Carter, John Donoghue, George Ellis, James Hartle, Craig Hogan, Don Page, Lee Smolin, William Stoeger and Frank Wilczek.

We organized two further meetings with the aforementioned Templeton support. The second one - entitled 'Fine-Tuning in Living Systems' - was held at St George's House, Windsor Castle, in August 2002. The emphasis of this was more on biology than physics, and we were much helped by having John Barrow on the Programme Committee. Although this meeting was of great interest in its own right - representing the rapidly burgeoning area of astrobiology - there was little overlap with the multiverse theme, so it is not represented in this volume. Also, the proceedings of the Windsor meeting have already been published as a special issue of the International Journal of Astrobiology, which appeared in April 2003.

The third meeting was held at Cambridge in September 2005. It was again hosted by Martin Rees, but this time at Trinity College, Martin having recently been appointed Master of Trinity. The title of the meeting was 'Expectations of a Final Theory', and on this occasion David Tong joined the Programme Committee. Most of the focus was on the exciting developments in particle physics - in particular M-theory and the string landscape scenario, which perhaps provide a plausible theoretical basis for the multiverse paradigm. Many of the talks were highly specialized and - since this volume was already about to go to press - it was anyway too late to include them. Nevertheless, the introductory talk by Steven Weinberg and the summary talk by Franck Wilczek were very general and nicely complemented the articles already written. I was therefore delighted when they both agreed - at very short notice - to produce write-ups for this volume. The article by Stephen Hawking also derives from his presentation at the Trinity meeting, although he had previously spoken at the 2001 meeting as well. It is therefore gratifying that both Cambridge meetings - and thus all three Templeton-supported meetings - are represented in this volume.

Although I have described the history behind this volume, I should emphasize that the articles are organized by topic rather than chronology. After the overview articles in Part I, I have divided them into three categories. Part II focuses on the cosmological and astrophysical aspects of the multiverse proposal; Part III is more relevant to particle physics and quantum cosmology; and Part IV addresses more general philosophical aspects. Of course, such a clean division is not strictly possible, since some of the articles cover more than one of these areas. Indeed, it is precisely the amalgamation of the cosmological and particle physical approaches which has most powered the growing interest in the topic. Nevertheless, by and large it has been possible to divide articles according to their degree of emphasis.

Although this book evolved out of a collection of conference papers, the articles are intended to be at semi-popular level (for example at the level of Science or Scientific American) and most of the contributions have been written by the authors with that in mind. However, there is still some variation in the length and level of the articles, and some more closely resemble in technicality the original conference presentations. Where papers are more technical, I have elaborated at greater length in my introductory remarks in order to make them more accessible. In my view, the inclusion of some technical articles is desirable, because it emphasizes that the subject is a proper branch of science and not just philosophy. Also it will hopefully broaden the book's appeal to include both experts and non-experts.

As mentioned in my Introduction, the reaction of scientists to the multiverse proposal varies considerably, and some dispute that it constitutes proper science at all. It should therefore be stressed that this is not a proselitizing work, and this is signified by the question mark in the title. I did briefly consider the shorter title 'Multiverse?' or even 'Multiverse' (without the question mark), but I eventually discarded these as being too unequivocal. In fact, the authors in this volume display a broad range of attitudes to the multiverse proposal - from strong support through open-minded agnosticism to strong opposition. The proponents probably predominate numerically and they are certainly more represented in Parts II and III. However, the balance is restored in Part IV, where many of the contributors are sceptical. Therefore readers who persevere to the end of this book are unlikely to be sufficiently enlightened to answer the question raised by its title definitively. Nevertheless, it is hoped that they will be stimulated by the diversity of views expressed. Finally, it should be stressed that perhaps the most remarkable aspect of this book is that it testifies to the large number of eminent physicists who now find the subject interesting enough to be worth writing about. It is unlikely that such a volume could have been produced even a decade ago!

Bernard Carr

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