Universe or Multiverse?

More than 2000 years ago, Epicurus taught that there are an infinite number of other worlds, both like and unlike ours, and Aristotle taught that there are none. Neither hypothesis can currently be falsified, and this issue of potential for falsification (that is testability) goes to the heart of many of the chapters in Carr's book. All but one of the 27 chapters, provided by 27 pundits (almost but not quite a one-to-one mapping) are written versions of talks given at one of three meetings, held between 2001 and 2005 at Stanford and Cambridge Universities and partly sponsored by the Templeton Foundation. Every reader will surely find some chapters interesting and informative, some provocative, and some rather vacuous. These will not be the same chapters for all readers. Two 'conflict of interest' statements: first, I spoke at one of these meetings, but was not one of those asked to provide a chapter. And, second, the first time I suggested in a lecture for scientists that 'many universes, either in temporal succession or embedded in higher dimensional space' was a possible explanation of the habitability of ours was fall 1974, shortly after Brandon Carter's first paper on anthropic principles and explanations, but before Bernard Carr and Martin Rees's 1979 Nature paper, which presented all the anthropic arguments then known and divided them into numbers that required no additional physics beyond the four standard forces (like the number of particles in a star) and those that seemed essential for life but not calculable (like the ratio of the electromagnetic to nuclear force constant). My other three possibilities were 'G.d has been very careful' (now called intelligent design), additional physics to be learned, and shear complexity. The core multiverse concept is that our universe (the 4-dimensional spacetime with which we are or could be connected and all its contents) is one of many, perhaps infinitely many, probably with different values of the constants of nature and other physical differences, which cannot communicate with ours even in principle. Such ensembles are predicted by some versions of inflation, string and M-theory. The anthropic principle is the idea that our universe has (or even must have) the structure, physics, chemistry and all required for me to be writing this and you to be reading it (editors are optional). Both concepts have firm supporters and firm opponents among the 26 male and one female authors. The woman, M-theorist Renata Kallosh, is for, and provides hints of how one might calculate, at least, the likelihood of our universe within an ensemble (a sort of testability). Her chapter is fairly heavy going in isolation, and readers who don't normally think about antisymmetric tensor gauge fields might want to start with John Donaghue, who explains what a particle physicist means by 'naturalness' and suggests that the known spectrum of quark and lepton masses might be a signature of multiverse origins. Given the Templeton sponsorship, you might reasonably want to know the extent to which 'progress in spirituality' has conditioned the topics covered. The answer is 'somewhat', in that authors range from the avowedly atheist (Stephen Hawking) to evangelical Christian quantum cosmologist Don Page, with stop-overs among the Jesuits (William Stoeger), philosophers of religion (Robin Collins), and the (I think) teleologists Paul Davies and John Barrow. There is also among the authors strong divergence of opinion on whether Hugh Everett's version of many worlds is (just) a quantum multiverse (Tegmark), almost certainly correct and meaningful (Page), or almost certainly wrong or meaningless (Carter). And two chapters, by Smolin and Weinberg, suggest that even the classic fine-tuning required for carbon to be formed from three helium nuclei may not be anthropically essential for a habitable universe. The last word belongs to Steven Weinberg. On previous occasions, Martin Rees has said that he has enough confidence in the multiverse to bet his dog's life on it, while Andrei Linde said he would bet his own life. Weinberg concludes his contribution by saying that he has just enough confidence in the multiverse to bet the lives of both Andrei Linde and Martin Rees's dog.