Today’s Wall Street Journal has a review I wrote of Sir Roger Penrose’s new book Cycles of Time. The review is aimed at a much wider audience than this blog, and is the product of substantial editing to get its length down and make it as readable as possible for as many people as possible, so here are some supplementary remarks.
I should make it clear that I’m not at all convinced by what Penrose is proposing. He needs the distant future of the universe to be conformally invariant, and this requires all particles to be massless. As far as we know the electron is completely stable, with unchanging mass, and this will always ruin conformal invariance. Penrose himself notes the problem. For this to be overcome, whatever our ultimate understanding is of how particles get mass must change so that these masses go to zero in the future. It’s also seems to me that the conformal anomaly of QCD will always be a problem, with quantization and the renormalization group always breaking conformal invariance and giving a mass scale, indefinitely far into the future.
The other main problem is the one shared by most “pre-big-bang” ideas: how do you ever test them? Penrose and a collaborator last year created a stir by claiming to see in the CMB patterns of the sort he argues might be expected from black hole decays late in an era before the Big Bang, but it’s not clear there’s a real prediction here, and others who have redone this analysis say they see nothing.
Attempts to get a Big Bang in our future as well as our past generally strike me as motivated by a very human desire to see in the global structure of the universe the same cyclic pattern of death and rebirth that govern human existence. To me though, deeper understanding of the universe leads to unexpected structures, fascinating precisely because of how alien they are to human concerns and experience. Just because we might find a cold, empty universe an unappealing future doesn’t mean that that’s not where things are headed.
The book is in many ways an unusual document. It includes an extensive appendix working out some of the details of the mathematics of his proposal. In some sense he has managed to get a trade publisher to put out a highly technical discussion of a speculative idea inside the covers of a popular book, instead of going the usual route of publishing this in a refereed journal. The only references I can find to other places where he has written some of this up are to chapters in this book and this one, as well as this contribution to a conference proceeding. The technical idea behind this, that the hypothesis of the vanishing of the Weyl curvature in the early universe leads to possible cosmological models that can be extended past the Big Bang singularity he attributes to this paper of K.P. Tod. There’s a nice recent exposition of this by Tod here.
So, I’m not convinced by the speculation about the far future, and for an evaluation of the ideas about extending back through the big bang singularity you’ll need someone more expert about cosmology than me. These topics are very clearly labeled in the book as speculative, without support from other physicists or any experimental evidence. The bulk of the book though is other material providing a background and context for the speculation, and it is this which I think makes it most valuable as a popular book. Penrose is a wonderful, elegant and clear writer, and he covers a lot of ground about physics beautifully here. Most remarkable are the illustrations, by far the best visual representations of a range of important ideas that I know of. Physicists and mathematicians work with lots of internal pictures in their minds representing important aspects of the concepts they are investigating, but very rarely do they have the technical skill to grasp some of the essence of these pictures and get them down on paper. Even more rarely do they make it into wide distribution in print, so I’m glad to see that happen here.