Yes, this multiverse business is tedious, but since it is becoming mainstream physics, with colloquium talks here at Columbia devoted to it, and the Columbia University Press publishing books about it, seems to me that someone at Columbia should be commenting on these, and I don’t see anyone else doing it. Will try to make this short.
Yesterday Matthew Kleban’s talk here was entitled Testing the Multiverse. The only part that actually really was about testing the multiverse was the part describing work on bubble collisions with other universes. This has been heavily advertised in the press, see here, here, here, here, here and many others. Kleban described some of these ideas, but when it came to the experimental testing part, he just briefly acknowledged that all searches for these things have come up empty. The only prospect for the future mentioned was the polarization data to be released later this year by Planck, which would give some new things to look at, but he seemed unenthusiastic that this would realistically lead anywhere. So, as far as the “testing” goes, it has been done and the tests failed.
The rest of the talk was about various inflationary models, including Kleban’s work on “unwinding inflation” (see here, here and here). Some of these models do have testable consequences, and many do lead to “eternal inflation”, so in such models you expect to continually produce new inflated universes, although with exactly the same physics. This is being sold as “testing the multiverse”, and string theory is brought in to justify lots of possible different physics in different universes, but this is not a testable part of these scenarios. What’s being advertised is a grandiose picture of the string landscape, laws of physics determined environmentally, etc., etc., but if you actually look at the product that you’re actually buying as “testable”, you don’t get any of the cool stuff. For slides of a somewhat similar recent talk by Kleban, see here.
A while back I acquired a copy of the new book Worlds Without End: the many lives of the Multiverse, started reading it and was planning on writing a detailed review. I soon got bogged down in the first half of the book, which is a detailed intellectual history of speculation about multiple universes (so lots about relevant parts of Plato, Aristotle, Lucretius, the Stoics, Augustine, Nicholas of Cusa, Giordano Bruno, Kant and others). Finally I realized I just didn’t have the energy to serious read this material. People with other interests and/or more time on their hands may find this quite worthwhile.
The second half of the book is devoted to the question of current speculation about physics, so more up my alley, but again I found it hard to focus on this. I fear it is only mildly interesting to see what a theologian/philosopher of religion makes of the current multiverse mania, not enough so though to do more than skim the text. From this skimming, what’s in the book is a lot of retelling (sometimes introducing misunderstandings) of the hype-laden tales of the multiverse told in dozens of books and magazine articles over the past decade or so.
Rubenstein ends the section about what physicists have to say with Tegmark, seen as having reached the final endpoint of the “Ultimate Multiverse”:
So some worlds will be linear, and some will be cyclical; some will be singular, and some will be plural; some will be infinite, and some will be finite; some will branch forward, and some will branch back. Some worlds will be manufactured, and some will be simulated; some designers will be kind and some will be cruel, some capable and some all but incompetent.
And, presumably, some of the set of all possible world will have a creator-god who breathes over primordial waters, who separates the sea from dry land.
How on earth did we get back here?
I take Tegmark’s vision as empty, so a good thing to ignore, but Rubenstein sees this as an opening for theologians to get back into the mainstream cosmology business, and the rest of the book focuses on this. With the boundaries between science and religion now gone, all sorts of possibilities open up for theologians. The final part of the book begins by invoking (just like Henrich Päs, who comes at it from the mind-altering drug rather than theological angle) Nietzsche:
Nietzsche concludes the Genealogy by expanding this vision, promising “all great things bring about their own destruction through an act of self-overcoming” (3.27, emphasis added). This promise then, has me wondering. If science can be regarded as the self-overcoming of a particular form of religion, might multiverse cosmologies be something like the self-overcoming of science? Might they mark the end of the fantasy that “science” has wrested itself free from “religion”, “objectivity” free from subjectivity, and matter free from meaning? After all, we have seen each of these multiverse cosmologies open onto metaphysics and mythology not in moments of lapse or weakness, but precisely where they are scientifically most compelling.
It seems that, unlike most authors, Rubenstein actually has got the story of multiverse mania right: it’s left conventional notions of science behind and entered into the realm of theology. We do, however, disagree about whether or not this is a good thing…
Update: Bogus claims about Multiverse “predictions” are now all the rage. For the latest, see the Caltech Quantum Frontiers blog, which has Yasunri Nomura writing about Making Predictions in the Multiverse. There of course are no predictions there, just mainly a discussion of the idea that many-worlds and the eternal inflation multiverse are somehow the same, an idea I continue to find unfathomable. Nomura doesn’t mention that he actually did have a prediction from the Multiverse (and someone made a movie about it…). The prediction was for a Higgs mass of 140 GeV, but of course when you’re in the multiverse business, wrong predictions are not a problem, they’re always true somewhere.
Update: For more on the multiverse front, Edward Frenkel has a review of the Tegmark book in this Sunday’s New York Times. He does a great job of explaining the problems with the way Tegmark is trying to use mathematics. John Preskill tweets in agreement (positive and negative).
Update: Nomura, when asked about experimental evidence for the multiverse, responded that the experimental situation is
not much different from some other situations—e.g. in the big-bang theory, inflationary cosmology, and Darwinism in biology
So, the scientific evidence for the multiverse is “not much different” than the evidence for evolution? And Tegmark thinks I’m the one in league with creationists…