Multiple Universes

A couple of my co-bloggers here thought I should say something about John Horgan’s comments on Richard Dawkins vs. Francis Collins in Time, berating Dawkins on his endorsement of multiple universes as a solution to fine-tuning issues.

My first inclination was to say something snide. After all, here are Dawkins and Collins, two non-physicists, arguing cosmology, and Horgan, a science journalist, pontificating on who is right and wrong. A perfect occasion for misunderstandings all around. Since I don’t believe in staying confined to one discipline myself, I’m not going to take too many cheap shots, but there’s clearly a lot that needs setting straight here, particularly since few seem to understand the physical reasons many physicists these days feel compelled to talk about the possibility of a multiverse.

The greatest misunderstanding is the notion that physicists invoke multiple universes just in order to sweep fine-tuning under the carpet. I see a good number of theologians suggesting this, plus scientists like Collins who go digging around for signs of intelligent design in the universe. This is bloody nonsense. Physicists take multiple universes seriously, but not because we necessarily like the idea (I, for one, would be happier avoiding it) or because it would get the intelligent design people off our back. We take it seriously as a possibility, mainly because it’s damn difficult to avoid if you play around with any kind of quantum cosmology. Starting with inflationary cosmologies, we have been playing around with scenarios involving either a vast number of universe-bubbles or huge numbers of at least metastable vacua for some time now. As I said, they’re difficult to avoid—that is, without invoking arbitrary principles for the sole purpose of sweeping multiple universes under the carpet.

Now, if we already have to consider multiple universes as a genuine possibility, it makes good sense to ask what relevance this might have to fine-tuning questions. It is bound to be significant, particularly since our universe is then not necessarily a respresentative sample of the population of universes. Our very presence means our sample is biased (on top of the usual problems with a sample of one).

One reason to worry about multiple universes is the question of how we test such an idea. Theologically-minded people usually suggest that multiple universes are untestable, ad-hoc inventions in comparison which an unobservable God is a positively restrained idea. Not quite. First of all, if multiple universes appear in the context of an overall well-supported theory we can certainly speak of indirectly testing the idea. This is not at all odd in ordinary scientific practice. And even now, when we are far from having cosmological theories that physicists would bet their right arm on, physicists do talk about testing certain multiple-universe theories by observation. Note that I am not saying every multiple universe idea makes contact with experiment in this way. For example, Leonard Susskind defends the string-theoretical “landscape” notion in his The Cosmic Landscape: String Theory and the Illusion of Intelligent Design. It’s a fascinating book, well worth reading, but (as he partially acknowledges) his version of anthropic argument has trouble suggesting more than a tenuous connection with experimental tests. Moreover, if you’re like me, and you’re inclined toward a degree of skepticism concerning string theory, you’ll be extra cautious because the whole discussion takes place in the context of a theory that has not been able to make contact with too many reality tests. (Unlike, say, basic cosmic inflation.)

If you want to find out more, check out my chapters on physics and cosmology in The Ghost in the Universe and Science and Nonbelief. Vic Stenger’s Timeless Reality has a nice, accessible discussion as well.

Now, back to Collins and Dawkins. As I mentioned before, Collins is way out of his depth when talking physics and cosmology. He doesn’t understand what physical cosmologists are doing, and what he says rarely goes beyond philosophical posturing. Dawkins, on the other hand, can be criticized for taking Susskind’s version of anthropic reasoning on board somewhat over-enthusiastically. Still, in reading The God Delusion, my impression was that Dawkins was fully aware that this was outside his area of expertise, and that he wasn’t leaning too heavily on it.