Welcome to the Multiverse

The October issue of Discover magazine has a new feature, a column by Sean Carroll, whose inaugural effort is now on-line as Welcome to the Multiverse. Sean makes the argument that opposition to multiverse mania is due to people having too naive an idea about what science is. They don’t realize that testing those parts of a theory you can directly observe allows you to draw conclusions about those parts you can’t directly observe:

A lot of people, both inside and outside the scientific community, are viscerally opposed to the idea of other universes, for the simple reason that we can’t observe them—at least as far as we know. It’s possible that another universe bumped into ours early on and left a detectable signature in the cosmic background radiation; cosmologists are actively looking. But the multiverse might be impossible to test directly. Even if such a theory were true, the worry goes, how would we ever know? Is it scientific to even talk about it?

These concerns stem from an overly simple demarcation between science and nonscience. Science depends on being able to observe something, but not necessarily everything, predicted by a theory. It’s a mistake to think of the multiverse as a theory, invented by desperate physicists at the end of their imaginative ropes. The multiverse is a prediction of certain theories­—most notably, of inflation plus string theory. The question is not whether we will ever be able to see other universes; it’s whether we will ever be able to test the theories that predict they exist.

Sean makes quite clear that multiverse mania is driven by string theory. Inflation is part of the story, but it’s not a fundamental theory by itself. All it can tell you is that your fundamental theory should have an inflaton field of some kind, with a potential satisfying certain properties. The big idea that justifies the multiverse is that:

In short, string theory predicts that the laws of physics can take on an enormous variety of forms, and inflation can create an infinite number of pocket universes. So the different laws of physics predicted by string theory might not be just hypothetical. They might really be out there somewhere among the countless parts of the multiverse. This is not a situation that cosmologists dreamed up in a flight of fancy; it is something we were led to by trying to solve problems right here in the universe we observe.

The problem that Sean doesn’t mention is one of circularity. Since you can’t observe anything about it directly, the multiverse must be justified in terms of another theory that can be tested and this is string theory. But if you talk to string theorists these days about how they’re going to test the unified theory that string theory is supposed to provide, their answer is that, alas, there is no way to do this, because of the multiverse. You see, the multiverse implies that all the things you would think that string theory might be able to predict turn out to be unpredictable local environmental accidents.

So, the multiverse can’t be tested, but we should believe in it since it’s an implication of string theory, but string theory can’t be tested because of the multiverse.

Until recently, string theorists would sometimes hold out hope that the LHC would see low-energy strings, extra dimensions, or supersymmetry, and that these discoveries would somehow pick out a predictive version of string theory from the landscape of the multiverse. This year’s data from the LHC has pretty much destroyed such hopes.

Sean ends with the inspirational admonition:

The proper scientific approach is to take every reasonable possibility seriously, no matter how heretical it may seem, and to work as hard as we can to match our theoretical speculations to the cold data of our experiments.

What’s going on in this story though is not a concerted effort to match theoretical speculation to experimental data, but something very different, a concerted effort to build a theoretical framework perfectly insulated from testability, and sell it to the rest of the physics community and the public, hoping no one notices the circularity.

Update: Besides the usual spam, this topic seems to attract mostly empty comments supposedly agreeing with me, and comment moderation is unusually annoying. I think I’ll turn off comments on this posting, and encourage people who want to discuss the topic to do so over at Cosmic Variance, where Sean has a posting devoted to this.