Scientific American in recent years seems to be quite fond of parallel universes, with major articles promoting the multiverse here, here and here (commentary on this blog here and here). Their latest issue continues in this vein with an article by Sean Carroll entitled Does Time Run Backward in Other Universes?, which advertises his 2004 work with Jennifer Chen claiming that the multiverse explains the arrow of time. For new blog entries about this, see here for something from Sean, here for a Lubos rant.

As with all claims about the multiverse, the problem is whether they are even in principle scientifically testable or not. If they’re not, they’re not science and promoting them to the public is a bad idea. The only thing I can find in the Scientific American article that addresses the testability issue at all is the following:

As of right now, the jury is out on our model. Cosmologists have contemplated the idea of baby universes for many years, but we do not understand the birthing process. If quantum fluctuations could create new universes, they could also create many other things—for example, an entire galaxy. For a scenario like ours to explain the universe we see, it has to predict that most galaxies arise in the aftermath of big bang–like events and not as lonely fluctuations in an otherwise empty universe. If not, our universe would seem highly unnatural.

This doesn’t seem to have anything to do specifically with the Carroll/Chen claims about the arrow of time, but rather is just a restatement of one of the desired properties of multiverse models, that they don’t lead to “Boltzmann Brains”.