After watching the Breakthrough Prize awards tonight, tomorrow night on the Science Channel you can watch a program that actually features physicists rather than Hollywood/Silicon Valley celebrities. There’s an hour long infomercial for the Multiverse, entitled “Which Universe Are We In?”. You get to hear from
- Max Tegmark starting and ending the show with a generic promotional spiel about how wonderful the multiverse is.
- Seth Lloyd about how weird QM is, and that it and cosmology provide strong experimental support for the multiverse.
- Anthony Aguirre explaining about seeing collisions of other universes in the sky, and about how evidence for the multiverse has now been seen (BICEP2), providing a huge leap forward for the multiverse.
- Laura Mersini-Houghton about the string landscape and how she has used it to make predictions, which are now becoming accepted.
The program ends kind of like a car commercial, with beautiful scenery and swelling music. A voice over mentions un-named fuddy-duddy critics, mainly to say that BICEP2’s “great support for the theory of the multiverse” has “given then something to think about”. It suggests that the answer to the question raised by all these different kinds of multiverse (“which one is true?”) can be answered by believing all multiverse models at once, no need to choose.
No mention of tedious things like dust. This multiverse is all new and shiny, slices, dices, provides every reality you could possibly want.
On a somewhat higher level, Quanta magazine followed up last week’s multiverse piece with a new one this past week, Multiverse Collisions May Dot the Sky from Jennifer Ouellette. Aguirre appears here too, working with collaborators on analyzing possibly observable consequences of bubble collisions. One of them is Hiranya Peiris, who explains that multiverse theory is like the theory of evolution:
Peiris acknowledges that this argument has its critics. “It can predict anything, and therefore it’s not valid,” Peiris said of the reasoning typically used to dismiss the notion of a multiverse as a tautology, rather than a true scientific theory. “But I think that’s the wrong way to think about it.” The theory of evolution, Peiris argues, also resembles a tautology in certain respects — “an organism exists because it survived” — yet it holds tremendous explanatory power. It is a simple model that requires little initial input to produce the vast diversity of species we see today.
A multiverse model tied to eternal inflation could have the same kind of explanatory power. In this case, the bubble universes function much like speciation. Those universes that happen to have the right laws of physics will eventually “succeed” — that is, they will become home to conscious observers like ourselves. If our universe is one of many in a much larger multiverse, our existence seems less unlikely.
The problem of course with bubble collision “predictions” are that they’re not falsifiable. As far as they’re concerned, you can only win: seeing nothing doesn’t disprove the multiverse. The most recent attempt to look for evidence in the CMB that I’m aware of is this, which found nothing in the WMAP-7 data. I haven’t seen anything using Planck data released so far. Presumably when new data is released later this month some kind of search for bubble collision evidence will be done, and Quanta magazine isn’t likely to report the likely outcome.
The Quanta piece isn’t an infomercial like the TV program, it does explain some of the problems with this whole endeavor, including this from Erick Weinberg:
“My own feeling is you need to adjust the numbers rather finely to get it to work,” Weinberg said. The rate of formation of the bubble universes is key. If they had formed slowly, collisions would not have been possible because space would have expanded and driven the bubbles apart long before any collision could take place. Alternatively, if the bubbles had formed too quickly, they would have merged before space could expand sufficiently to form disconnected pockets. Somewhere in between is the Goldilocks rate, the “just right” rate at which the bubbles would have had to form for a collision to be possible.
Researchers also worry about finding a false positive. Even if such a collision did happen and evidence was imprinted on the CMB, spotting the telltale pattern would not necessarily constitute evidence of a multiverse. “You can get an effect and say it will be consistent with the calculated predictions for these [bubble] collisions,” Weinberg said. “But it might well be consistent with lots of other things.” For instance, a distorted CMB might be evidence of theoretical entities called cosmic strings. These are like the cracks that form in the ice when a lake freezes over, except here the ice is the fabric of space-time. Magnetic monopoles are another hypothetical defect that could affect the CMB, as could knots or twists in space-time called textures.
Weinberg isn’t sure it would even be possible to tell the difference between these different possibilities, especially because many models of eternal inflation exist. Without knowing the precise details of the theory, trying to make a positive identification of the multiverse would be like trying to distinguish between the composition of two meteorites that hit the roof of a house solely by the sound of the impacts, without knowing how the house is constructed and with what materials.
There’s also the problem that even if you did see something, it really would tell you pretty much nothing about the supposed other universe:
Should a signature for a bubble collision be confirmed, Peiris doesn’t see a way to study another bubble universe any further because by now it would be entirely out of causal contact with ours. But it would be a stunning validation that the notion of a multiverse deserves a seat at the testable physics table.
Update: One problem with arguing that the multiverse is like the theory of evolution that physicists should keep in mind: creationists love it.
