New Scientist today has a feature article headlined
How to think about… The multiverse
The idea of an infinite multitude of universes is forced on us by physics.
It starts off quoting Sean Carroll:
“One of the most common misconceptions is that the multiverse is a hypothesis,” says Sean Carroll at the California Institute of Technology in Pasadena. In fact, it is forced upon us.”It is a prediction of theories we have good reason to think are correct.”
The problem with this claim is that it’s simply not true. There is no model that “we have good reason to think [is] correct” that predicts a multiverse of universes with different physics (i.e. fundamental constants). I’ve written about this many times, see for instance Theorists Without a Theory. In case you were thinking of interpreting Carroll’s claim in some other way, the article goes on to invoke Alexander Vilenkin:
“The so-called constants of nature, like the mass of the electron or Newton’s gravitational constant, will have different values in different bubbles,”
To be fair to New Scientist, I haven’t read beyond the headline and first few paragraphs of this article, since the rest is behind a paywall. Maybe the later part of the article (which most people can’t read) explains what is wrong with the Carroll and Vilenkin claims.
For the latest on the models supposed to give us different physics in different parts of the multiverse, you might want to take a look at this new paper on the arXiv, and Cumrun Vafa’s talk about it this week at Strings 2018. The paper and talk conjecture that the supposed metastable dS solutions of the string landscape don’t really exist (they are in the “swampland” of things that aren’t solutions of string theory). If this is true and you want to save string theory, as Vafa explains, you need to invoke different sorts of supposed solutions to string theory, with the CC replaced by a “quintessence” mechanism.
At the end of the talk (1:01), Eva Silverstein tries to explain what is wrong with Vafa’s arguments. He responds “I’m not saying you’re wrong, you might be right, this might be also be right”. This shows clearly the fundamental problem of the subject: there is no well-defined theory here, just a bunch of conjectures about what one might be, with no way to tell whether Vafa or Silverstein is right, and no way to extract well-defined predictions from the mass of possible conjectures.
Update: Thanks to those who sent me a copy of the full New Scientist article. It’s short, and the part behind the paywall is even worse than the part publicly available, just adding to the confusion by invoking “many-worlds”, with more from Sean Carroll. Our doppelgangers doing exciting stuff in other universes make an appearance, although Carroll expresses a lack of interest in what they’re up to.
Update: The Strings 2018 talks and videos are available here or at this Youtube channel, and 4 gravitons has a blog posting. As usual with Strings 20XX conferences, very little about actual string theory there. For an overview of the state of the field, you might want to watch this video of the 50 years of string theory session, moderated by David Gross. Dan Harlow was the only speaker raising the elephant in the room question: “is string theory still a useful candidate as a theory of HEP physics?” (and also asked whether they should finally rename the conference series). Gross read off submitted questions for the panel, most of which were asking about the elephant in the room. The panelists each found a different way of avoiding dealing with the question. Other questions asked about the hot “is there a dS string vacuum?” issue, responses were “maybe yes, maybe no”, with no indication of any way to resolve this.