In short, it’s the idea that our Universe, and all that’s contained within it, is just one small region of a larger existence that includes many similar, and possibly many different, Universes like our own. On the one hand, if our current theories of physics are true, the Multiverse absolutely must exist. But on the other hand, as Sabine Hossenfelder rightly points out, it’s unlikely to teach us anything useful.
Writing in NPR, Sabine Hossenfelder is right to criticize that approach, stating, “Just because a theory is falsifiable doesn’t mean it’s scientific.” But just because variants of the Multiverse are falsifiable, and just because the consequences of its existence are unobservable, doesn’t mean that the Multiverse isn’t real. If cosmic inflation, General Relativity, and quantum field theory are all correct, the Multiverse likely is real, and we’re living in it.
Just don’t expect it to solve your most burning questions about the Universe. For that, you need physics you can put to an experimental or observable test. Until that day arrives, the consequences of a Multiverse will likely remain in the realm of science fiction: where they presently belong. It’s okay to speculate, but if you insist on attributing a physics problem’s solution to an untestable feature of the Universe, you’re essentially giving up on physics. More.
So is Ethan Siegel saying that fellow astrophysicist Sean Carroll, a multiverser, is giving up on physics?
Probably. But maybe physics is best off to just be whatever physicists currently consider a community of practice as long as it isn’t claiming any special status as a field of knowledge.
See also: Sean Carroll and Brute Facts (Barry Arrington)
Theoretical physicist: Reasons to be skeptical of the multiverse Bookmark this for the next airhead invasion of your local Great Ideas discussion group.
Theoretical physicist: Multiverse not based on sound science reasoning Good points But what if multiverse theory is simply a means of fending off the impasses that fully naturalist theoretical physics is in? It doesn’t need to make sense, any more than bollards do.