Science

Interview with ‘An Infinity of Worlds’ author, Will Kinney

On cosmic inflation, multiverses, and Michio Kaku.

July 3, 2023

AIPT Science contributor Julian recently reviewed cosmologist Will Kinney’s popular book An Infinity of Worlds: Cosmic Inflation and the Beginning of the Universe , which digs deep into the lesser known process that many scientists think helped the Big Bang jump-start the universe. Dr. Kinney was kind enough to answer some of Julian’s questions through email, presented with some light editing below.

AIPT: Why did you write the book An Infinity of Worlds?

Will Kinney: Well, the the short answer is, I wrote the book because MIT Press got in touch with me and asked me to write it. But it was a neat opportunity to dive into a really interesting topic, and one that I spend most of my time doing my research in, but presented for a lay audience. It’s a real challenge in particular with inflation, because it’s a very abstract thing.

You’ll know that Giordano Bruno features fairly prominently in the book. He was an interesting fellow, and in many ways was hundreds of years ahead of his time, and in many other ways was like a complete nut. But I found that his spirit and the ideas that he had really illuminated a lot of what I had to say in the book, including taking the title, which is a quote from Bruno.

[I was] trying to write a science book that was a little different than a lot of other science books, in terms of the way it approached difficult material.

Interview with 'An Infinity of Worlds' author, Will Kinney

AIPT: Diving into An Infinity of Worlds a little bit more, you wrote, “Despite its quantum mechanical origin, the multiverse generated by eternal inflation is in no way related to the many worlds interpretation of quantum mechanics.” Could that mean we exist in multiple multiverses?

WK: Yeah, I think so. I mean, essentially, these two things [the inflationary multiverse and the many worlds interpretation of quantum mechanics] are about fundamentally different questions. In the Everettian many worlds idea, every time a quantum state is reduced from a superposition down to one choice […] that creates a sort of a multiverse in which the universe is constantly branching every time a quantum choice is made. So you have this enormous, higher dimensional thing, where all outcomes are happening.

The inflationary multiverse is a little bit more pedestrian than that, in that it’s just saying that you have our universe embedded in a larger spacetime which contains sort of other universes like our own. They’re actually spatially separated from us. In the Everettian multiverse, it’s not that these other quantum options are spatially separated by a span of space. They just all sort of coexist. In this way they’re fundamentally different. I suppose you could have an Everettian interpretation in which you could also have an inflationary multiverse. I don’t see any reason that those two concepts couldn’t exist simultaneously.

AIPT: In An Infinity of Worlds, you wrote, “Recently proposed conjectures postulate that any vacuum energy long-lived enough to support cosmological inflation beyond a few doubling times must also lie in the [string theory] swampland. If this is true, it means that the infinite multiverse of eternal inflation is almost certainly an artifact of an inherently ill posed theory.” What’s your opinion about this bottom-up approach to quantum gravity?

WK: The idea of the string swampland has raised a lot of interesting questions, and certainly is an active area of thought right now, especially among fundamental theorists.

There’s this fundamental tension between me as a cosmological model builder — who wants to use de Sitter spaces, things with a positive energy — and the string theorists who say no, these are necessarily unstable and that the only real ground states here are anti-de Sitter things or the negative vacuum energy. Those don’t really match the universe we seem to live in, which likes to have a positive energy.

I think most cosmologists’ response to this tension is to just ignore the string theorists and go ahead and build models anyway with positive vacuum energy. But it remains to be seen exactly how this tension will be resolved. I do not think that these swampland conjectures really pose a problem for constructing consistent cosmological models, and until they can be brought in to really explain what we see in the real world, then I’ll maintain a bit of skepticism about the validity of at least those particular conjectures of the swampland program.

AIPT: Where is your research taking you now?

WK: Well, a question that I’ve been really getting interested in lately is this question of whether or not the universe really had a beginning.

In the standard Big Bang model, the universe arises out of a singularity, where spacetime is infinitely dense. Our laws of physics break down there, and we can’t say what happens at a finite time in the past. Inflation complicates this, because inflation really removes that singularity, and it takes place before the hot Big Bang itself. It’s a period of whole vacuum-dominated expansion that later on decays and turns into this hot soup of particles of the thermal Big Bang as we know it.

I write about this in the book. Formally, the mathematical property that you’re testing is called geodesic completeness: does every observer see the universe infinite into the past? Or are there observers who see it as finite?

I’ve been thinking about these questions in a larger context. My graduate student, Nina Stein, and I wrote a paper a couple of years ago looking at bouncing cosmologies in particular, as a way to get around [the requirement of a beginning], so you could have the universe that was cyclic in some way, so it could cycle indefinitely into the future and into the past. We managed to show that those particular universes that were proposed in that model, or a very large and related class of them, all had to be finite into the past, too. Doing these bouncing universes did not save you from having to have an initial condition.

So this starts to bring out interesting questions. We’ve been working on […] trying to understand what spacetimes can be past infinite, and what ones had to have a beginning in the context of general relativity.

'An Infinity of Worlds' cover

AIPT: Theoretical physicist Michio Kaku recently appeared on Joe Rogan’s podcast to discuss his new book Quantum Supremacy. This has led to an outcry from some in the physics community. What do you think about this situation?

WK: I haven’t read Michio Kaku’s book … so I’m criticizing it from the point of view of listening to his public statements about it, and particularly some of the clips from that Rogan interview. I think that in this case his latest book is … I think it’s hard to describe it as anything other than a cash grab. It’s in this school of popular science writing that is mostly, like, lots of breathless buzzwords and not much substance behind it.

And it sells, right? He sells a lot more copies of books than I do [laughs]. So it’s hard to criticize it on that front. But I find that sort of writing and that approach to science communication to be very shallow and lazy, and I think we need less of it.

AIPT Science is co-presented by AIPT and the New York City Skeptics.