Books on the multiverse

Hi friends,

Can anyone rec a good, recent nonfiction book on the multiverse? My last science class was in undergrad, so I’m no expert, but I don’t want too basic a book or too condescending a voice (looking at you, Brian Greene). Something like Hawking, maybe?

One problem, there are at least two completely different multiverse ideas, at least as I understand it.

One is all about quantum mechanics. The standard (Copenhagen) interpretation of quantum mechanics is very odd because the model is a probability distribution and the one outcome within that distribution that we observe is only singled out when the observation occurs. This leads to the rather philosophical question of what counts as an observer and why. The multiverse theory avoids this problem by saying that all of the possibilities in the distribution in fact do occur, all at once, in infinite different universes.

The other is mostly about cosmology but also quantum mechanics and sometimes string theory. The big bang theory of cosmology is very good at matching observation, except when it isn't. One of the main ways it isn't good is that the observed distribution of matter is too uniform to have been produced by simple expansion (don't ask me to explain that more). Instead, it requires a (very early, very short) period of accelerating expansion known as inflation. The problem is evidently that when quantum mechanics is thrown in the mix it's unclear how inflation would ever stop. One solution is to say that mostly it doesn't stop except in certain edge conditions. Accelerating expansion would mean that pretty soon everything is too far apart for causal relationships. So only in the edge conditions where inflation stopped is any kind of observable physics possible. Our universe is one of these, forever cut off from an infinite number of other such universes by the infinite expansion of space in between. Some people like to hook in string theory. String theory set out to incorporate general relativity and quantum mechanics. It ended up predicting a vast array of equally good (mathematically) physics models with no way of saying why our universe has the physics we actually observe. One answer proffered is to say all of the possibilities do occur, but in the infinite different universes produced by endless inflation.

So a real part of the trouble is that understanding why the multiverse idea has become fashionable requires a review of cosmology, particle physics, quantum mechanics, and string theory.

I don't think much of the multiverse under either definition and I can't provide sympathetic references. Sabine Hossenfelder's Lost in Math is a rather caustic popular review but I think she does lay out the basic ideas and their intellectual histories fairly well. A far more detailed review (certainly over my head in terms of math I can critically understand) can be found in Roger Penrose's Fashion, Faith, and Fantasy in the New Physics of the Universe. Penrose does sum up in more general terms in between the details.

Thank you for the detailed reply! Lost in Math is on my wishlist already (and I didn’t even realize it was about the multiverse!), so I’ll definitely check it out.

As you say, there are so many angles to come at it from, it’s hard to know where to start reading about it outside of fiction.

I really appreciate the time you took answering my question.