Time to get back into a little combination book blogging/science blogging! Let’s talk a little bit about Existential Physics by Sabine Hossenfelder, published in 2022.

I read this book recently due to somewhat curious circumstances. My friend Mark at my university that I’ve known since I became a professor some 15 years ago has been running a book club with colleagues from a number of departments. They settled on reading Hossenfelder’s Existential Physics as their next book, and since they didn’t have a physicist in the group, Mark asked if I was interested in joining them? I was, because the book club sounded fun, and I’m also Twitter friends with Sabine and was quite confident that the book would be an interesting read! So in discussing the book, I can not only talk about my reactions as a physicist, but also the reactions of colleagues in theater, linguistics, software and information systems.

It is undeniable that modern physics has become extremely weird, with attempts to explain things like quantum physics, general relativity and the standard model of particle physics leading to even weirder hypotheses. Existential Physics is an introduction and exploration of some of the most profound questions that physics has attempted to answer, and a discussion of whether these explanations are plausible or even count as physics.

In the book, you will find chapters on some of the following heady questions, which are also titles of chapters: Does the past still exist? How did the universe begin? How will it end? Are you just a bag of atoms? Has physics ruled out free will? Was the universe made for us? Does the universe think? Is consciousness computable? Can we create a universe?

To give you an example of one of these discussions, “Does the universe think?” begins with the observation that there is a remarkable similarity between the large-scale filament structure of the universe and the neuron structure of the human brain. Perhaps, then, the universe is itself something like a giant brain, doing its own neural processing. Hossenfelder notes that, according to the laws of physics as we understand them, this seems unlikely, because size matters: the universe is (to the best of our knowledge) constrained by a speed limit, the speed of light. In our own tiny brains, this isn’t an issue, but the universe is so large that it’s hard to imagine that more than a score of signals might have traveled the length of the “universal brain” during the entire history of the universe to date!

Hossenfelder turns around, though, and points out that there could be a loophole, in the form of literal holes in spacetime: wormholes. If there are shortcuts through space, and some hypotheses speculate that many, many wormholes might have been created in the early universe and persist to this day, faster “communication” of the universal brain might happen through these wormholes. But this is completely speculation at this point.

This gives you an idea of the overarching theme of the book, which separates out the “big questions” into things that we know are true, things that are hypotheses that might be true but are unproven, and things that are purely speculation based on philosophical concerns. It makes an effort to explain to the lay reader that some of the strangest speculations in physics are outside of physics itself and more in the realm of natural philosophy, including things that could never in principle be proven.

To give you an idea of how things break down, here’s an example I used to try to clarify things, which wasn’t in the book but seemed to help my book club. Einstein’s general relativity predicts the existence of black holes and wormholes. Black holes were once entirely theoretical, but have been proven to exist with almost absolute certainty through astronomical observations. Wormholes are also predicted by relativity, and we have mathematical models of what a wormhole looks like, and since we know that Einstein’s theory works remarkably well it is a plausible physical hypothesis that wormholes exist. We haven’t seen them yet, however, and it is also possible that wormholes are not allowed due to some laws of physics that we don’t yet understand. Then we have things like the hypothesis that quantum physics can be explained by a multiverse. The multiverse is not predicted by known physics; it is an attempt to explain bizarre properties of quantum physics and make them “make sense” to us, though it is quite plausible that the strangeness of quantum physics can be made to make sense without a multiverse. The multiverse falls more on the side of philosophical speculation, especially since there is no known way to test for a multiverse. Wormholes, in contrast, could in principle be observed.

To Hossenfelder’s credit, she does not criticize these deeper philosophical speculations into the nature of the universe. She recognizes that we’re always going to have a need to make sense of things, and that there’s nothing wrong with profound speculation. She works hard, though, to differentiate those strange things in physics that we’ve experimentally tested and proven, those things that are plausible, and those things that are pure speculation.

Existential Physics is bolstered by several interviews with famed scientific thinkers, giving additional questions to ponder and a different perspective on some of the questions being discussed. Hossenfelder definitely has opinions on the plausibility of various ideas put forth in the book, but she recognizes that in many cases her guess is as good as another — because they all lie outside of what can be tested.

Existential Physics is a fun book! My book club has really enjoyed it, and there have been very lively and entertaining discussions of the book’s contents and topics like free will, multiverses, and Einstein’s relativity. Hossenfelder does a good job of presenting the basic ideas of physics for the layperson, though it is helpful if the reader has at least some passing familiarity with things like relativity and quantum weirdness.

Perhaps one of the best compliments I can give for the book is that I learned a lot while reading it, and it gave me a lot to think about! I’m sure I’ll be pondering some of the deep questions presented for the rest of my days. I can wholeheartedly recommend Existential Physics.