#### 7-minute read

I guess it was inevitable that in my wider reading on subjects such as astronomy and physics I would eventually bump into quantum mechanics. Where I have encountered it so far, I have admitted it went straight over my head. It might thus seem foolhardy for a biologist to try and tackle a book like this. Then again, the hallmark of good communicators is that they make complex topics understandable. And theoretical physicist Sean Carroll’s previous books have been lauded, some even winning prizes. Are you ready to get down and dirty with quantum mechanics?

Carroll’s opening salvo, and one of the main reasons to write this book, is that physicists do not understand quantum mechanics. That’s right. They know how to use it to design new technology or predict the outcomes of experiments, but they do not truly understand it. And for a theory that was pretty much formulated by 1927 that is more than a bit embarrassing. What is worse, Carroll writes, is that many physicists scoff at the idea of spending time and effort on understanding its foundations.

Carroll thinks we have made good progress with this, though, and throughout this book, he champions the Everett or Many-Worlds formulation of quantum mechanics that was developed in 1957 by Hugh Everett.

This is where we reach a fork in the road. I could go ahead and try and summarize the ideas put forward in this book, leaving you with the impression that I understood everything he is talking about.

### “Carroll’s opening salvo, and one of the main reasons to write this book, is that physicists do not understand quantum mechanics. That’s right.”

I could write about the contrast between classical mechanics where particles have a location and a velocity, and quantum mechanics where these are replaced by a cloud of probability. Here, particles exist in a superposition of all possible locations and velocities that only take on a single value when we observe them, i.e. quantum wave functions appear particle-like when observed. (Actually, that bit I did understand.) I could write of Werner Heisenberg’s uncertainty principle and Niels Bohr’s concept of complementarity: particles have a wave function and you can describe either a particle’s position or its momentum (its velocity), but not both simultaneously. This was empirically shown by the famous double-slit experiment.

I could write about quantum entanglement and whether or not, depending on which theorem you follow, particles interacting at a distance means that information is travelling faster than light. Or about Everett’s assertion that the universe can be described by a single wave function, evolving according to the Schrödinger equation. Or the fact that parts of the universe interacting (two particles for example) lead to them becoming entangled. That this is called decoherence and, according to Everett, means the wave function branches into multiple worlds, representing all the different ways in which such interactions could have played out. And that the space of all possible wave functions is called the Hilbert Space which may or may not have an infinite number of dimensions (I apologise in advance to any physicists reading this if I have butchered above brief summaries).

I could do all of this. But I won’t. It will probably bring a smile to Carroll’s face if I say that I would like to think that there is a branch of the wave function where I did write that review.

### “[…] nature is quantum by definition – classical physics is what we observe because our brain cannot observe wave functions, the same way four-dimensional space flummoxes our brain.”

The truth is that most of the concepts explained here still seem esoteric, abstract, and counterintuitive to me. Partially that is the nature of the beast. As Carroll writes, nature is quantum by definition – classical physics is what we observe because our brain cannot observe wave functions, the same way four-dimensional space flummoxes our brain. But we can still calculate and theorise with it. And partially it reflects that even physicists are still trying to wrap their heads around all of the above.

But here are three reasons why this did not matter to me and why, despite the mental gymnastics involved, I still enjoyed this challenging book.

First, even though this book is not a history book, Carroll gives a short overview of how these ideas developed and, importantly, corrects common stories told about, for instance, Albert Einstein. Such as that by the time of the 1927 Solvay conference, where leading physicists met to discuss quantum theory, Einstein had grown old and conservative in his ideas, lost the debates with Bohr, and could not get to grips with Heisenberg’s uncertainty principle. That is not how Carroll reads this history.

### “The universe does not copy itself at every interaction like a foam bath bubbling over. “Branches” and “worlds” are just convenient shorthands for us humans to talk about the evolving wave function of the universe.”

Second, in spite of the mind-bending concepts, Carroll employs excellent metaphors and examples, uses handy diagrams, and even includes a staged dialogue between a theoretical physicist daughter and her sceptical particle physicist father. And he is balanced enough to consider some alternatives to Everett’s Many-Worlds formulation. All of this makes matters if not understandable at least graspable. I found myself reading along and thinking “I sort of see where you are going with this”. Carroll’s casual writing style and occasional humour help much in this regard. And though he throws in the occasional equation as an illustration, he does not bog the reader down with impenetrable calculations.

Third and final, Carroll devotes space to providing a reality check on what quantum mechanics actually means. Over the decades, a lot of ideas have leaked into mainstream thinking where they have started leading a life of their own and are being taken out of context. Does the Many-Worlds formulation mean that there are a near-infinite number of copies of you and me out there, each corresponding to all the possible choices we could have made at every turn in our lives? Not quite. The universe does not copy itself at every interaction like a foam bath bubbling over. “Branches” and “worlds” are just convenient shorthands for us humans to talk about the evolving wave function of the universe.

*Something Deeply Hidden* is a book that pleasantly stretches the mind. It even makes previous books I have read, such as *The Case Against Reality*, a bit more understandable. Although, as an aside, my impression is that Hoffmann’s assertion of conscious agents being at the base of everything (Carroll calls it idealism on pages 223-224) is totally at loggerheads with Carroll’s understanding of quantum mechanics. Even though the finer points might still elude me (perhaps I should take it down a notch and pick up *Quantum: A Guide For The Perplexed*), Carroll convinces that the study of quantum foundations is a worthwhile and interesting academic pursuit. And given his pleasant writing style, I am certainly tempted by his other book, *The Big Picture*.

Disclosure: The publisher provided a review copy of this book. The opinion expressed here is my own, however.

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