The Big Idea: Sean Carroll

I’ve been aware of the “Many Worlds” interpretation of physics for some time — longtime readers of mine know it’s intimately connected with space travel in my “Old Man’s War” series of novels. But in the real world, how does it connect to the actual physics we know and (profess to) understand? Actual physicist Sean Carroll knows, and in his new book, Something Deeply Hidden, he delves right into it.


If there’s one thing the world doesn’t need, it’s another book on quantum mechanics. I mean, who hasn’t written one? In preparation for writing Something Deeply Hidden, I searched on Amazon for books with titles of the form “Quantum X,” and was rewarded with Quantum Eating, Quantum Touch, Quantum Leadership, and many more.

The existence of these books reflects the widespread conviction that quantum mechanics, however scientists might think about it, is fundamentally profound and deeply mysterious, so it could mean just about anything. You might expect to find a countervailing stream of books by sober-minded physicists and science writers, doggedly explaining that quantum mechanics isn’t really all that inexplicable after all. It’s science, not mysticism.

That’s not exactly what you find. Sure, there are valiant attempts to dispel the worst kinds of quantum woo. But even the most hardnosed quantum books seem to agree that the subject is unavoidably murky, something so bizarre and ill-understood that it’s not meant to be grasped by mere human beings. This or that quantum phenomenon is trotted out, accompanied by an implicit shaking of the head – “Can you believe it? This makes no sense at all!”

So my big idea for Something Deeply Hidden was: quantum mechanics is understandable.

To be clear, the challenge is not just that quantum mechanics is complicated or recondite, like general relativity or the standard model of particle physics. It’s that physicists themselves, who are supposed to be experts, don’t understand it, and in their more honest moments they admit it.

Quantum mechanics sits at the absolute heart of all of modern physics; it’s the deepest, most important idea that physicists have. But what we teach our students, as philosopher Tim Maudlin has put it, is a recipe, not a theory.

We can set up a quantum system, like an electron orbiting the nucleus of an atom. And we can measure something about it, like its position. The quantum recipe tells us the probability of getting any particular measurement outcome. And that recipe has been tested to enormous precision, and has come through with flying colors every time.

What we can’t actually tell you is what happens when you measure a quantum system. What counts as “measuring”? How quickly does it happen? Do you have to be conscious?

All of these questions are, in the standard textbook formulation, left entirely vague. The resulting recipe is good enough for government work, or for building incredibly complex technologies, but it falls well short of the clarity and rigor we would expect of a well-defined scientific theory.

Thus, in the words of Richard Feynman, “I think I can safely say that nobody understands quantum mechanics.”

At least, nobody thinks that other physicists understand quantum mechanics. Some of us, including myself, think we do understand the basics. The problem is that there is more than one honest, rigorous physical theory that reproduces the textbook quantum recipe in the appropriate regime. So we have multiple approaches, and have to decide which one is the best description of Nature; but that’s what scientists are always supposed to do.

So in the book I explain my favorite approach to quantum mechanics, the Many-Worlds formulation. It has a bad reputation, as it sounds a little science-fiction-y, or at least like you’re tacking on a bunch of extra stuff (entire universes worth) just to solve an irritating problem in quantum measurement. But the truth is the opposite: the theory is lean and mean, getting enormous mileage out of very few basic assumptions. The extra worlds are predicted by the theory, not tacked onto it.

One of the goals of Something Deeply Hidden is to make all that clear. But the broader, more important message is the one above: that quantum mechanics is understandable. Maybe my favorite understanding will turn out to be the right one, or maybe one of the various competitors. But they’re all ultimately intelligible, not ineffable.

It took me a while to come to this conclusion. I didn’t start out to be a rebel, fighting against the entrenched establishment of physicists who don’t want to face up to the quantum measurement problem. But the more I’ve worked in the field, and the more I’ve thought about it myself, the more irritating and embarrassing it is that we haven’t figure out quantum mechanics once and for all, and for the most part we haven’t even been trying. I’m hoping my book does its small part in changing that.


Something Deeply Hidden: Amazon|Barnes & Noble|Indiebound|Powell’s

Read an excerpt. Visit the author’s site. Follow him on Twitter.

17 Comments on “The Big Idea: Sean Carroll”

  1. Pleasantly surprised to see Sean Carroll featured in your Big Idea series. I’ve been reading and listening to him for years now and have found him to be smart, honest, unflappable, thorough, funny and compassionate. I haven’t read this one yet, but it’s on my reading list. He has a knack for writing in a way that is easy to read, much like John in that respect, and also has a knack for explaining complex science in a way that allows a general audience to get some glimmer of an understanding.

  2. I’m an on and off reader here, but is this the first time a nonfiction book has gotten a Big Idea slot?

  3. “the Many-Worlds formulation. It has a bad reputation”

    Well, its been out doing god knows what until the wee hours of the morning, comes home loopy as the day is long, sleeps all day, cant keep a job, wont move out of its parents basement, and may have gotten that poor girl from the wrong side of the galaxy pregnant.

    I mean, doesnt it kinda *deserve* a bad rap?

  4. a) the Woo Theory fundamental particle is the ‘wargle’. It’s anti-particle is known as the ‘bargle’. The Weakminded Force that binds them is mediated by Whackalons and they group together to form Bozons – not to be confused with the more sensible Bosons of normal matter.
    b) why oh why oh why did this stream of me-consciousness have to end up in the world group where Trump, Johnson, Bolsonaro, Orban etc didn’t get eaten by Mr Angleton?

  5. Well, if a hundred monkeys on typewriters can eventually write Shakespeare, it seems logical that the quantum foam eventually burbling along chaotically will just happen to fall into a configuration that wakes up, looks around, observes the universe and determines it by saying “Let there be light” — said happenstance-created observer locking all the stuff we can see into stable existence.

    “It’s not the physical act of measurement that seems to make the difference, but the “act of noticing”, as physicist Carl von Weizsäcker (who worked closely with quantum pioneer Werner Heisenberg) put it in 1941.”

  6. [Deleted because not only off-topic but referring to something that didn’t even originate on the site — JS]

  7. I do have Nick Herbert’s 1985 book Quantum Reality, which spends some time on Bell’s Inequality, and describes eight different interpretations (of which Many-Worlds is just one), while also noting that all eight of them are actually consistent with the math or ‘recipes’ of quantum mechanics (though some, like Bohmian ‘neorealism’, are difficult to make consistent due to the faster-than-light communication.required to do so). Granted, most scientists really don’t like the von Neumann ‘consciousness creates reality’ approach, but it is still consistent with the math.

    Ethan Siegel, of the ‘Starts With a Bang’ blog, really doesn’t seem to like the Many Worlds interpretation, but some of his articles seem to have confused ‘many worlds’ arising out of quantum possibilities with ‘many worlds’ arising from different Big Bang/inflationary scenarios starting from the same background, which aren’t the same thing.

    For all its other flaws, James P. Hogan’s The Proteus Operation seemed to be operating on a decent version of the Many Worlds Interpretation, where a world was essentially a point riding a quantum wavefront, and any lost connection between worlds would be impossible to re-establish precisely.

  8. The problem I have with Sean Carroll is that he confuses epistemology and metaphysics. He uses the words “wave function” to mean the mathematical description of reality, but he also uses it as a short-hand for reality itself – as though reality is just a mathematical equation. It’s not always possible to tell which he is talking about. One hopes that this is more clear in the book.

    OTOH, I find his arguments for the Everett formulation compelling and the discussions on his podcast fascinating. I’ll never grasp the mathematics, but I agree with him that the reality which quantum mechanics describes needs to be better understood and that we won’t make progress if we don’t try.

  9. @jayarava: “…. as though reality is just a mathematical equation

    But surely, it is!

    I’m a bit disappointed, as I read this thinking it was a discussion of the Big Idea in an SF novel, and I thought “this is my kind of SF!” But, I’ll be just as happy to read it as non-fiction. I’ve long been disappointed that the Many Worlds interpretation “has a bad reputation”, because I have always thought it made the most sense. Occam’s Razor and all that.

    @Jenna: Yeah, most of Hogan’s flaws are due to the fact that he was an utter nut job, but I still love his novels, and The Proteus Operation is a favourite

  10. @Greg
    I worked for a startup that had prospects like that. Lots of fun, but when the wave function collapsed so did we.

  11. I wonder if this will be anything like “We Have No Idea” by Jorge Cham and Daniel Whiteson, which did a similar thing. Break down really complicated concepts in Quantum Theory and Dark Matter into something easier to understand. I really enjoyed that one, maybe I’ll try this too.

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