Sean Carroll’s new (available in stores early September) book, Something Deeply Hidden, is a quite good introduction to issues in the understanding of quantum mechanics, unfortunately wrapped in a book cover and promotional campaign of utter nonsense. Most people won’t read much beyond the front flap, where they’ll be told:
Most physicists haven’t even recognized the uncomfortable truth: physics has been in crisis since 1927. Quantum mechanics has always had obvious gaps—which have come to be simply ignored. Science popularizers keep telling us how weird it is, how impossible it is to understand. Academics discourage students from working on the “dead end” of quantum foundations. Putting his professional reputation on the line with this audacious yet entirely reasonable book, Carroll says that the crisis can now come to an end. We just have to accept that there is more than one of us in the universe. There are many, many Sean Carrolls. Many of every one of us.
This kind of ridiculous multi-worlds woo is by now rather tired, you can find variants of it in a host of other popular books written over the past 25 years. The great thing about Carroll’s book though is that (at least if you buy the hardback) you can tear off the dust jacket, throw it away, and unlike earlier such books, you’ll be left with something well-written, and if not “entirely reasonable”, at least mostly reasonable.
Carroll gives an unusually lucid explanation of what the standard quantum formalism says, making clear the ways in which it gives a coherent picture of the world, but one quite a bit different than that of classical mechanics. Instead of the usual long discussions of alternatives to QM such as Bohmian mechanics or dynamical collapse, he deals with these expeditiously in a short chapter that appropriately explains the problems with such alternatives. The usual multiverse mania that has overrun particle theory (the cosmological multiverse) is relegated to a short footnote (page 122) which just explains that that is a different topic. String theory gets about half a page (discussed with loop quantum gravity on pages 274-5). While the outrageously untrue statement is made that string theory “makes finite predictions for all physical quantities”, there’s also the unusually reasonable “While string theory has been somewhat successful in dealing with the technical problems of quantum gravity, it hasn’t shed much light on the conceptual problems.” AdS/CFT gets a page or so (pages 303-4), with half of it devoted to explaining that its features are specific to AdS space, about which “Alas, it’s not the real world.” He has this characterization of the situation:
There’s an old joke about the drunk who is looking under a lamppost for his lost keys. When someone asks if he’s sure he lost them there, he replies, “Oh no, I lost them somewhere else, but the light is much better over here.” In the quantum-gravity game, AdS/CFT is the world’s brightest lamppost.
I found Carroll’s clear explanations especially useful on topics where I disagree with him, since reading him clarified for me several different issues. I wrote recently here about one of them. I’ve always been confused about whether I fall in the “Copenhagen/standard textbook interpretation” camp or “Everett” camp, and reading this book got me to better understanding the difference between the two, which I now think to a large degree comes down to what one thinks about the problem of emergence of classical from quantum. Is this a problem that is hopelessly hard or not? Since it seems very hard to me, but I do see that limited progress has been made, I’m sympathetic to both sides of that question. Carroll does at times too much stray into the unfortunate territory of for instance Adam Becker’s recent book, which tried to make a morality play out of this difference, with Everett and his followers fighting a revolutionary battle against the anti-progress conservatives Bohr and Heisenberg. But in general he’s much less tendentious than Becker, making his discussion much more useful.
The biggest problem I have with the book is the part referenced by the unfortunate material on the front flap. I’ve never understood why those favoring so-called “Multiple Worlds” start with what seems to me like a perfectly reasonable project, saying they’re trying to describe measurement and classical emergence from quantum purely using the bare quantum formalism (states + equation of motion), but then usually start talking about splitting of universes. Deciding that multiple worlds are “real” never seemed to me to be necessary (and I think I’m not the only one who feels this way, evidently Zurek also objects to this). Carroll in various places argues for a multiple world ontology, but never gives a convincing argument. He finally ends up with this explanation (page 234-5):
The truth is, nothing forces us to think of the wave function as describing multiple worlds, even after decoherence has occurred. We could just talk about the entire wave function as a whole. It’s just really helpful to split it up into worlds… characterizing the quantum state in terms of multiple worlds isn’t necessary – it just gives us an enormously useful handle on an incredibly complex situation… it is enormously convenient and helpful to do so, and we’re allowed to take advantage of this convenience because the individual worlds don’t interact with one another.
My problem here is that the whole splitting thing seems to me to lead to all sorts of trouble (how does the splitting occur? what counts as a separate world? what characterizes separate worlds?), so if I’m told I don’t need to invoke multiple worlds, why do so? According to Carroll, they’re “enormously convenient”, but for what (other than for papering over rather than solving a hard problem)?
In general I’d rather avoid discussions of what’s “real” and what isn’t (e.g. see here) but, if one is going to use the term, I am happy to agree with Carroll’s “physicalist” argument that our best description of physical reality is as “real” as it gets, so the quantum state is preeminently “real”. The problem with declaring “multiple worlds” to be “real” is that you’re now using the word to mean something completely different (one of these worlds is the emergent classical “reality” our brains are creating out of our sense experience). And since the problem here (classical emergence being just part of it) is that you don’t understand the relation of these two very different things, any argument about whether another “world” besides ours is “real” or not seems to me hopelessly muddled.
Finally, the last section of the book deals with attempts by Carroll to get “space from Hilbert space”, see here, which the cover flap refers to as “His [Carroll’s] reconciling of quantum mechanics with Einstein’s theory of relativity changes, well, everything.” The material in the book itself is much more reasonable, with the highly speculative nature of such ideas emphasized. Since Carroll is such a clear writer, reading these chapters helped me understand what he’s trying to do and what tools he is using. From everything I know about the deep structure of geometry and quantum theory, his project seems to me highly unlikely to give us the needed insight into the relation of these two subjects, but no reason he shouldn’t try. On the other hand, he should ask his publisher to pulp the dust jackets…
Update: Carroll today on Twitter has the following argument from his book for “Many Worlds”:
Once you admit that an electron can be in a superposition of different locations, it follows that person can be in a superposition of having seen the electron in different locations, and indeed that reality as a whole can be in a superposition, and it becomes natural to treat every term in that superposition as a separate “world”.
“Becomes natural” isn’t much of an argument (faced with a problem, there are “natural” things to do which are just wrong and don’t solve the problem). To me, saying one is going to “treat every term in that superposition as a separate “world”” may be natural to you, but it doesn’t actually solve any problem, instead creating a host of new ones.
Update: Some places to read more about these issues.
David Wallace’s book, The Emergent Multiverse.