Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime
By Sean Carroll
Dutton, 2019

When attempting to understand Sean Carroll’s new book Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime, we might as well start with every dog-lover’s favorite thought experiment: Schrödinger’s Cat. In the simplified version, and you know you’re in the deep end of the pool when even the thought experiment needs to be simplified, there’s a cat locked in a box with a vial of poison and a radioactive source. If a monitor inside the box detects radioactive energy, the vial is shatters and the cat dies a slow, horrible death. If no radioactivity is detected, the cat survives and may live in ill-tempered indolence for many years. An outside observer will only know which fate is real by opening the box; before that, the cat is in something called superposition, potentially either dead or alive. Its reality will only ‘collapse’ into one alternative or the other upon being observed. 

The physicist Erwin Schrödinger came up with his famous thought experiment as a way of illustrating one of the many peculiarities of quantum mechanics, and the cat in the box provides Carroll with a very well-known hypothetical to correct out of existence. The problem with the thought experiment, Carroll maintains, is that only the cat represents the quantum interpretation of reality; the observer deciding whether or not to lift the lid of the box is still existing very comfortably in the classical Newtonian world we see all around us. The central unnerving contention of quantum physics is that such a comfortable Newtonian world is at best an illusion. Carroll’s contention is that it’s not just a question of the poison, the cat, and the box; the observer is part of the same wave function affecting everything else. 

In Something Deeply Hidden, the equation Schrödinger developed to describe such wave functions rules the discussion (the book helpfully provides a stripped-down layman’s version of this equation, which will thoroughly baffle most of those laymen). “To a physicist,” Carroll writes, “a good equation makes all the difference.” 

Schrödinger’s equation describes the wave functions of quantum mechanics, and Carroll stresses, in a continuation and augmentation of earlier physicists such as Hugh Everett, that includes the collapse of those wave functions. In reference to the famous cat, a wave function collapses to a set of facts in which the observer has opened the box and the cat inside has died a gruesome, horrible death, but there’s a wave function collapse in which the observer has opened the box and been disappointed to find the cat still alive. At its essential kernel, this is the “Many Worlds” idea, that the resolution of each quantum wave function splits off a separate reality, what Carroll refers to as a ghost world. This world is conceptually necessitated by the operation of Schrödinger’s equation, and regardless of how it offends intuition, Carroll contends that it’s the simplest choice. “The Many-Worlds formulation of quantum mechanics removes once and for all any mystery about the measurement process and collapse of the wave function,” he writes. “We don’t need special rules about making an observation: all that happens is that the wave function keeps chugging along in accordance with the Schrödinger equation.”

Nevertheless, the intuition persists in being offended. Surely, it objects, any resolution that would rather posit the instantaneous formulation of countless new worlds than opt for a fundamental misunderstanding of one equation is not only ignoring Occam’s Razor but exchanging it for Occam’s Deluxe Riding Leaf-Blower? Surely, equation or no equation, there’s no actual proof for a ghost world and there could never be? 

Carroll’s book is so comprehensive, and he’s such a fantastic teacher, that he anticipates such objections: 

Whatever your feelings might be about Many-Worlds, its simplicity provides a good starting point for considering alternatives. If you remain profoundly skeptical that there are good answers to the problem of probability, or are simply repulsed by the idea of all those worlds out there, the task you face is to modify Many-Worlds in some way. Given that Many-Worlds is just “wave functions and the Schrödinger equation,” a few plausible ways forward immediately suggest themselves: altering the Schrödinger equation so that multiple worlds never develop, adding new variables in addition to the wave function, or re-interpreting the wave function as a statement about our knowledge rather than a direct description of reality. All of these roads have been enthusiastically walked down. 

As a smart and intensely readable undergraduate class in the history of quantum theory and the nature of quantum mechanics, Something Deeply Hidden could scarcely be improved. Carroll has a natural teacher’s knack for democratizing the conversation without sacrificing his own authority. I don’t believe for one femtosecond that possibility is the same thing as actuality; I don’t believe in ghost worlds and don’t think there’s a single word in Carroll’s book that should make anybody else believe in them. And I’d very much rather not believe there are a novemdecillion Schrödinger cats lounging around the multiverse because I turned left instead of right one morning. But since the Many Worlds by their very definition don’t and can’t impinge on my own visible reality in any way, I think of them exactly as I’ve always thought of Bester and Bradbury and Dunsany: it sure is fun to imagine.

Steve Donoghue is a founding editor of Open Letters Monthly. His book criticism has appeared in The Boston Globe, The Wall Street Journal, The Historical Novel Society, and The American Conservative. He writes regularly for The National, The Washington Post, The Vineyard Gazette, and The Christian Science Monitor. His website is http://www.stevedonoghue.com.