Originating in the 1950s, the many-worlds theory posits that parallel worlds constantly branch off from each other, moment by moment.Credit: Shutterstock

Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime Sean Carroll Oneworld (2019)

At the beginning of Something Deeply Hidden, Sean Carroll cites the tale of the fox and the grapes from Aesop’s Fables. A hungry fox tries to reach a bunch of grapes dangling from a vine. Finding them beyond his grasp, but refusing to admit failure, the fox declares the grapes to be inedible and turns away. That, Carroll declares, encapsulates how physicists treat the wacky implications of quantum mechanics.

The father of parallel universes

Carroll wants that to stop. The fox can reach the grapes, he argues, with the many-worlds theory. Originated by US physicist Hugh Everett in the late 1950s, this envisions our Universe as just one of numerous parallel worlds that branch off from each other, nanosecond by nanosecond, without intersecting or communicating. (The many-worlds theory differs from the concept of the multiverse, which pictures many self-contained universes in different regions of space-time.)

Six decades on, the theory is one of the most bizarre yet fully logical ideas in human history, growing directly out of the fundamental principles of quantum mechanics without introducing extraneous elements. It has become a staple of popular culture, although the plots of the many films and television series inspired by it invariably flout the theory by relying on contact between the parallel worlds, as in the 2011 movie Another Earth.

In Something Deeply Hidden, Carroll cogently explains the many-worlds theory and its post-Everett evolution, and why our world nevertheless looks the way it does. Largely because of its purely logical character, Carroll calls Everett’s brainchild “the best view of reality we have”.

Catch a wave

Quantum mechanics is the basic framework of modern subatomic physics. It has successfully withstood almost a century of tests, including French physicist Alain Aspect’s experiments confirming entanglement, or action at a distance between certain types of quantum phenomena. In quantum mechanics, the world unfolds through a combination of two basic ingredients. One is a smooth, fully deterministic wave function: a mathematical expression that conveys information about a particle in the form of numerous possibilities for its location and characteristics. The second is something that realizes one of those possibilities and eliminates all the others. Opinions differ about how that happens, but it might be caused by observation of the wave function or by the wave function encountering some part of the classical world.

Many physicists accept this picture at face value in a conceptual kludge known as the Copenhagen interpretation, authored by Niels Bohr and Werner Heisenberg in the 1920s. But the Copenhagen approach is difficult to swallow for several reasons. Among them is the fact that the wave function is unobservable, the predictions are probabilistic and what makes the function collapse is mysterious.

Hugh Everett (second from right) originated the many-worlds theory. (Also pictured, left to right: Charles Misner, Hale Trotter, Niels Bohr and David Harrison.)Credit: Alan Richards/AIP Emilio Segre Visual Archives

What are we to make of that collapsing wave? The equations work, but what the wave function ‘is’ is the key source of contention in interpreting quantum mechanics. Carroll outlines several alternatives to the Copenhagen interpretation, along with their advantages and disadvantages.

One option, the ‘hidden variables’ approach championed by Albert Einstein and David Bohm, among others, basically states that the wave function is just a temporary fix and that physicists will eventually replace it. Another tack, named quantum Bayesianism, or QBism, by Christopher Fuchs, regards the wave function as essentially subjective. Thus it is merely a guide to what we should believe about the outcome of measurements, rather than a name for a real feature of the subatomic world. Late in his life, Heisenberg proposed that we have to change our notion of reality itself. Reaching back to a concept developed by Aristotle — ‘potency’, as in an acorn’s potential to become an oak tree, given the right context — he suggested that the wave function represents an “intermediate” level of reality.

Einstein, Bohr and the war over quantum theory

Carroll argues that the many-worlds theory is the most straightforward approach to understanding quantum mechanics. It accepts the reality of the wave function. In fact, it says that there is one wave function, and only one, for the entire Universe. Further, it states that when an event happens in our world, the other possibilities contained in the wave function do not go away. Instead, new worlds are created, in which each possibility is a reality. The theory’s sheer simplicity and logic within the conceptual framework of quantum mechanics inspire Carroll to call it the “courageous” approach. Don’t worry about those extra worlds, he asserts — we can’t see them, and if the many-worlds theory is true, we won’t notice the difference. The many other worlds are parallel to our own, but so hidden from it that they “might as well be populated by ghosts”.

Branching cats

For physicists, the theory is attractive because it explains many puzzles of quantum mechanics. With Erwin Schrödinger’s thought experiment concerning a dead-and-alive cat, for instance, the cats simply branch into different worlds, leaving just one cat-in-a-box per world. Carroll also shows that the theory offers simpler explanations of certain complex phenomena, such as why black holes emit radiation. Furthermore, the theory might help to develop still-speculative ideas about conundrums such as how to combine quantum mechanics with relativity theory.

A realist takes on quantum mechanics

Something Deeply Hidden is aimed at non-scientists, with a sidelong glance at physicists still quarrelling over the meaning of quantum mechanics. Carroll brings the reader up to speed on the development of quantum physics from Max Planck to the present, and explains why it is so difficult to interpret, before expounding the many-worlds theory. Dead centre in the book is a “Socratic dialogue” about the theory’s implications. This interlude, between a philosophically sensitive physicist and a scientifically alert philosopher, is designed to sweep away intuitive reservations that non-scientists might have.

Nevertheless, non-scientists might have lingering problems with Carroll’s breezy, largely unexamined ideas about “reality”. Like many physicists, he assumes that reality is whatever a scientific theory says it is. But what gives physicists a lock on this concept, and the right to say that the rest of us (not to mention, say, those in extreme situations such as refugees, soldiers and people who are terminally ill) are living through a less fundamental reality? Could it be that we have to follow Heisenberg’s lead? That is, must we rely on tools for talking about the complexities of reality that philosophers have developed over millennia to explain why the fox has such a tough time reaching those grapes?

What a wacky idea.