Paul Halpern
Basic Books (US, out now; UK, 8 February)
IN THE battle for the public imagination, the multiverse reigns supreme. Fuelled by the Marvel Cinematic Universe and other franchises, the idea that an infinite number of universes might exist alongside ours (and may even be contactable) has never been so popular. Why do we find this concept endlessly fascinating? And how promising is multiverse science?
Paul Halpern, a professor of physics at Saint Joseph’s University, Pennsylvania, tackles these questions in The Allure of the Multiverse: Extra dimensions, other worlds, and parallel universes. In it, he explains how and why the multiverse came to be taken seriously in some quarters.
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He also corrects a wealth of misconceptions, chief among which is the notion that there is just one way of thinking about the multiverse. Perhaps fittingly, rather than adherents having settled on one story, they are divided among many possibilities – none of which quite resembles the dimension-hopping antics seen on screen.
These options include the many-worlds interpretation, in which all possible outcomes of a quantum measurement are realised in some universe, and eternal inflation, in which the universe’s expansion continues forever in some regions, leading to a multiverse. What’s more, these multiverse ideas are an island of misfit toys in a sea of more mainstream physics.
The idea of parallel, unreachable worlds stretches back all the way to the ancient Greeks. Under the Pythagorean view of cosmology, Earth was thought to orbit a “Central Fire” that obscured humanity’s view of a “Counter-Earth” on the other side. More recently, the 19th-century socialist Louis-Auguste Blanqui argued in his treatise Eternity Through the Stars that, in a universe of unlimited size but with a finite number of elements, identical and near-identical Earths were bound to exist somewhere.
Modern formulations have sprung up in the past century. Halpern reveals how multiverses have emerged as potential solutions to the biggest questions in physics, such as the hunt for a theory of everything.
Take the observer effect. In quantum mechanics, a wave function represents the many possible states of a given particle. When that particle is measured by an observer, the wave function collapses into a single state, which is the reality that the observer experiences.
Yet what constitutes an observer remains debated, with detractors seeing this as a fundamental flaw in the concept. Some feel that the branching universes of the many-worlds interpretation solve this issue: here, copies of the observer experience each possible outcome, all encompassed by a universal wave function that never collapses.
It might seem surprising to find eminent physicists on both sides of this debate, but Halpern links this fissure to a wider question: should scientists focus on observable, verifiable pursuits or seek out that which is invisible – for now?
His own view is “cautiously open-minded”: he doesn’t rule out a possible multiverse, but argues that there “needs to be a strong explanatory benefit that overcomes the stark disadvantage of a lack of direct detectability” if such notions are to be taken seriously.
The Allure of the Multiverse is ambitious. While Halpern is a lively, clear writer who includes helpful analogies for difficult concepts, the sheer volume of information he provides can overwhelm. Likewise, the “allure” of the book’s title suggests a greater focus on the cultural fascination with the multiverse than it provides, with this discussion largely limited to the book’s conclusion.
Nevertheless, this is a terrific guide to mind-bending physics and will reward rereading. Its complexity is admirable, and, in a cultural landscape where the multiverse is often oversimplified, I hope the teams behind the next multiverse-spanning blockbluster give it a read.
