The Allure of Many Worlds Robert P
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Originating in the 1950s, the many-worlds theory posits that parallel worlds constantly branch off from each other, moment by moment. QUANTUM PHYSICS The allure of many worlds Robert P. Crease enjoys Sean Carroll’s foray into a 60-year-old theory. t the beginning of Something Deeply the multiverse, which the theory by relying on contact between the Hidden, Sean Carroll cites the tale of pictures many self- parallel worlds, as in the 2011 movie Another the fox and the grapes from Aesop’s contained universes Earth. AFables. A hungry fox tries to reach a bunch of in different regions of In Something Deeply Hidden, Carroll SHUTTERSTOCK grapes dangling from a vine. Finding them space-time.) cogently explains the many-worlds theory beyond his grasp, but refusing to admit fail- Six decades on, the and its post-Everett evolution, and why our ure, the fox declares the grapes to be in edible theory is one of the world nevertheless looks the way it does. and turns away. That, Carroll declares, most bizarre yet fully Largely because of its purely logical charac- encapsulates how physicists treat the wacky logical ideas in human ter, Carroll calls Everett’s brainchild “the best implications of quantum mechanics. history, growing view of reality we have”. Carroll wants that to stop. The fox can directly out of the fun- Something Deeply Hidden: Quantum CATCH A WAVE reach the grapes, he argues, with the many- damental principles of Worlds and the worlds theory. Originated by US physicist quantum mechanics Emergence of Quantum mechanics is the basic frame- Hugh Everett in the late 1950s, this envisions without introducing Spacetime work of modern subatomic physics. It has our Universe as just one of numerous extraneous elements. SEAN CARROLL successfully withstood almost a century parallel worlds that branch off from each It has become a staple Oneworld (2019) of tests, including French physicist Alain other, nanosecond by nanosecond, without of popular culture, Aspect’s experiments confirming entangle- intersecting or communicating. (The many- although the plots of the many films and tel- ment, or action at a distance between certain worlds theory differs from the concept of evision series inspired by it invariably flout types of quantum phenomena. In quantum 30 | NATURE | VOL 573 | 5 SEPTEMBER 2019©2019 Spri nger Nature Li mited. All ri ghts reserved. ©2019 Spri nger Nature Li mited. All ri ghts reserved. BOOKS & ARTS COMMENT mechanics, the world unfolds through a combination of two basic ingredients. One is a smooth, fully deterministic wave func- Books in brief tion: a mathematical expression that conveys information about a particle in the form of Meat Planet numerous possibilities for its location and Benjamin Aldes Wurgaft UNIVERSitY OF CALIFORNIA PRESS (2019) characteristics. The second is something that In 2013, physiologist Mark Post wowed world media with a lab- realizes one of those possibilities and elimi- grown burger, cultured from bovine muscle cells at a cost of nates all the others. Opinions differ about nearly US$300,000. Yet that brave new food is still a fledgling how that happens, but it might be caused by biotechnology. Historian Benjamin Wurgaft explores this “small, observation of the wave function or by the strange world” in a thoughtful study mixing science reportage with wave function encountering some part of the philosophical meditations. He interviews Post, visits hotspots such as classical world. ‘cellular agriculture’ institute New Harvest in New York City, and even Many physicists accept this picture at face muses over the cultivation of human cells for consumption. In vitro value in a conceptual kludge known as the meat, he shows, opens horizons as much moral as environmental. Copenhagen inter- “Carroll argues pretation, authored that the many- by Niels Bohr and The Number of the Heavens worlds theory Werner Heisen- Tom Siegfried HARVARD UNIVERSitY PRESS (2019) is the most berg in the 1920s. The philosopher Aristotle saw the idea of multiple worlds as illogical. In straightforward But the Copenha- 1277, Étienne Tempier, bishop of Paris, countered by insisting that God approach to gen approach is could create any number of them. In this sparkling history, science understanding difficult to swallow journalist Tom Siegfried follows manifestations of the multiverse, from quantum for several reasons. Renaissance scholar Nicholas of Cusa’s worlds “without number” to mechanics.” Among them is the the “different heavens” theorized by René Descartes, the discovery of fact that the wave galaxies, inflationary cosmology and a pick-and-mix of multiverses, be function is unobservable, the predictions are they ‘quilted’ or holographic. Whether you find the concept liberating probabilistic and what makes the function or scientifically absurd, this is a mind-bending journey. collapse is mysterious. What are we to make of that collapsing wave? The equations work, but what the Higher and Colder wave function ‘is’ is the key source of con- Vanessa Heggie UNIVERSitY OF ChiCAGO PRESS (2019) tention in interpreting quantum mechan- The North Pole, South Pole and ‘third pole’, Mount Everest, were ics. Carroll outlines several alternatives to prime twentieth-century expeditionary challenges. To physiologists the Copenhagen interpretation, along with such as Nello Pace and Kåre Rohdal, they were also labs for their advantages and disadvantages. probing the physical impacts of extreme cold and altitude. But their One option, the ‘hidden variables’ scientific record, as medical historian Vanessa Heggie reveals in approach championed by Albert Einstein this insightful study, is overwhelmingly Eurocentric. The fieldwork and David Bohm, among others, basically sparked technological advances, but was tainted by racist ‘science’ states that the wave function is just a tem- and practice, from taking anthropometric measurements of Arctic porary fix and that physicists will eventually peoples to erasing Nepalese Sherpas’ contributions from papers. replace it. Another tack, named quantum Bayesianism, or QBism, by Christopher Fuchs, regards the wave function as essen- Opium tially subjective. Thus it is merely a guide to John Halpern and David Blistein HACHEttE (2019) what we should believe about the outcome Opium has been entwined with society for millennia. Here, of measurements, rather than a name for a psychiatrist John Halpern and writer David Blistein trace its path real feature of the sub atomic world. Late in from Mesopotamia through ancient Egypt, Greece and Persia, finally his life, Heisenberg proposed that we have to reaching Britain and the United States by the nineteenth century. Both change our notion of reality itself. Reaching countries then fomented opium wars with China. By the twentieth back to a concept developed by Aristotle — century, US “drug hysteria and race-based enforcement” was rife, ‘potency’, as in an acorn’s potential to become setting the stage for unwinnable drug wars. Now, the US opioid crisis an oak tree, given the right context — he sug- is killing tens of thousands a year. It is time, the authors argue, to treat gested that the wave function represents an addiction as a curable illness — and learn the lessons of history. “intermediate” level of reality. Carroll argues that the many-worlds theory is the most straightforward approach The Artist in the Machine to understanding quantum mechanics. It Arthur I. Miller MIT PRESS (2019) accepts the reality of the wave function. In Can artificial intelligence (AI) attain the intellectual prowess of, say, fact, it says that there is one wave function, astrophysicist Subrahmanyan Chandrasekhar? Arthur Miller probes and only one, for the entire Universe. Fur- that knotty question. Identifying seven hallmarks of creativity, he ther, it states that when an event happens in explores today’s AI innovation landscape. Phillip Isola’s app Pix2Pix our world, the other possibilities contained ‘translates’ one image to another; Rebecca Fiebrink’s Wekinator lets in the wave function do not go away. Instead, users make music with gestures; composer Eduardo Miranda plays new worlds are created, in which each pos- duets with a slime-mould biocomputer. There are AI-scripted films, sibility is a reality. The theory’s sheer sim- musicals, poetry — and, in Miller’s view, many reasons to be cheerful plicity and logic within the conceptual about computational creativity, now and in the near future. Barbara Kiser ©2019 Spri nger Nature Li mited. All ri ghts reserved. ©2019 Spri nger Nature Li mited. All ri ghts reserved.5 SEPTEMBER 2019 | VOL 573 | NATURE | 31 COMMENT BOOKS & ARTS framework of quantum mechanics ASTROPHYSICS 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, Zwicky: new lens on an 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 elusive astrophysicist be populated by ghosts”. Jaco de Swart enjoys a biography of the scientist who BRANCHING CATS pioneered findings on dark matter and supernovae. For physicists, the theory is attractive because it explains many puzzles of quan- tum mechanics. With Erwin Schrödinger’s ritz Zwicky advanced astronomy over presented his findings thought experiment concerning a dead- much of the twentieth century, pio- in the journal of the and-alive cat, for instance, the cats simply neering findings on constituents of Swiss Physical Soci- branch into different worlds, leaving Fthe cosmos from supernovae and neutron ety. However, it took just one cat-in-a-box per world. Carroll stars to dark matter and compact galaxies. another three dec- also shows that the theory offers simpler He worked at two of the world’s most his- ades for the phenom- explanations of certain complex phenom- torically important observatories: Mount enon to be observed ena, such as why black holes emit radia- Wilson and Palomar in California.