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Embrace the unknown in an exploration of the cosmos

The Unknown Universe: What We Don't Know About Time and Space in Ten Chapters

Stuart Clark
Head of Zeus
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Every clear night, 300 quadrillion particles of light are fired by an enormous laser at mirrors less than half a meter wide on the Moon, allowing scientists to measure the Moon’s orbital motion with extreme accuracy and precision. The handful of particles that make it back to Earth deliver vital information: whether or not Einstein’s theory of gravity continues to correctly describe nature. So far, it does. But as Stuart Clark suggests in The Unknown Universe, breakthroughs happen when a “brave scientist [throws] away a cherished assumption,” rather like Einstein himself did in overturning Newton’s understanding of gravity.

As the current gravitational framework has been verified to one part in 1013, the scientists conducting the lunar laser ranging experiment are certainly brave for maintaining confidence that there’s “a deeper theory of gravity to be found.” According to Clark, they exemplify “the true gold standard for science: constant self-questioning.”

The book’s scope is admirably broad: Instead of delving directly into modern cosmology, Clark journeys outward through concentric circles of human discovery of the cosmos, highlighting unknowns at every scale: from the Moon, whose origins are still uncertain, to the Sun, whose magnetic cycles remain baffling. He proceeds from our solar system—which, according to some astronomers, likely contains undiscovered giant planets—to galaxies and the universe at large, whose dynamics are so perplexing that scientists have invoked “dark matter” and “dark energy”—so named because of their wholly mysterious nature—as explanations. Clark’s narration is accessible to any reader with basic mathematical understanding, but it is also enriching for specialists, providing ample references to technical papers for further reading.

Multiple eras of human history inform Clark’s description, connected by common threads: continual subversion of established scientific dogma and deep, lingering questions. A central theme of the book is the incompleteness and necessary dynamism of science: that ever-shifting landscape where the only constant is the “unknown.” By emphasizing the “hole[s] in our understanding of the Universe,” Clark provides an incisive critique of scientific entrenchment: the tendency of some modern scientists to describe our theories as “almost perfect” even when mysteries remain.

A laser beam is fired into the night sky from NASA's Goddard Space Flight Center in Greenbelt, Maryland.lbirke

A laser beam is fired into the night sky from NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

Eschewing the simplistic “lone genius” narrative, Clark details a broad cast of players, emphasizing their interconnectedness. For example, he discusses how Isaac Newton’s foundational theory of gravity rested upon a serendipitous visit from Edmond Halley (of Halley’s Comet fame).

Female scientists could have been treated more prominently, however. For instance, while Clark lauds William Herschel for contributing to our understanding of the universe’s structure through his observations of galaxies, which he called “nebulae,” Hershel’s sister Caroline, who actually made the first observations of “nebulae” with William’s telescope, is described merely as her brother’s “amanuensis.” Jocelyn Bell, discoverer of pulsars—small, rapidly rotating remnants of exploded stars—is also conspicuously absent.

While the strength of the book is its comprehensive treatment of the history and nature of the astrophysical sciences, readers hungry for exotic theories will be sated. The book discusses other universes, 11-dimensional worlds, and the notion of a “timescape,” a universe where time runs differently in each location, such that some parts may be 5 billion years older than others.

Physics and cosmology are often triumphantly presented as staid things. Clark recounts their charged history and cites instances of scientific “hubris run aground,” instead painting a portrait of a universe that continues to surprise, defy understanding, and derail popular belief. The Unknown Universe reminds us that the quest for knowledge demands uncompromising skepticism and abundant humility alongside the insatiable curiosity that has always characterized the human heart.

About the author

The reviewer is at the Rudolf Peierls Centre for Theoretical Physics, University of Oxford, Oxford OX1 3NP, UK.