Gravity's Century From Einstein's Eclipse to Images of Black Holes

When the movie Interstellar was released in 2014, I thought its depiction of a black hole was one of the most hauntingly beautiful scenes. And with input from prominent astrophysicist Kip Thorne, there was plenty of science to this piece of science fiction (see The Science of Interstellar). Amazingly, we only had to wait five more years for an actual image of a black hole – or really its event horizon – to be published. But these astounding images have been a long time coming. With Gravity’s Century, science writer Ron Cowen traces the story back to Albert Einstein and provides an accessible and compact overview of the century-long quest in physics to better understand gravity.

Cowen starts off gently enough, introducing Einstein’s general theory of relativity. Although a cornerstone of modern physics, its descriptions of reality seem counterintuitive in comparison to our day-to-day experience of how the world functions. And thus Cowen has the unenviable task of explaining such concepts as the constant speed of light (no matter how fast an observer tries to race along with it), time dilation, spacetime fabric and how it warps, gravitational lensing, and, of course, black holes. But, with useful diagrams and clear examples, clarifies them he does.

The first few chapters focus on Einstein and his theory. Cowen provides a pleasant mix of relevant biographical information and science history, charting how the thinking of Einstein and others developed. He paints a very human portrait of the man, mentioning both his triumphs and his tribulations. Einstein’s theory required that he immerse himself in an unfamiliar branch of mathematics, something he struggled with tremendously. He made mistakes, sometimes stubbornly opposing others, only to later come around to their point of view (e.g. on the question of whether the universe is expanding or not). And he did not think much of black holes. Nobody ever said brilliant minds never get the wrong end of the stick.

For all his great theorizing, it fell to others to test Einstein’s ideas in practice, and that is the other main thread in Cowen’s story. One of the first ideas to be tested was gravitational lensing, that is, whether gravity can bend light. Or, really, as Cowen clarifies, whether spacetime near a massive object is curved. One way to check this is to compare the position of stars in the sky when they appear close to the Sun versus when they do not. Doing that successfully requires the rare conditions afforded by a solar eclipse when the Sun’s bright light is momentarily prevented from drowning out the much fainter light of distant stars. Against the backdrop of World War I, Cowen provides a vivid story of the efforts involved back in 1919 to try and do this, reminding the reader just how much the state of infrastructure and technology meant this was a long and laborious process.

From there, Cowen walks the reader through other milestones and conjectures, such as the accidental discovery of the sought-after cosmic microwave background (the leftover heat of the Big Bang), further observations of gravitational lensing, the formulation of dark matter and dark energy (see e.g. The 4 Percent Universe), the discovery of black holes (see e.g. Black Hole and Einstein’s Monsters) and the realisation that one of them lies at the heart of our own galaxy (see Revealing the Heart of the Galaxy). Cowen also provides detailed coverage of the more recent detection of gravitational waves (see e.g. Einstein's Unfinished Symphony, Black Hole Blues and Other Songs from Outer Space, Ripples in Spacetime, and Gravity’s Kiss), before coming to the current pièce de résistance: the collaborative effort by the Event Horizon Telescope to image the spectacular light show that occurs right at the edge of a black hole. (As a side note, it looks like this book went to the printers just before that first image was released on April 10th)

Cowen manages to cover all these exciting topics at a brisk clip in just over 160 pages, with nary an equation in sight. And where they appear, he explains their working and relevance, such as Einstein’s famous equation (no, not that one, this one). Most chapters feature short “deeper dive” sections which go into just that little bit more technical detail. As a biologist who was last taught physics in high school twenty years ago (plus whatever I have gleaned from subsequent reading), I found the writing accessible.

The only chapter that went a bit over my head in places was that on quantum gravity. But perhaps I should take heart from the fact that even Einstein did not succeed in unifying gravity and quantum theory (see also the just published Something Deeply Hidden). But even here, Cowen is enlightening. I had heard of the models that postulate that the universe is a hologram. But what does that really mean? Well, not that we are living in The Matrix. The analogy Cowen mentions of a three-dimensional video game being read off a two-dimensional chip was very helpful and gave me a basic idea of what some theorists mean when they conjecture that all actions and physical laws of our four-dimensional universe are governed by a system that resides on the boundary of the cosmos.

Gravity’s Century is a great introduction to Einstein’s theory of general relativity and the century of research that has been testing his ideas since. Cowen is an enthusiastic storyteller that knows how to communicate complex topics effectively. Readers new to the fields of cosmology and astrophysics are well served with this springboard.