How We Brought Our Understanding of Gravity Down to Earth

Few things are as familiar and yet as elusive as gravity. You can’t see or hear it, though you constantly witness its effects. It’s everywhere, and it’s inescapable.

In December 1968, the Apollo 8 astronauts left the Earth’s gravitation grip for the first time — but they certainly didn’t escape gravity itself. They merely traded our planet’s gravitational pull for that of the moon: As they approached the moon, they eventually entered the zone in which the moon’s gravity dominates that of Earth.

BOOK REVIEW — “The Trouble With Gravity” by Richard Panek (Houghton Mifflin Harcourt, 256 pages).

The Earth’s pull is diminished, but it never reaches zero. Even the so-called “zero gravity” experienced by the crew of the International Space Station is a misnomer: There’s plenty of gravity where the ISS is; that’s what holds it in its orbit. (The crew doesn’t feel gravity’s effects because they, along with the station and everything in it, are “falling” at the same rate as one another, as they orbit the Earth.)

Ubiquitous as it may be, gravity has proven to be frustratingly difficult to comprehend — a paradox that lies at the heart of science writer Richard Panek’s new book, “The Trouble with Gravity: Solving the Mystery Beneath Our Feet.”

It wasn’t until the work of Isaac Newton, in the second half of the 17th century, that we really started to make sense of it. Panek points out that Newton kept two notebooks. In one, he confessed his various sins, including making pies on the Sabbath and having “uncleane thoughts words and actions and dreamese.” In the other one, he addressed “certain philosophical questions.”

Roughly speaking, he had a “God notebook” and an “earthly matters” notebook. These days science is a secular pursuit, but for Newton, understanding the workings of the universe couldn’t be separated from an attempt to understand — to borrow a phrase that Stephen Hawking would use some 300 years later — “the mind of God.” And so, for Panek, the two notebooks serve as a useful metaphor for Newton’s conflicting urges.

In explaining gravity — and along with it, the structure of the solar system — Newton was torn between a purely physical account and one which required, or at least reflected, the divine. As he would write in his masterwork, the “Principia,” the “most elegant system of the sun, planets, and comets could not have arisen without the design and dominion of an intelligent and powerful being.”

Newton was struggling to understand gravity, space, and time. Space, like God, seemed to permeate the universe — he described it as God’s “sensorium” — and gravity somehow held everything together.

Newton showed that gravity attracted every object to every other object, throughout the universe. He figured out the equation governing this attraction, which we now call Newton’s law of universal gravitation. But how, exactly, did gravity work? How could it reach across empty space? Newton, stymied, wrote: “I have not as yet been able to deduce from phenomena the reason for these properties of gravity, and I do not feign hypotheses.”

Newton’s “Principia” might be familiar territory for the sort of reader Panek is likely to reach; less well known is a peculiar book called “Somnium” (Latin for “The Dream”), written by German mathematician and astronomer Johannes Kepler, who pre-dated Newton by more than half a century. “Somnium,” written in 1608 (but only published later, by Kepler’s son), vividly imagines the people and creatures that might live on the Moon, as well as what the view of Earth would look like from so far away; it is, arguably, the first work of science fiction.

Isaac Newton’s copy of Principia, in which the author has written corrections for the second edition, resides in the Wren Library at Trinity College, Cambridge. Visual: Wikipedia Commons

Remarkably, Kepler gets the relevant physics more or less right, even though he’s clearly struggling to make sense of the force we now call gravity. (And if you think it’s hard to understand now, imagine how hard it was before Newton came along.) “Somnium” requires some context, and Panek provides it: Kepler, he explains, was writing at a time when reports of new lands were reaching European ears, and his audience was primed for stories about strange new worlds. Kepler’s trick was to place those new worlds above our heads. In this way, he was able to slip an astronomy lesson into a tale that reads like a fantasy novel. Clearly we’re in the history-of-science trenches here; to Panek’s credit, the story is entertaining enough that we don’t realize we’re in the trenches.

Readers who habitually plow through popular science books (especially those that focus on physics and astronomy) will guess where things are heading: We know we’ll eventually meet Einstein and curved space-time, and we know we’ll hear about black holes and gravitational waves. But Panek’s amicable, almost breezy style makes the journey, familiar as parts of it may be, fun.

Consider black holes. You know, or at least you’ve heard, that they have intense gravitational fields; their gravitational pull is so strong that not even light can escape. But what would you actually see if you fell into one? As it turns out, you’d see time “out there” — out beyond the edge of the black hole — speeding up. In fact, as you plunged to your doom, you’d witness “the entire future of the universe,” Panek writes. He justifiably dwells for several paragraphs on just how mind-boggling this is. By confessing his own amazement, he invites the reader to share in the adventure.

Black holes boggle the imagination — and so too does the so-called multiverse. The very idea of multiple universes has sparked controversy for decades, and today’s physicists are sharply divided over whether to take the idea seriously. But in Panek’s hands, the idea seems almost plausible: “Aristotle’s spheres defined our farthest view; then Galileo’s congeries of stars within our galaxy; then Hubble’s congeries of galaxies; then superclusters of galaxies, all hanging by a dark-matter thread; then a web of threads billowing from dark energy’s breath,” he writes. “Why not other universes?”

There are occasional heady ideas here, and the reader could easily get bogged down without a skilled tour guide. Panek largely succeeds, not by shouting facts with a megaphone, but by inviting us to follow along as he explores the alleyways of the often-convoluted history of the physical sciences — and he does so with humility and humor.

Often, I wanted to play along. Panek lists the movies in which gravity plays a key role (think of characters clinging for dear life to the edges of cliffs) and mentions “North by Northwest,” “Saboteur,” and “Vertigo”; I wanted to add “Blade Runner,” “Die Hard,” “Cliffhanger,” and (of course) “Gravity.” (I caught just one mistake: Panek tells of Einstein lecturing on general relativity to the Prussian Academy in the fall of 1915, but places the Academy in Vienna rather than Berlin.)

“The Trouble with Gravity,” not surprisingly, is about more than gravity; it’s about our continuing struggle to understand how the universe works. But the two are inexorably intertwined. Gravity governs the motion of everything from golf balls to galaxies. It has guided the first 14 billion years of cosmic evolution, and will guide its future as well. This book is a good place to get to know it a little better.

Dan Falk (@danfalk) is a science journalist based in Toronto. His books include “The Science of Shakespeare” and “In Search of Time.”

Dan Falk (@danfalk) is a science journalist based in Toronto and a senior contributor to Undark. His books include “The Science of Shakespeare” and “In Search of Time.”