The Clockwork Universe: Isaac Newton, the Royal Society, and the Birth of the Modern World - Edward Dolnick (2011)
Part III. Into the Light
Chapter 51. Just Crazy Enough
The second reason that the Principia was so baffling is more easily stated—the theory made no sense. This is not to deny that the theory of gravitation “works.” It works astonishingly well. When NASA sent a man to the moon, every calculation along the way turned out precisely as Newton would have forecast centuries before. Nor does the model break down when applied to the farthest corners of the universe or the largest structures in nature. A theory that Newton devised by pondering the solar system and its one sun turns out to apply to galaxies made up of billions upon billions of suns, galaxies whose existence was unknown in Newton’s day.
But early scientists found themselves bewildered even so. The problem was that the theory predicted, but it did not explain. Why do rocks fall? “Because of gravity,” the world has said ever since Newton, but that answer only pins a name to our ignorance. Molière long ago made fun of the doctor who explained that opium makes us sleepy because it has a “dormitive potency.” When Newton published the Principia, many scientists hailed his mathematics but denounced “gravity” as the same sort of empty explanation. They demanded to know what it meant to say that the sun pulled the planets. How did it pull them? What did the pulling?
Another difficulty cut deeper. Today we’ve grown accustomed to thinking of modern science as absurd and unfathomable, with its talk of black holes and time travel and particles that are neither here nor there. “We are all agreed that your theory is crazy,” Niels Bohr, one of the twentieth century’s foremost physicists, once told a colleague. “The question that divides us is whether it is crazy enough to have a chance of being correct.” We think of classical science, in contrast, as a world of order and structure. But Newton’s universe was as much an affront to common sense as anything that modern science has devised, and Newton’s contemporaries found his theory crazy, too.
One of the great mysteries of modern science is where consciousness comes from. How can a three-pound hunk of gray meat improvise a poem or spin out a dream? In Newton’s day, gravity was just as bewildering.52 How could it be that every hunk of matter pulls every other? Newton’s scheme seemed fantastically elaborate—the Alps pulled the Atlantic Ocean, which pulled back and pulled the Tower of London at the same time, which pulled Newton’s pen, which pulled the Great Wall of China. How could all those pulls also stretch to the farthest corners of space, and do so instantly? How does gravity snag a comet speeding outward past the farthest planets and yank it back toward us?
Every aspect of the picture was mystifying. Gravity traveled across millions of miles of empty space? How? How could a force be transmitted with nothing to transmit it? Leibniz was only one of many eminent thinkers who hailed the brilliance of Newton’s mathematics but scoffed at his physics. “He claims that a body attracts another, at whatever distance it may be,” Leibniz jeered, “and that a grain of sand on earth exercises an attractive force as far as the sun, without any medium or means.”
It was Newton’s notion of “action at a distance” that particularly galled Leibniz and many others. Newton agreed that there was no resolving this riddle, at least for the time being, but he put it to one side. “Mysterious though it was,” historian John Henry writes, by way of summarizing Newton’s view, “God could make matter act at a distance—to deny this was to deny God’s omnipotence.”
The skeptics were not so easily satisfied. Without some mechanism that explained how physical objects pulled one another, they insisted, this new theory of universal gravitation was not a step forward but a retreat to medieval doctrines of “occult forces.” Proper scientific explanations involved tangible objects physically interacting with other tangible objects, not a mysterious force that flung invisible, undetectable lassos across endless regions of space. Invoking God, said Leibniz, was not good enough. If gravity was a force that God brought about “without using any intelligible means,” then that would not make sense “even if an angel, not to say God himself, should try to explain it.”
Nor was the mystery merely that gravity operated across vast distances. Unlike light, say, gravity could not be blocked or affected in any way whatsoever. Hold your hand in front of your eyes and the light from a lamp on the other side of the room cannot reach you. But think of a solar eclipse. The moon passes between the Earth and the sun and blocks the sun’s light, but it certainly doesn’t block the gravitational force between Earth and sun—the Earth doesn’t fly out of its orbit. The force seems to pass through the moon as if it weren’t there.
The closer you examined Newton’s theory, the more absurd it seemed. Consider, for instance, the Earth in its orbit. It travels at a fantastic speed, circling the sun at about 65,000 miles per hour. According to Newton, it is the sun’s gravitational pull that keeps the Earth from flying off into space. Now imagine a giant standing atop the sun and swinging the Earth around his head at that same speed of 65,000 miles per hour. Even if the titan held the Earth with a steel cable as thick as the Earth itself, the steel would snap at once, and Earth would shoot off into the void. And yet, with no sort of cable at all, gravity holds the Earth in an unbreakable grip.
Seen that way, gravity seems incredibly powerful. But compared with nature’s other forces, like electricity and magnetism, it is astonishingly feeble. If you hold a refrigerator magnet a tiny distance from the fridge, the magnet leaps through the air and sticks to the door. Which is to say, the magnetic pull of a refrigerator door outmuscles the gravitational pull of the entire Earth.
It was just as hard to understand how gravity could cut instantaneously across the cosmos. Newton maintained that it took no time whatsoever, not the briefest fraction of a second, for gravity to span even the vastest distance. If the sun suddenly exploded, one present-day physicist remarks, then according to Newton the Earth would instantly change in its orbit. (According to Einstein, we would be every bit as doomed, but we would have a final eight minutes of grace, blithely unaware of our fate.)
None of this made sense. Kepler and Galileo, the first great scientists of the seventeenth century, had toppled the old theories that dealt with an everyday, commonsensical world where carts grind to a halt and cannonballs fall to Earth. In its place they began to build a new, abstract work of mathematical architecture. Then Newton had come along to complete that mathematical temple.
So far, so good. Other great thinkers of the day, such men as Leibniz and Huygens, shared those mathematical ambitions. But when those peers and rivals of Newton looked closely at the Principia, they drew back in shock and distaste. Newton had installed at the heart of the mathematical temple not some gleaming new centerpiece, they cried, but a shrine to ancient, outmoded, occult forces.
Curiously, Newton fully shared the misgivings about gravity’s workings. The idea that gravity could act across vast, empty stretches of space was, he wrote, “so great an absurdity that I believe no man who has in philosophical matters any competent faculty of thinking can ever fall into it.” He returned to the point over the course of many years. “To tell us that every Species of Things is endow’d with an occult specific Quality [like gravity] by which it acts and produces manifest Effects, is to tell us nothing.”
Except … except that the theory worked magnificently. Newton’s mathematical laws gave correct answers—fantastically accurate answers—to questions that had long been out of reach, or they predicted findings that no one had ever anticipated. No one until Newton had explained the tides, or why there are two each day, or why the Earth bulges as it does, or why the moon jiggles as it orbits the Earth.
Description and prediction would have to do, then, and explanation would have to wait. How the universe managed to obey the laws he had discovered—how gravity could possibly work—Newton did not claim to know. He would not guess.
He painted himself as the voice of hardheaded reason, Leibniz as the spokesman for airy speculation. When Leibniz rebuked him for proposing so incomplete a theory, Newton maintained that restraint was only proper. He would stick to what he could know, even though Leibniz talked “as if it were a Crime to content himself with Certainties and let Uncertainties alone.” Newton opted for caution. “Ye cause of gravity is what I do not pretend to know,” he wrote in 1693, “& therefore would take more time to consider of it.”
Twenty years later, he had made no progress. “I have not been able to discover the cause of those properties of gravity,” Newton wrote in 1713, “and I frame no hypotheses.” Another two centuries would pass before Albert Einstein framed a new hypothesis.
In the meantime, Newton declared his peace with his own considerable achievement. “And to us it is enough that gravity does really exist, and act according to the laws which we have explained,” he wrote, in a kind of grand farewell to his theory, “and abundantly serves to account for all the motions of the celestial bodies, and of our sea.”