IN 1665, an outbreak of plague forced Cambridge University to close. Isaac Newton, a 23-year-old mathematics student who had just earned his B.A., returned to his family's estate in Lincolnshire, where he spent the next 18 months thinking and studying alone. Those 18 months are without parallel in the past 1,000 years.
To begin with, Newton discovered the composition of light. He proved that white light is a blend of all colors and that color becomes visible only when light rays are refracted at different angles. All of modern optics builds upon Newton's discovery. He turned his attention to movement and inertia and worked out the three essential laws of motion. The most famous is the third, known to everyone: For every action, there is an equal and opposite reaction. All of modern physics grows out of Newton's laws. The mathematics required to derive these laws — which involve multiple variables with continuously changing quantities — did not exist in Newton's day. So he invented an entirely new mathematical discipline: the calculus. (Independently, the German scholar Gottfried Leibniz would later devise it as well.) Modern mathematics without the calculus is impossible to imagine.
Any of these achievements would have assured Newton's fame. But the heights he scaled were greater still.
In his garden one day, an apple really did fall. Newton pondered the force that pulled it to the earth. It was a force that seemed to operate even at great distances — an apple dropping from the highest tree imaginable would still hit the ground. How far out did this force reach? All the way to the moon? Yet the moon didn't fall to the earth, but traveled around it instead. Why?
The problem of celestial movement vexed the intellectuals of Newton's day. They envisioned a globe being swung on a chain, circling round and round, centripetal force holding it in a steady orbit. Cut the chain and the circling stops — the globe flies off in a straight line. But heavenly bodies don't fly off in straight lines. Though untethered by chains, they move in fixed orbits. How could that be?
Alone in Lincolnshire, Newton solved the puzzle: He discovered the law of gravity. The same force that pulls an apple to the ground holds distant planets in their paths. That was the chain linking the moon to the earth and the planets to the sun. Gravity couldn't be seen or touched, but it could be proved with an elegant mathematical formula. And that formula, Newton said, "allows me to explain the system of the world."
For nearly 20 years, Newton told no one of his discovery. When he finally published the Principia, his great treatise on motion and gravitation, the effect was seismic. Newton's discovery, says Alan Charles Kors, a renowned scholar of Western thought, "is one of the most extraordinary scientific syntheses in the history of the human mind."
The Principia rocked Western civilization, for it showed that the universe was lawful, logical
— above all, knowable. To a deeply pious Europe, it meant that mere mortals could perceive the very blueprint of Creation. To study the world empirically, to understand its workings, was to come closer to God. Newton's legacy was nothing less than the triumph of modern science.
In the words of Alexander Pope's famous epitaph:
"Nature and Nature's laws lay hid in night,
God said, Let Newton be! — and all was light."
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Jeff Jacoby is a columnist for The Boston Globe.
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