| It's easy to forget that before Newton, the motion of objects on Earth and in the heavens was almost totally unexplained, with many believing that our fates were determined by the malevolent designs of spirits and demons. Witchcraft, sorcery, and superstition were heatedly debated even at the most learned centers of learning in Europe. Science as we know it did not exist. Greek philosophers and Christian theologians, in particular, wrote that objects moved because they acted out of human-like desires and emotions. To the followers of Aristotle, objects in motion eventually slowed down because they got "tired." Objects fell to the floor because they "longed" to be united with the earth, they wrote. ... In 1666, when Newton was twenty-three years old, he banished the spirits that haunted the Aristotelian world by introducing a new mechanics based on forces. Newton proposed three laws of motion in which objects moved because they were being pushed or pulled by forces that could be accurately measured and expressed by simple equations. Instead of speculating on the desires of objects as they moved, Newton could compute the trajectory of everything from falling leaves, soaring rockets, cannonballs, and clouds by adding up the forces acting on them. This was not merely an academic question, because it helped to lay the foundation for the Industrial Revolution, where the power of steam engines driving huge locomotives and ships created new empires. Bridges, dams, and towering skyscrapers could now be built with great confidence, since the stresses on every brick or beam could be computed. So great was the victory of Newton's theory of forces that he was justly lionized during his lifetime, prompting Alexander Pope to acclaim: Nature, and Nature's laws lay hid in night, God said, Let Newton be! and all was light. Newton applied his theory of forces to the universe itself by proposing a new theory of gravity. He liked to tell the story of how he returned to the family estate of Woolsthorpe in Lincolnshire after the black plague forced the closing of Cambridge University. One day, as he saw an apple fall off a tree on his estate, he asked himself the fateful question: if an apple falls, then does the moon also fall? Can the gravitational force acting on an apple on Earth be the same force that guides the motion of heavenly bodies? This was heresy, since the planets were supposed to lie on fixed spheres that obeyed perfect, celestial laws, in contrast to the laws of sin and redemption that governed the wicked ways of humanity. In a flash of insight, Newton realized he could unify both earthly and heavenly physics into one picture. The force that pulled an apple to the ground must be the same force that reached out to the moon and guided its path. He stumbled upon a new vision of gravity. He imagined himself sitting on a mountaintop throwing a rock. By throwing the rock faster and faster, he realized that he could throw it farther and farther. But then he made the fateful leap: what happens if you throw the rock so fast that it never returns? He realized that a rock, falling continually under gravity, would not hit the earth but would circle around it, eventually returning to its owner and hitting him on the back of his head. In this new view, he replaced the rock with the moon, which was constantly falling but never hit the ground because, like the rock, it moved completely around the earth in a circular orbit. The moon was not resting on a celestial sphere, as the church thought, but was continually in free fall like a rock or apple, guided by the force of gravity. This was the first explanation of the motion of the solar system. Two decades later, in 1682, all of London was terrified and amazed by a brilliant comet that was lighting up the night sky. Newton carefully tracked the motion of the comet with a reflecting telescope (one of his inventions) and found that its motion fit his equations perfectly if it was assumed to be in free fall and acted on by gravity. With the amateur astronomer Edmund Halley, he could predict precisely when the comet (later known as Halley's comet) would return, the first prediction made on the motion of comets. The laws of gravity that Newton used to calculate the motion of Halley's comet and the moon are the same ones NASA uses today to guide its space probes with breathtaking accuracy past Uranus and Neptune. | — Michio Kaku, Einstein's Cosmos: How Albert Einstein's Vision Transformed Our Understanding of Space and Time (Great Discoveries) | Indexes/22 |
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