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10Gravity
Cruise your radio dial today and try to find any popular song that
would have been imaginable without Louis Armstrong. By introducing solo
improvisation into jazz, Armstrong took apart the jigsaw puzzle of popular
music and fit the pieces back together in a different way. In the same way,
Newton reassembled our view of the universe. Consider the titles of some
recent physics books written for the general reader: The God Particle,
Dreams of a Final Theory. When the subatomic particle called the neutrino
was recently proven for the first time to have mass, specialists in cosmology
began discussing seriously what effect this would have on calculations of the
ultimate fate of the universe: would the neutrinos’ mass cause enough extra
gravitational attraction to make the universe eventually stop expanding and
fall back together. Without Newton, such attempts at universal understand-
ing would not merely have seemed a little pretentious, they simply would
not have occurred to anyone.
This chapter is about Newton’s theory of gravity, which he used to
explain the motion of the planets as they orbited the sun. Whereas this
book has concentrated on Newton’s laws of motion, leaving gravity as a
dessert, Newton tosses off the laws of motion in the first 20 pages of the
Principia Mathematica and then spends the next 130 discussing the motion
of the planets. Clearly he saw this as the crucial scientific focus of his work.
Why. Because in it he showed that the same laws of motion applied to the
heavens as to the earth, and that the gravitational force that made an apple
fall was the same as the force that kept the earth’s motion from carrying it
away from the sun. What was radical about Newton was not his laws of
motion but his concept of a universal science of physics.
Gravity is the only really important force on the cosmic scale. Left: a false-color image of saturn’s rings, composed of innu-
merable tiny ice particles orbiting in circles under the influence of saturn’s gravity. Right: A stellar nursery, the Eagle Nebula.
Each pillar of hydrogen gas is about as tall as the diameter of our entire solar system. The hydrogen molecules all attract
each other through gravitational forces, resulting in the formation of clumps that contract to form new stars.