Physics builds from observations. No physical theory can succeed if it is not confirmed by observations, and a theory strongly supported by observations cannot be denied. For us, these are almost truisms. But early in the seventeenth century these lessons had not yet been learned. The man who first taught that observations are essential and supreme in science was Galileo Galilei.
Galileo first studied the motion of terrestrial objects, pendulums, free-falling balls, and projectiles. He summarized what he observed in the mathematical language of proportions. And he extrapolated from his experimental data to a great idealization now called the “inertia principle,” which tells us, among other things, that an object projected along an infinite, frictionless plane will continue forever at a constant velocity. His observations were the beginnings of the science of motion we now call “mechanics.”
Galileo also observed the day and night sky with the newly invented telescope. He saw the phases of Venus, mountains on the Moon, sunspots, and the moons of Jupiter. These celestial observations dictated a celestial mechanics that placed the Sun at the center of the universe. Church doctrine had it otherwise: Earth was at the center. The conflict between Galileo's telescope and Church dogma brought disaster to Galileo, but in the end the telescope prevailed, and the dramatic story of the confrontation taught Galileo's most important lesson.
Galileo died in 1642. In that same year, his greatest successor, Isaac Newton, was born. Newton built from Galileo's foundations a system of mechanics based on the concepts of mass, momentum, and force, and on three laws of motion. Newton also invented a mathematical language (the “fluxion” method, closely related to our present-day calculus) to express his mechanics, but in an odd historical twist, rarely applied that language himself.
Newton's mechanics had—and still has—cosmic importance. It applies to the motion of terrestrial objects, and beyond that to planets, stars, and galaxies. The grand unifying concept is Newton's theory of universal gravitation, based on the concept that all objects,