Newton explained that apples fell from trees by virtue of the same universal attractive natural force that caused the planets to orbit the skies.
In his 1687 book, Philosopiae Naturalis Principia Mathematica, Newton presented complex mathematical formulae that described the observed orbits of the known planets fairly accurately. Newton also provided an explanation for why the attractive force of gravity did not cause the planets to fall in on themselves the way the apple falls to the ground. Since all the planets and stars in the universe exerted mutually attractive force and because there were an infinite number of planets distributed uniformly throughout the universe, there was no "center" of the universe and the planets and stars are all pulled in many directions, all of which, in effect, cancel out their tendency to fall together (Hawking, 1991).
Almost eighty years earlier, in 1609, Galileo Galilei invented the world's first optical telescope and began carefully observing the planets in the solar system. Galileo noticed immediately that like earth, the planet Jupiter also seemed to have moons orbiting it, which directly conflicted with the traditional belief that all objects in the solar system orbited the earth in the explanation supported by the church. In 1632, Galileo published his Dialogue Concerning the Two Chief World Systems, with the permission of the pope, because Galileo had agreed not to favor the Copernican view over the model proposed by Aristotle, as well as to conclude that God's work could never be fully understood by man (Feynman, 1995; Hawking, 2002). When the book had the effect of convincing people that Copernicus had been right all along, the pope condemned the work, brought Galileo before the inquisitors, forced him to publicly denounce his mistaken views, and condemned the world's first astronomer to house arrest for the remainder of his life (Hawking, 2002).
Albert Einstein and the Modern Theory of Gravity:
In 1905, an obscure Swiss patent clerk named Albert Einstein conducted a series of revolutionary thought experiments in which he envisioned himself riding along on a beam of light in conjunction with which he published a scientific paper on what he called Special Relativity, based on the principle that the speed of light is a constant on nature and that the observations of a hypothetical traveller on a beam of light would differ substantially from those of a stationary observer in a different relative position with respect to that beam of light (Feynman, 1995; Goldsmith, 1997). As a consequence of the implications of Einstein's special relativity, time is linked to the constant, unchanging speed of light and decreases with increased velocity with respect to a stationary observer; moving objects grow in mass while shrinking in size in the direction of travel and in proportion to their velocity; and nothing can ever reach the speed of light because its mass would become infinitely large while it would contract to zero size, and time would stop altogether (Goldsmith, 1997). Finally, special relativity also solved one of the problems with Newtonian mechanics by reconciling the observed deviations in the orbit of the planet Mercury
Twelve years later, Einstein published his General Theory of Relativity that applied the implications of the speed of light and special relativity to gravity and also suggested that matter and energy were different forms of the same thing (Goldsmith, 1997; Hawking, 2001). In principle, general relativity fundamentally changed the way that we understand gravity because it describes gravity as a property of space-time rather than an independent force that acts on planets and objects in a mechanical way. While it is an imperfect representation, that property of space-time is often depicted as a flexible, rubber-like grid in which larger planets "warp" the very shape of space, causing the orbital phenomenon of planetary motion (Hawking, 2001; 2002).
Einstein spent the rest of his life trying to unify gravity with the other three fundamental forces (electromagnetism, and the strong and weak nuclear forces). More recent theories of gravity have succeeded in mathematically unifying only the weak nuclear force and electromagnetism, but not gravity and the strong nuclear force. Contemporary science suggests that at the subatomic level, gravity is actually a property attributable to subatomic particles called Gravitons (Hawking, 2001; 2002).
Feynman, R. (1995). Six Easy Pieces. New York: Helix.
Goldsmith, D. (1997). The Ultimate Einstein. New York: Simon & Schuster.