Gravity Is the Force Responsible

Gravity is the force responsible for the fact that dropped objects fall to earth and also for the orbital motion of the planets and all the stars in the solar system. Early man observed the motions of the planets and believed that they could be explained by the fact that the earth was the center of "God's" universe and that all the heavenly objects necessarily revolved about the earth. Religious authorities especially liked that explanation because it did not contradict biblical scripture and denounced theoretical models that suggested that the earth and the other planets in the solar system orbited the sun instead of the earth and that the earth was not the center of the universe at all.

By the time of the invention of the telescope, scientists realized that other planets also had moons that orbited them and that models of the solar system in which the sun rather than the earth was at the center matched the observed orbital motions of the planets much more accurately than the earth-centered models promoted by the Church. The mechanical models devised to explain the planetary orbits described what was observed fairly accurately, in addition to proposing an explanation for the effects of gravity, if not for the manner in which it came about.

Those mechanical concepts recognized that gravity is an attractive force that acts over infinitely far distances and in proportion to both relative distance and the mass of objects, but failed to explain apparent inconsistencies, such as deviations in the orbit of Mercury. That mechanical concept of gravity was eventually replaced by a much more radical view in which gravity represents a property of space and time rather than a force acting in space, and in which gravity is one of four fundamental forces of nature attributable to subatomic particles and interactions.

Aristotle:

More than three-hundred years before the birth of Christ, the Greek philosopher Aristotle published his book, On the Heavens in which he explained two other specific reasons for believing that the earth was round. First, Aristotle realized that during lunar eclipses, the earth passes between the sun and moon and always casts a perfectly round shadow on the moon. This could only happen if the earth were round. Second, Aristotle explained that because the North Star always remained above the North Pole, the fact that appeared lower in the sky as one travelled south and higher in the sky as one travelled north implied that the earth was round (Hawking, 2002).

The Ancient Greeks also suspected that the earth was round, partly because they noticed the way that ships disappear gradually from view over the distant horizon. They believed that the earth was the center of the universe and that the sun and all the other planets circled the earth. The believed intuitively that circular motion was the most nearly perfect of all motions and apparently never considered any other motion such as ellipses but did not attempt to explain the process by which the planets moved about the earth (Hawking, 2002).

Ptolemy and Copernicus:

Ptolemy postulated a three-dimensional model to represent the gravitational motion of the sun and the planets in the solar system, and like Aristotle five hundred years earlier, he positioned the earth at the center of the universe and arranged the known planets in two-dimensional circular planes or "celestial spheres" around the earth, with each plane corresponding to the orbital motion of each respective planets. In turn, each circular plane fit inside a smaller celestial sphere corresponding to the proximity of each planet to the earth at the center. That model was adopted by the Catholic Church because it fit the biblical belief of God's earth at the center of the universe and it remained largely unchallenged for more than one-thousand years. In the 26th century, Copernicus, himself a priest, proposed that the earth circled the sun rather than vice-versa, although he did not publicize that belief as his own during his lifetime, out of fear of the consequences at the hands of the Church (Feynman, 1995).

Isaac Newton:

In the 17th century, Isaac Newton thought about the phenomenon of falling objects and that falling objects all travel downward at the same rate, regardless of their size or relative mass, provided their shape did not result in air resistance. Newton tested his hypothesis rolling balls of different weights down inclined planes (Feynman, 1995; Hawking, 1991). Newton proposed the Law of Universal Gravitation, according to which all physical objects are drawn to one another by an invisible force that is proportional to their size and distance from one another. 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).

Galileo Galilei:

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