Gravity is the force of attraction between any two objects in the universe. That force depends on two factors: the mass of each object and the distance between them.
The story behind English physicist Isaac Newton's (1642–1727) discovery of the gravitational force is one of the most fascinating in all of science. It begins in ancient Greece in the period from the sixth to the third century B.C. During that time, a number of Greek philosophers attempted to explain common observations from the natural world—such as the fact that most objects fall to the ground if they are not held up in some way.
Aristotle. Among the explanations developed for this tendency was one offered by Greek philosopher Aristotle (384–322 B.C. ). Aristotle developed a grand scheme of natural philosophy stating that all objects "belonged" in one place or another. Heat belonged in the atmosphere because it originally came from the Sun (as Aristotle taught). For that reason, heat rises. Objects fall toward Earth's surface, Aristotle said, because that's where "earthy" objects belong. Aristotle's philosophy was an attempt to explain why objects fall.
Galileo and Newton. Aristotle's philosophy dominated the thinking of European scholars for nearly 2,000 years. Then, in the sixteenth century, Italian physicist Galileo Galilei (1564–1642) suggested another way of answering questions in science. Scientists should not trouble themselves trying to understand why things happen in the natural world, Galileo said. Instead, they should focus simply on describing how things occur. Galileo also taught that the way to find out about the natural world is not just to think logically about it but to perform experiments that produce measurable results.
One of the most famous experiments attributed to Galileo was the one he conducted at the Leaning Tower of Pisa. He is said to have dropped two balls from the top of the tower and discovered that they both took the same time to reach the ground. Galileo's greatest achievements were not in defining the true nature of gravity, then, but in setting the stage for the work of Isaac Newton, who was born the year Galileo died.
Newton's accomplishments in the field of gravity also are associated with a famous story. Legend has it that Newton was hit on the head by an apple falling from a tree. That event got him wondering about the force between two objects on Earth (the apple and the ground) and the force between two objects in the universe (the force between a planet and the Sun).
Gravity on Earth and in the heavens. The connection between gravitational forces on Earth and in the heavens is a very important one. Measuring the force of gravity on Earth is very difficult for one simple reason. Suppose we want to measure what happens when an object falls on Earth. In terms of gravity, what actually happens is that the object and the planet Earth are attracted toward each other. The object moves downward toward Earth, and Earth moves upward toward the object. The problem is that Earth is so much larger than the object that it's impossible to see any movement on the part of the planet.
The situation is quite different in the heavens. The reason planets travel in an orbit around the Sun, Newton said, is that they are responding to two forces. One force is caused simply by their motion through the skies. Just imagine that at some time in the past, someone grabbed hold of Mars and threw it past the Sun. Mars would be traveling through space, then, because of the initial velocity that was given to it.
But Mars does not travel in a straight line. It moves in a circle (or nearly a circle) around the Sun. What changes Mars's motion from a straight line to a curve, Newton asked? The answer he proposed was gravity. The gravitational force between the Sun and Mars causes the planet to move out of a straight line and towards the Sun. The combination of the straight line motion and the gravitational force, then, accounts for the shape of Mars's orbit.
Mass: A measure of the amount of matter in a body.
Orbit: The path followed by a body (such as a planet) in its travel around another body (such as the Sun).
Proportionality constant: A number inserted into an equation to make both sides equal.
Weight: The gravitational attraction of Earth on an object.
But a huge point in Newton's favor was that he already knew all the main points about Mars and its orbit around the Sun. He had a good idea as to how fast the planet was traveling, its mass, the mass of the Sun, and the size of its orbit. Furthermore, the difference in size between Mars and the Sun was great—but not nearly as great as the difference between an apple and Earth.
So Newton derived his idea of the gravitational force by studying the orbit of the planets. He applied that idea to what he knew about the planets and found that he was able to predict almost perfectly the orbits followed by the planets.
Cavendish's findings. Proving the gravitational law on Earth was somewhat more difficult. Probably the most important experiment conducted for this purpose was one carried out by English chemist and physicist Henry Cavendish (1731–1810) in 1798. Cavendish suspended a light rod horizontally from a silk thread. At each end of the rod he hung a lead ball. Then he brought a third lead ball close to one of the two lead balls suspended from the rod. He was able to notice that the two lead balls attracted each other. As they did so, they caused the metal rod to pivot slightly on its silk thread. The amount by which the rod pivoted, Cavendish found out, depended on how closely the lead balls were brought next to each other and how much the two balls weighed (what their masses were). Cavendish's results turned out to confirm Newton's predictions exactly.
Einsteinian gravity. Newton's description of gravitational forces proved to be satisfactory for almost two and a half centuries. Then, observations began to appear in which his gravitational law turned out to be not exactly correct. The differences between predictions based on Newton's law and actual observations were small—too small to have been noticed for many years. But scientists eventually realized that Newton's law was not entirely and always correct.
In the early 1900s, German-born American physicist Albert Einstein (1879–1955) proposed a solution for problems with Newton's law. Interestingly enough, Einstein did not suggest modifications in Newton's law to make it more accurate. Instead, he proposed an entirely new way to think about gravity.
The way to think about gravitational forces, Einstein said, is to imagine that space has shape. Imagine, for example, a thin sheet of rubber stretched very tightly in all directions. Then imagine that the rubber sheet has indentations in it, similar to the depressions caused by pushing in on the sheet with your thumb. Finally, imagine that this dented rubber sheet represents space.
Using this model, Einstein suggested that gravity is nothing other than the effect produced when an object moving through space approaches one of these indentations. If a planet were moving through space and came close to an indentation, for example, it would tend to roll inward toward the dent. The effect to an outside observer would be exactly the same as if the planet were experiencing a gravitational force of attraction to the center of the dent.
Finally, Einstein said, these dents in space are caused by the presence of objects, such as stars and planets. The larger the object, the deeper the dent. Again, the effect observed is the same as it would be with Newtonian gravity. An object traveling through space will be pulled out of its orbit more by a deep dent (a heavy object) than it will be by a shallow dent (a lighter object).
So what's the point of thinking about gravity in Einstein's terms rather than Newton's? The answer is that the mathematics used by Einstein does everything that Newton's law of gravitation does plus it solves all of the problems that Newtonian gravity cannot explain.
Physicists now believe that all forces in the universe can be reduced to one of four fundamental forces: gravitation, electromagnetism, and the strong and weak force. The strong and weak force are forces discovered in the twentieth century; they are responsible for the way atoms and particles smaller than the atom interact with each other. Electromagnetic forces affect charged or magnetic particles. And the gravitational force affects all bodies of any size whatsoever. Of the four forces, the gravitational force is by far the weakest and probably least understood.
One of the great efforts among physicists during the twentieth century was the attempt to show how all four fundamental forces are really different symptoms of a single force. They have been successful in doing so for the electromagnetic and weak forces, which are now recognized as two forms of a single force. The attempts to unify the remaining forces, including gravitation, however, have been unsuccessful so far.
[ See also Celestial mechanics ]