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