Gravity
What goes up, must come down; that's a simple statement for a very complex force that has been a puzzle for millennia.
The ancient Greeks had a simple explanation for gravity: the elements (earth, air, fire, water) always sought their natural place. Objects composed of Earth elements had no choice but to fall, with heavier objects falling faster than lighter ones. The Greeks had a profound influence on science, and their teachings went unchallenged for 1,500 years.
Danish physicist Isaac Beeckmann gave considerable thought to gravitation. In 1613 he suggested the principle of inertia and, five years later, combined that with his law of uniformly accelerating bodies in a vacuum. He determined the distance an object falls was related to the square of the amount of time it was falling. Galileo came up with a novel idea for the time: the use of experimentation to verify hypotheses. Legend has it that in 1590 he experimented by dropping objects of various sizes from the top of the Leaning Tower of Pisa. He found there was no fundamental difference in the attraction of gravity on objects of different mass. This shattered Aristotle 's contention that heavier objects fell faster. No one questioned this before Galileo because questioning authority, especially the learned Greeks, was simply not the thing to do. Galileo rediscovered the law of uniform acceleration in 1604 by rolling balls down an inclined plane. Acceleration due to gravity turns out to be about 32 feet (9.8 m) per second squared.
The great giant of gravitational theory is Isaac Newton. A famous story relates that Newton was sitting in an orchard in the countryside in 1666 and saw an apple fall to the ground. It made him wonder why objects fell toward the Earth, and he surmised that it was because all matter attracts other matter to it.
In 1687 Newton published his Principia Mathematica which, among other things, revolutionized the understanding of gravity. Johannes Kepler had determined planets move in elliptical orbits around the Sun, but Newton realized that the gravitational force between a planet and the Sun was dependent on the mass of each object divided by the square of the distance separating them. This became known as the inverse square law. Newton's general theory of gravitation explained the universal attraction between any two objects. The same force acting on the apple kept the moon in orbit around the Earth. Planets, the moons of planets, comets, stars and galaxies all follow the same inverse square law. At this point, Newton ran afoul of his countryman Robert Hooke.
In the mid-1600s, the concept that gravity existed between objects was becoming popular. Physicist Hooke had suggested in 1664, a year before Newton saw the falling apple, that an object would be pulled into an orbit around a larger object by the force of gravity. In 1679 he went another step, arguing that the force of gravity was dependent on an inverse square law. Hooke later claimed credit for the discovery of gravitation, which Newton hotly contested. While Hooke was essentially correct in his theory, it appears he was notorious for not following through, and it was Newton who worked out the details.
The first actual measurement of the gravitational force between two objects was performed in 1798 by Henry Cavendish. He suspended a pair of 12 in. (30 cm) lead balls near a pair of 2 in. (5 cm) lead balls, and was able to calculate the force of attraction. His result came within one percent of the modern value!
As great as Newton's theory of gravitation was, it could not account for a puzzling anomaly in the orbit of Mercury. It was yet another giant, Albert Einstein, who discovered the solution. According to Einstein's theory of relativity, first announced in 1905, the energy and momentum associated with an object contributes to its mass. Because of Mercury's eccentric orbit, it receives a tiny additional gravitational push that was not accounted for in Newton's theory.
Einstein's general theory and special theory of relativity allowed scientists to consider the effect of gravity upon something that had been thought to be immune to its influence-- light. If light passed close to an object with enough mass, the gravitational pull would force the light to bend from its straight path. This was proven to be the case on May 29, 1919, during a total eclipse of the Sun.
The greater the mass of an object, the greater its gravitational force. What about an object that has an incredibly high mass, confined to a very small area? The gravitational force would be so great that even light could not move fast enough to escape it. Light would be bent into an orbit around the object. Such massive things are believed to exist; they are called black holes. According to theory, gravity waves should be generated by the disturbance created by material falling into black holes, but because gravity is a comparatively weak force in nature, gravitational radiation will be very difficult to detect, though attempts to accomplish this are underway.
The force of gravity has major cosmological implications. If enough invisible " dark matter" can be discovered, its gravitational attraction might be great enough to slow, or even reverse, the expansion of the universe that started with the " big bang" billions of years ago.
It is remarkable to realize that even after 400 years of theories, scientists still do not know what actually causes gravity.
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