Gravity

Gravity is a force of attraction that exists between objects based on their mass. First described by Isaac Newton in the seventeenth century, the law of gravity is one of the most fundamental principles of physics. While the precise details of how gravity works are still being debated today, the concept of gravity remains at the very heart of humankind's understanding of the physical universe and the laws by which it is governed.

Historical Background

Many scientists throughout history studied the observable effects of gravity on Earth. However, it was not until the seventeenth century that English physicist and mathematician Sir Isaac Newton developed an accurate theory on the subject. According to legend, Newton's historic breakthrough came as the result of an accident. While sitting beneath an apple tree one day, a falling apple struck Newton on top of his head. This incident allegedly led Newton to ponder what attracted the apple from the tree to the ground. While the authenticity of this story is debatable, the theory that Newton subsequently devised became very important to the understanding of gravity.

In his theory, which he published in the Principia Mathematica in 1687, Newton proposed the existence of a force that was directly tied to mass and distance. Specifically, he argued that one object attracts another with a force (gravity) that is directly proportional to the product of the objects' masses and inversely proportional to the square of the distance between them.

Newton's theory remained the only widely accepted explanation of gravity for several centuries until another scientist offered his own opinion on the matter. In the early twentieth century, famed physicist Albert Einstein developed another theory on gravity, the general theory of relativity, which greatly differed from Newton's theory. Since then, several other scientists have developed theories on gravity.

Theories on Gravity

From the Newtonian perspective, gravity is one of the four basic forces, alongside electromagnetism (the magnetic field produced by a current of electricity), strong nuclear force, and weak nuclear force. In general, gravity is a force of attraction between two bodies. The strength and effect of that force depends on the mass of the two bodies and the distance between them. As a result, the force of gravity grows stronger either when the mass of one of the bodies increases or when the bodies move closer to each other. This explains why the effects of gravity are only felt when one of the bodies is extraordinarily large.

On Earth's surface, gravity is most easily observed as a force that attracts things toward the ground. This happens because Earth's mass is great enough for it to have a strong gravitational attraction. For example, if a bowling ball is dropped from the top of a building, it will immediately fall to the ground because Earth's gravitational force will pull it in that direction. Though the bowling ball has its own gravitational force, it is pulled to the ground because Earth's significantly larger mass gives Earth a much stronger gravitational force.

The effects of gravity are felt beyond Earth. In space, gravity keeps planets in orbit around the stars and moons in orbit around those planets. Gravity also binds the stars into galaxies, galaxies into clusters, and more. It has even been suggested that gravity may be responsible for controlling the rate of expansion of the universe itself.

While the Newtonian theory of gravity appears to be sound in most respects, occasional inconsistencies have emerged. These inconsistencies drove scientists to search for other possible explanations of the nature of gravity. Einstein came to the conclusion that gravity is not a force but a distortion of space-time. He theorized that space-time is essentially flat, like the surface of a trampoline. If a person stands on a trampoline, the part of the surface on which he or she is standing becomes dimpled. A ball placed on the trampoline with that person would naturally be drawn into the dimpled area where the person is standing. According to Einstein's general theory of relativity, large bodies such as stars or planets create a similar kind of dimple in space-time. Thus, on a space-time scale, gravity is the equivalent of the dimpling effect that draws the ball toward the person on the trampoline.

Some scientists have developed other theories on the nature of gravity, many of them extensions of Einstein's general relativity. One such theory holds that gravity is transmitted by special particles called gravitons, which attract objects to one another, much like electromagnetism is transmitted by photons. Another theory suggests that the acceleration of an object due to gravity produces gravitational waves—the first experimental confirmation of which came in September 2015, when the Laser Interferometer Gravitational-Wave Observatory (LIGO) observed gravitational waves being emitted from two massive black holes. Whatever the explanation, gravity remains an important principle of physical science.

Bibliography

Balchin, Jon. Science: 100 Scientists Who Changed the World. New York: Enchanted Lion, 2003. Print.

Carroll, Susan. "All Physics Is Local." Atlantic. Atlantic Monthly Group, 12 Feb. 2016. Web. 19 May 2016.

Litka, Arthur H. "Gravity." Space Sciences. Ed. Pat Dasch. Vol. 2. New York: Macmillan, 2002. 63–65. Print.

Nordtvedt, Kenneth L. "Gravity." Encyclopaedia Britannica. Encyclopaedia Britannica, 3 May 2016. Web. 19 May 2016.

Rosen, Joe and Lisa Quinn Gothard.Encyclopedia of Physical Science. New York: Facts On File, 2010. Print.

Rovelli, Carlo. Seven Brief Lessons on Physics. Trans. Simon Carnell and Erica Segre. 2015. New York: Riverhead, 2016. Print.