# Gravity: What is Gravity?

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What is Gravity?

There are many articles here in the *Encyclopedia of the Cosmos* focusing on various aspects of gravity, as listed in "Related EoC Articles" below. However, to begin, let's explore first some basic definitions of what gravity is, or at least our understanding of it at this point in the human quest for knowledge and understanding of the universe around us.

## Some Relatively Simple Answers to the Question

### "StarChild" Question of the Month for February 2001

We don't really know what gravity "is." We can define what it is as a field of influence, because we know how it operates in the Universe. Some scientists think that gravity is made up of particles called gravitons which travel at the speed of light. However, if we are to be honest, we do not know what gravity "is" in any fundamental way – we only know how it behaves.

Here is what we do know... Gravity is a force of attraction that exists between any two masses, any two bodies, any two particles. Gravity is not just the attraction between objects and the Earth. It is an attraction that exists between all objects, everywhere in the Universe. Sir Isaac Newton (1642 — 1727) discovered that a force is required to change the speed or direction of movement of an object. He also realized that the force called "gravity" must make an apple fall from a tree, or humans and animals live on the surface of our spinning planet without being flung off. Furthermore, he deduced that gravity forces exist between all objects.

Newton's "law" of gravity is a mathematical description of the way bodies are observed to attract one another, based on many scientific experiments and observations. The gravitational equation says that the force of gravity is proportional to the product of the two masses (m

_{1}and m_{2}), and inversely proportional to the square of the distance (r) between their centers of mass. Mathematically speaking,F=Gm

_{1}m_{2}/r^{2},where G is called the Gravitational Constant. It has a value of 6.6726 x 10

^{-11}m^{3}kg^{-1}s^{-2}.The effect of gravity extends from each object out into space in all directions, and for an infinite distance. However, the strength of the gravitational force reduces quickly with distance. Humans are never aware of the Sun's gravity pulling them, because the pull is so small at the distance between the Earth and Sun. Yet, it is the Sun's gravity that keeps the Earth in its orbit! Neither are we aware of the pull of lunar gravity on our bodies, but the Moon's gravity is responsible for the ocean tides on Earth.

(Source: The "StarChild" site is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC).)

### Gravity

Gravity(or "gravitation"), is one of 4 main forces in the universe. The others are the electro-magnetic force, and 2 types of nuclear force. Every mass exerts a gravitational pull on any other mass, inversely proportional to the square of the distance between them. This was first proposed by Sir. Issac Newton, based on his calculation of orbits of planets and of the Moon. (Source: David P. Stern's "Educational Web Sites on Astronomy")

## Related EoC Articles

- Gravity: Canonical Quantization
- Gravity: Covariant Quantization
- Gravity: Dimensionally-Extended
- Gravity: Gauge Field
- Gravity: Graviton
- Gravity: Kaluza-Kline Theory
- Gravity: Quantization ca. 1990
- Gravity: Quantum
- Gravity: Renormalization
- Gravity: String Theory
- Gravity: Superspace Quantization
- Gravity: Supersymmetry
- Gravity: What is Gravity?

## External Links

- Earth's Gravity - Wikipedia.
- Gravitation - Citizendium.
- Gravity Basics - NASA's Astronomy Cafe, by Dr. Sten Odenwald. This page contains basic information and FAQs about the force we call gravity.
- Gravity Probe B - Lockheed Martin/NASA/Stanford University.
- Gravity Recovery and Climate Experiment (GRACE) -
- Newton's Theory of "Universal Gravitation" - David P. Stern, D Sc.
- What is Gravity? - Northwestern University, Qualitative Reasoning Group.
- What are Gravitational Waves? - The Laser Interferometer Space Antenna (LISA), NASA/ESA.

## Image Preview

Gravitational waves are propagating gravitational fields, "ripples" in the curvature of space-time, generated by the motion of massive particles, such as two stars or two black holes orbiting each other. Gravitational waves cause a variable strain of space-time, which result in changes in the distance between points, with the size of the changes proportional to the distance between the points. Gravitational waves can be detected by devices which measure the induced length changes. Waves of different frequencies are caused by different motions of mass, and difference in the phases of the waves allow us to perceive the direction to the source and the shape of the matter that generated them. (Source: NASA-The Laser Interferometer Space Antenna (LISA).)

Citation

Wallace, Matthew H. (Contributing Author); Bernard Haisch (Topic Editor). 2008. "What is Gravity?" In: Encyclopedia of the Cosmos. Eds. Bernard Haisch and Joakim F. Lindblom (Redwood City, CA: Digital Universe Foundation). [First published November 15, 2008.].

<http://www.cosmosportal.org/articles/view/135643/>

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