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Union University Department of Physics

The Science Guys

Science Guys > January 2001

January 2001

What is gravity or how does gravity work?

Imagine two or more extra dimensions of space curled up into regions too small for us to directly sense. Imagine a chunk of gravity as a vibrating string that wanders through all of the 5+ dimensions of space. Imagine space itself twisted and curved near a black hole. This is the amazing world of gravitation research.

Models of gravity began with Aristotle (384-322 BC) who thought that the natural place for an object was "down" and this described gravity. Galileo (1564-1642), by experiment, showed that gravity caused all objects to fall to Earth at the same rate but was unable to explain why. Isaac Newton (1642-1727) provided us with his Universal Gravitational Law. While Newton greatly added to our understanding of how gravity works, we still don’t know why gravity works.

Newton’s law stated that every object in the universe with mass attracts every other object in the universe that has mass. This force is proportional to the product of the two masses and inversely proportional to the square of the distance between their centers. Thus two coffee cups have an attractive force on each other, and two planets also have an attractive force on each other, as will a planet and a coffee cup. All objects possess inertia (a tendency to maintain their state of motion). Objects with greater mass have greater inertia. So when the coffee cup and the planet are free to move, the cup does the moving because the planet has too much inertia to move any appreciable amount. Thus the cup "falls" to the planet. The force of gravity on an object on Earth is the "weight" of the object.

While gravitational forces act on things that are close together, they also act on things that are at great distances apart. For example the Earth and Moon are 240,000 miles apart but gravity creates an immense force (over a billion, trillion pounds) between the two bodies. This is sometimes referred to as action at a distance. Although it is hard philosophically to image a force exerted over such a distance through empty space, such is Newton’s Gravitational Law. If it were not for gravity, the Moon would move off from Earth in a straight line because of its motion and inertia.

In this past century Einstein (1879-1955) explained gravity with his general relativity in terms of a warping or bending of space in the presence of an object. The more massive an object the more space is warped. This theory is arguably the most mathematically fierce in all of physics and cannot be understood without mathematics. If objects are not too massive, both Newton and Einstein’s theories give similar results. However for very dense or massive objects, Einstein’s theory provides a more accurate description. Scientists are still working on improving or further understanding the theory of gravity. The next level of understanding seems to be quantum gravity. Today considerable money and effort is being invested in an attempt to find gravity waves and/or particles called gravitons. Research is also under way to determine if Newton’s theory must be modified because of the presence of extra dimensions. We will probably never have a perfect theory of gravity.

Scientists are essentially model builders. As data is collected and understanding expands, scientists constantly improve the model. Just as Galileo improved on Aristotle’s theory, Newton improved on Galileo’s, Einstein improved on Newton's, and so on it will go. New models are adopted if they better predict nature and are more aesthetically pleasing. People may someday regard our models of gravity as childish, but they work for us now and allow us to make predictions about the universe. As for a definitive answer, only the One who created gravity knows exactly how and why it works.