The Theories of Gravity and Relativity

Gravity is one of the four fundamental forces of nature, the others being electromagnetism and the two nuclear forces - the weak and the strong. Although the effect of gravity is easily perceived, how gravity works is not completely understood. There are two different interpretations that physicists use: the Newtonian and Einstein's general theory of relativity. Newtonian physics tells us that the force of gravity is proportional to the square of the distance between them. In other words, the larger the mass the greater the force of gravity; the force of gravity falls off quite rapidly as the objects are moved further apart. Newtonian physics accounts for the observed orbits of planets and moons, the motion of comets, how all other celestial bodies are attracted to one another, the motion of falling objects at the Earth's surface, the ocean tides, etc. (Line 8)

Einstein's General Theory of Relativity is a fundamental concept of the nature of space, time, and gravitation that has profoundly influenced how humankind views the universe. Einstein's theory views gravity as a property of space rather then as a force between bodies. As a result of the presence of matter, space becomes curved and bodies follow the line of least resistance among curves. Gravity then is viewed as the consequence of the curvature of space induced by the presence of a massive object. (Line 13)

Einstein's theory of relativity did not overthrow Newton's, but rather modified some of its most fundamental concepts. According to Einstein, gravity is not, in fact, an action-at-a-distance force, operating in a simple world of three dimensions and absolute time. He deduced, from two of his most basic insights (that the laws of physics are independent of the motion of an observer and that nothing can travel faster that light)¹, that space and time must be " handcuffed" together. This line of thinking was crystallized by the German mathematician Hermann Minkowski in the concept of space-time, a four dimensional sheet stretching into infinity, with space set out along the familiar three dimensions and time along the fourth. This is difficult to visualize, accustomed as we are to thinking in terms of only three dimensions. Einstein's theory showed that the four-dimensional " sheet" has a warped and pliable topography, filled with hill and valleys that are continuously being reshaped by the motion of objects within it. Matter tells space-time how to curve, and conversely, the geometry of space-time tells matter how to move. (23)

In space-time, then, there is no unseen force of attraction between objects; what appears to be gravity is merely a consequence of the curvature of the universe. This conclusion may, at first, seem bizarre, but consider this simple analogy: two great circles, or geodesies, are drawn on a globe, both passing through the North Pole and intersecting the equator at points A and B. If two particles, one at A and the other at B, were to move upward along the geodesies, the distance between them would shrink as they approached the pole. It might appear as if they were being attracted to one another, but, actually, they draw closer only because of their movement along the curved lines. (Line 30)

The notion that gravity is integral to the malleable geometry of the universe has been elaborated by a variety of astronomical observations. It accounts for the way light from distant stars bends as it passes massive objects in space, for example. It also explains, with a degree of tail unmatched by Newton's equations, how planets and stars move in relation to one another. Thus, Einstein's theory of relativity accurately describes the large-scale structure of the universe. (Line 35)

Throughout the years, since Einstein formulated the theory, the evidence gathered has supported his deductions and predictions. Interestingly, at this very writing, two scientists (Formalont and S. Kopelki) reported that in September 2002 they were able to measure the speed of gravity and confirm Einstein's assumption that it travels at the speed of light2. Thus, there may be confirmation of the well-known idea that if gravity traveled at the speed of light and the suddenly vanished from the solar system, the Earth would remain in orbit for about eight minutes, time taken for light to travel from the sun to our planet. Then, in the absence of gravity, the Earth would move off in a straight line. It should be noted, however, that not all scientists agree with their method of confirmation³. (Line 42)