Einstein's Theories

EINSTEIN'S THEORIES

Einstein and Relativity

Albert Einstein electrified the physics community in the United States with his special theory of relativity in 1905 and his general theory of relativity in 1916. In his special theory of relativity Einstein posited that space and time are not absolute and independent realities. Indeed, the word relativity indicates that motion, space, and time cannot be measured from a fixed point but are relative to the observer measuring them. The speed of light, however, is a constant in all frames of reference. As an object approaches the speed of light, it appears to contract in the direction of motion; its mass increases; and time, as measured by a clock moving with the object, slows. Einstein asserted that mass and energy are interchangeable properties: E=mc2, where energy is said to be equivalent to mass times the square of the speed of light. Since the speed of light is 186,000 miles per second, Einstein's theory suggested that a small mass could be transformed into a enormous amount of energy (his theory was shown to be true with the fission reaction of the atomic bomb). His 1905 theory applied only to systems that move at a constant velocity relative to each other.

The General Theory of Relativity

Einstein's general theory of relativity extended his work of eleven years earlier to account for systems moving relative to each other at any velocity, even if those velocities are changing. It is principally concerned with the large-scale effects of gravitation. Recognizing that inertial mass (the mass derived from measurements of acceleration) and gravitational mass (mass from measurements of gravitation intensity) were the same, Einstein asserted that material bodies—planets, stars, and other masses—produce curvatures in space-time. The space surrounding massive bodies is distorted, or curved. Einstein's general theory holds that clocks run more slowly in stronger gravitational fields and even light is bent in the curvature of space associated with a powerful gravitational field.

A New Physics

Einstein noted that the laws of classical physics, developed by Sir Isaac Newton in the seventeenth century, usually gave results close to those predicted by general relativity. But in some instances he suggested that there might be observable differences in the two theories. Einstein predicted that the perihelion (closet point of orbit) of a planet's orbit would be slightly further from the body it orbited than classical physics predicted. Just such a difference in perihelion was observed in the orbit of Mercury. The views of the general theory were given further credence when A. S. Eddington measured the curvature in starlight passing near the sun during a solar eclipse in May 1919.

Sources:

Peter G. Bergmann, The Riddle of Gravitation (New York: Scribners, 1968);

Albert Einstein, Relativity: The Special and General Theory, fifteenth edition (London: Methuen, 1954);

Einstein and Leopold Infield, The Evolution of Physics: The Growth of Ideas from Early Concepts to Relativity and Quanta (New York: Simon & Schuster, 1938);

Max Jammer, The Conceptual Development of Quantum Mechanics (New York: McGraw-Hill, 1966).