Einstein's theory of relativity.
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Relativity Einstein's theory of relativity has caught the imagination of the average person more than any other physical theory in history. Yet the theory of relativity, unlike many other results of physical science, is not easily understood by the average person. We can understand the relativity theory fully only by means of the mathematical formulas which make it up. Without mathematics, we can only state some of its basic ideas and quote, but not prove, some of its conclusions. The relativity theory deals with the most fundamental ideas which we use to describe natural happenings. These ideas are time, space, mass, motion, and gravitation. The theory gives new meaning to the old ideas that these words represent. It is basically made up of two parts. One is the special, or restricted, relativity theory, published by Albert Einstein in 1905. The general relativity theory was put forward by Einstein in 1915. Special theory of relativity This theory is called the special relativity theory because it refers to a special kind of motion. This is uniform motion in a straight line, that is, with constant velocity. Suppose we are on a smoothly running railway train which is moving at a constant velocity. In this train you may drop a book, play catch, or allow a pendulum to swing freely. The book will appear to fall straight down when it is dropped; the ball will travel directly from the thrower to the catcher. All these activities can be carried on in much the same way and with the same results by people standing still on the ground outside the train. So long as the train runs smoothly, with constant velocity, none of our mechanical activities will be affected by its motion. On the other hand, if the train stops or speeds up abruptly, our activities may be changed. A book may be jarred from a seat and fall without being dropped.
This amount is still about a million times greater than the energy released in the burning of chemical fuels. Various experiments have proved the truth of many of these conclusions about relativity. In 1938, H. E. Ives used a hydrogen atom as a moving clock. He found that a fast-moving hydrogen atom does slow down in its rhythm, just as Einstein predicted the moving clock would do. This slowing down could be shown by a change in the frequency of the line given off in its spectrum. The changes of mass as predicted by the special theory of relativity are observed in machines that are used to accelerate electrons and nuclear particles to the high speeds necessary to study nuclear properties. The mathematician H. Minkowski gave a mathematical form to the special relativity theory in 1907. A line involves only one dimension. We can locate any point on a sheet of paper by measuring from that point to any two sides of the paper that are perpendicular to each other. Therefore, we can say that any point on a sheet of paper involves two dimensions. All points in space involve three dimensions: height, length, and breadth. But there is one other important fact involved. In physics as well as history we must deal with events. When and where did the French Revolution start, for example? When and where does the earth have the smallest velocity in its movement about the sun? Events must be characterized by four numbers, bringing in the idea of a fourth dimension. Three of these numbers answer the question where; one must answer the question when. Answering the question when involves the idea of time. Then we consider things in terms of four dimensions. This question of answering when and where an event took place becomes more complicated, according to the theory of special relativity, because rods can change their lengths, and clocks change their rhythms, depending on the speed at which they operate when they are in motion.
For example, the general theory predicts the existence of waves that "carry" the force of gravity, just as electromagnetic waves carry light. Experimenters have not yet been able to detect these gravitational waves. Scientists are also trying to combine electromagnetic and gravitational forces in a theory called the unified field theory. Relativity and other ideas The ideas of relativity form a framework which can embrace all laws of nature. Relativity has changed the whole philosophical and physical notions of space and time. It has influenced our views and speculation of the distant worlds and stars and of the tiny world of the atom. Some of this speculation is still going on. Does our universe, regarded as a whole, resemble a plane surface or a sphere? It is not possible to answer this question, because there are many different theories and much uncertainty about the distribution of matter in the universe. All the theories try to describe the universe as a whole and are based upon the mathematical principles of general relativity. According to some theories, a light ray sent from an arbitrary point in space returns, after a very long time interval, to the point of departure, like a traveller in a journey around our earth. Thus, if you were to start from your home and travel into space along a straight line, you would eventually return to the point from which you started. According to other theories, however, a light ray or a traveller would continue an endless journey through space. In spite of all these successes of the relativity theory, it is not right to say that Newtonian physics is wrong. Newtonian physics holds true if the velocities of the objects being studied are small compared with the velocity of light. Such objects are found every day in our own experience, and therefore classical physics can still be applied to our daily problems. Astronomers have found that Newton's theory of gravitation still holds true in their calculations. But the relativity theory does limit the area to which the Newtonian physics can be successfully applied.
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