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# Einstein's theory of relativity.

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Introduction

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.

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Middle

Two events judged as taking place at the same time by the observer in the train may not be simultaneous for the observer on the ground.

The length of every object resting in the train appears to the observer outside to be shortened in the direction in which the train is moving.

Perhaps the most important of these deductions is the fact that mass is not unchangeable.  The mass of an object increases with its velocity.  Theoretically, the mass of an object would become infinite if its velocity became the velocity of light.  This mass increase has been observed with experiments.  A small particle of matter accelerated to 86 per cent of the speed of light has twice as much mass as it does when it is at rest.

The theory also shows a relation between a body's mass and its energy (E = mc-squared).  This relation has great practical importance in the liberation of the energy in the nucleus of an atom.  When energy is liberated from the nucleus of the uranium atom and atoms of other elements are formed, the total mass of these atoms is less than the total mass of the uranium atom.  This means that some of the mass of the nucleus of the uranium atom has been transformed into energy.  The E = mc-squared law shows that the energy in a single uranium nucleus is 220,000,000,000 electronvolts, providing that all its mass could be converted to energy.  However, splitting the uranium nucleus, a process known as fission, releases only 0.1 per cent of the total energy content.  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.

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Conclusion

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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