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.  A ball will travel differently.  

One way of stating the principle of this theory is to say that the laws of mechanics are the same for an observer in a smoothly moving train as for the observer at a fixed position on the ground.  Physicists would say: if two systems move uniformly relative to each other, then all the laws of mechanics are the same in both systems.  This principle may be called the classical relativity principle.  This principle is as old as the ideas of mechanics and physics.  

Suppose we have a long train much like the train in the previous example.  But instead of rolling along at a normal speed, it will be moving uniformly at a speed of, let us say, 32,000 kilometres per second.  Instead of having two persons playing catch on the train, we will have a radio aerial on the train sending out radio waves, or an electric torch sending out light signals.  Observers on the train will measure the velocity of the radio waves and light signals.  On the ground we will also have an aerial or electric torch, and observers measuring the velocity of the signals.  Is the velocity of the radio or light waves the same for those on the ground as it is for those on the train?  If we had asked this question of physicists in the late 1800's, they would have said no.  They would have said the classical relativity principle holds true for mechanical activities, but not for those of electromagnetic waves, that is, not for radio or light waves.  

A physicist would have said that radio and light waves travel through ether at a velocity of 299,792 kilometres per second.  Ether was a substance that scientists imagined to fill all space, to account for the transmission of light in outer space.  The physicist would have said that the stars, sun, planets, and our imaginary moving train move through the ether sea at different speeds.  Thus, the velocity of light will be different for an observer on the sun, on the earth, and on the train.  Just as the earth changes velocity during the year in which it completes its journey around the sun, the speed of light for the observer should change too.  

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Scientists of that time held the theory that the ether through which all objects of the universe were believed to move provided a nonmoving frame of reference.  All other motions could be judged from this frame of reference.  Ether was looked upon as a fluid or elastic solid.  It was believed to occupy the spaces between the atoms that made up matter.  It offered no resistance to the earth's movement.  

Among the many experiments which helped destroy the ether theory, the most famous is that of Michelson and Morley in 1887.  Their measurements of the speed of ...

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