The theories of relativity, general and special, by Albert Einstein intend to make two others compatible: the mechanics of Isaac Newton and the electromagnetism of James Clerk Maxwell.
According to the laws of the movement established for the first time in detail by Isaac Newton around 1680-89, two or more movements are added according to the rules of elementary arithmetic.
Suppose a train passes by our side at 20 kilometers per hour and that a child throws a ball from the train at 20 kilometers per hour in the direction of train movement. For the child, who moves along with the train, the ball moves at 20 kilometers per hour. But for us, the movement of the train and that of the ball add up, so that the ball will move at the speed of 40 kilometers per hour.
As it turns out, you can't talk about the speed of the ball dry. What counts is its speed with respect to a particular observer. Any theory of movement that attempts to explain the way in which speeds (and related phenomena) seem to vary from one observer to another would be a "theory of relativity."
Einstein's theory of relativity was born from the following fact: what works for balls thrown from a train does not work for light. In principle it could be assumed that the light would spread, either in favor of the earthly movement, or against it. In the first case it would seem to travel faster than in the second (in the same way that an airplane travels faster, in relation to the ground, when it has tailwind than when it is facing it). However, very careful measures showed that the speed of light never varied, whatever the nature of the movement of the source that emitted the light.
Einstein said then: suppose that when you measure the speed of light in a vacuum, it is always the same value (about 299,793 kilometers per second), under any circumstances. How can we arrange the laws of the Universe to explain this phenomenon? Einstein found that to explain the constancy of the speed of light it was necessary to accept a series of phenomena, in principle, unexpected.
He found that the objects had to be shortened in the direction of the movement, all the more the greater their speed, until finally reaching a null length at the limit of the speed of light; that the mass of moving objects had to increase with speed, until it became infinite in the limit of the speed of light; that the passage of time in a moving object was getting slower as the speed increased, until it came to a stop at that limit; and, finally, that mass was equivalent to a certain amount of energy and vice versa.
All this was elaborated in 1905 in the form of the "special theory of relativity", which dealt with bodies with constant velocity. In 1915 he extracted even more subtle consequences for objects with variable velocity, including a description of the behavior of gravitational effects. It was the "general theory of relativity."
The changes predicted by Einstein are only noticeable at high speeds. Such speeds have been observed among the subatomic particles, seeing that the changes predicted by the brilliant scientist were really happening, and with great accuracy. Moreover, if Einstein's theory of relativity were incorrect, particle accelerators could not work, atomic bombs would not explode and there would be certain astronomical observations that would be impossible to do.
But at current speeds, the predicted changes are so small that they can practically be ignored. In these circumstances the elementary arithmetic expressed by the laws of Isaac Newton governs; and, as we are accustomed to the operation of these laws, they already seem to us of "common sense", while the theories of the relativity of Albert Einstein seem to us "strange" and difficult to understand.
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