At this point at the latest, our comparison with the ball game is no longer accurate. The speed of our ball with perhaps two meters per second is simply no longer comparable with the speed of light, which in a second about 300,000 kilometers through a passage of time to question to save the speed of light as a natural constant. That was indeed a stroke of genius. According to Einstein's theory, time, our reliable measure for all events, was as elastic as a rubber band. No matter how controversial his confusing views about time might be in the beginning, in the future it should prove to be true piece by piece.
During the mathematical elaboration of his theory, Albert Einstein also came across other physical quantities that had to change the faster their reference system moved. If his assumptions were correct, then not only had lengths to shorten, which Lorentz already recognized, but also the mass of the moving bodies had to increase. When the masses of bodies increase at high speeds, there had to be a relationship between mass and energy.
The result of these considerations was his famous formula E=MC².
That this formula explained the loss of masses in a nuclear fission or fusion and that the vanished masses would become free in the form of enormous energies, nobody suspected at that time.
But Einstein's theory was not yet complete. What was missing was gravity. If this force dominates the whole universe, then it also affects every reference system, then there was no reference system that moved in a straight line with constant speed according to the specifications of the special relativity theory. Because when gravity acts on a mass, it is accelerated. So how could accelerated movements be incorporated into the principle of relativity?
Albert Einstein owed the largest case of his life to an accident in his neighborhood. As he later writes: "While working on a house roof, a painter crashes and falls into the deep. While falling," he later reports, "he hadn't felt any weight. So if, during a free fall in the gravitational field of the earth, all things behave as if the gravitational field did not exist at all, as if one were moving weightlessly in a force-free space, then perhaps gravitation was not an independent force at all. Perhaps it was only a geometric property of space?".
The thought had far-reaching consequences. On the one hand, he conclusively explained Galileo's observation of why all things fall equally fast in a vacuum, no matter how heavy they are, on the other hand, how could one imagine a space in which gravitation was hidden as a geometric property?
Part 4 
Part 6 
