Once upon a time, there was a poor Chinese who worked from the morning to the night for some healthy people. At the end of the day he was paid, went back to his modest home, had dinner, drunk a little bit then went to sleep. The following day he would start again. He worked consciously, owners praised him, all of his co-workers appreciated him, his wife loved him and his children were healthy. But our Chinese always sighed: “How hard I have to work to earn my daily bread!” If I would be Mandarin, people will worship me, I will wear expensive clothes, I will eat and drink only goodies, I will have a harem. The Gods heard him and made him Mandarin. Suddenly, from illiterate he is now literate.
But he destroyed his peace. Daily, there were several attempts to his life, messengers bring him only news about war threatened, at night he couldn’t sleep anymore, he couldn’t feel the food’s taste and his harem was untouched. Then the Chinese said: “I got worse than I was”. I ate poor food but it had taste, I slept unmoved every night, I did my duty as a man but now I can’t do anything. So, he addressed to Gods again: “If I knew what means to be Mandarin, I wouldn’t have asked you that. Better make me the Sun.”
The Gods listened and made him the Sun. But, everyone was swearing at him. Sometimes it was to sunny, sometimes it was hidden behind the clouds. Anyway, he couldn’t resist anymore and asked the Gods to make him a simple illiterate worker. Now, our Chinese man wasn’t sad anymore, he was radiating happiness and said: “Why I wasn’t happy? I had everything I wished: happiness, health, great work capacity, healthy children, my wife loved me and had real friends. Why I had to be Mandarin?”
His co-workers couldn’t understand why he is so happy. He said: “I am the happiest man on the Earth. I do not give myself for any Mandarin, neither on the Sun from the sky.”
Perhaps you are asking what is the whole idea of this.
From the position that Chinese is, poor and illiterate, that is all it can be seen: the Mandarin didn’t make physical effort in his work, wear expensive clothes, have good food and drinks, have a entire harem, etc. Everything else couldn’t be seen. Changing his position, he could see the other side.
It is the same in Physics, existence or nonexistence of some forces (the gravity force, the acceleration force etc) depends of the observer’s position (in what reference system it is located, if it is in the origin of the system or not, the speed of the system, if acceleration is positive or negative, if it is tangential or normal, etc).
The contraction or dilatation of time and lengths of Relativistic Mechanics is just an impression of the inertial observer. That’s why most of the people don’t understand the Theory of Relativity.
In other words, everything depends on what side of the mirror you are. The theory is valid in any circumstances.