Paradoxes of the theory of relativity
The Twin Paradox
There is one way to get around the problem of multiple clocks. Let's do another gedanken experiment. This time we have two twins. One twin gets in a space ship and flies to alpha centauri and back at high speed. Her twin brother on earth knows that her clock and her life processes move slower than his.
When she returns to earth he is not surprised to see that she is younger than he is. But shouldn't she see the same thing? That is, since he (on earth) was moving relative to her space ship, shouldn't she see that he is younger? Well, the correct answer is that she is younger than he is. The symmetry of the situation is broken because she had to accelerate to fly away, decelerate to turn around at alpha centauri, and then accelerate and decelerate again to return home.
Therefore, her clocks behave differently. After all, Einstein's postulates apply only to coordinate systems travelling at a constant velocity. Actually, this gedanken experiment has really been done. In very careful experiments, two atomic clocks were flown around the earth in opposite directions.
When they returned to the original laboratory and were compared with "stay-at-home" clocks, they were slower (younger) by about a tenth of a microsecond, just the amount Einstein would have predicted (actually, the effects of gravity had to be taken into account since the planes were flying high above the earth; more about this later)!
