The Twin Paradox
Among all the paradoxes of Special Relativity,
the most famous one is the Twin Paradox.
According to Special Relativity, a moving clock runs slower,
so a person in a high speed moving vehicle will get old slower.
Suppose E(arth) and T(raveler)
are twins on Earth. At the age of 20, T starts a space trip.
When he returns to Earth 10 years later, T will be 30 and E will be much older, let's say 40.
The reason is that the high speed motion makes time pass slower in the spaceship,
thus T can be much younger than E.
But the conclusion can also be the reverse.
The most important part of Special Relativity is the principle of relativity.
From the standpoint of T, E is in a high speed motion relative to him,
thus the clock on Earth should run slower, and E should be much younger.
This is the twin paradox.