What is Special Relativity (1)
A swimmer is swimming in the river. He wants to swim upstream from point A to point B, then swim back.
If his speed in still water is c, and the speed of the river water is v, what is the average speed of his round-trip?
Let s be the distance between A and B, then
Time from A to B:
Time from B to A:
So the average speed is less than c.