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Which Twin Is Older?

A simplified account of special relativity and the twins paradox.

Take a Message Soldier

A messenger runs from the rear to the head of a marching column and back. When he gets back, the rear is where the head was when he set off. What is the ratio of his speed to that of the column?


Age 16 to 18
Challenge Level


In times gone by men moved very heavy stones by using logs as rollers. Assuming all the logs have the same radius and that the logs roll and do not slide explain why the stone moves in the direction the men push it twice as fast as the logs. (Try a similar experiment yourself, for example with some pencils and a heavy book, and you will find you leave the 'logs' behind at the back and have to move them into position in front of the 'stone'.)