I'm going to use units of light years for distance and years for time. In these units, the speed of light equals one. So most of the time I don't need to include the speed of light in the sums. Where it does pop up, it will be represented by the usual letter "c".

Let's start by filling in some numbers. B is going to travel to a soon to be discovered brown dwarf star exactly one light year away. It will travel there at 4/5 of the speed of light.

From the point of view of twin A, this is all very simple. B whizzes off at 4/5 c and travels a distance of one light year. It will therefore take B 5/4 years to complete the journey there and the same amount of time to get back. Total journey time will be 10/4 or 2 1/2 years.



At least, that's how A sees things using his own clock. The clock on B's ship appears to show something quite different.

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