## Suppose Betty and Barney are twins and Betty of them travels to a star 4 light years away at a speed that would result in a time and length contraction of a factor of two (0.866c)

## Betty sees the distance as 2 light years, so the round trip will take 4.62 years. During the trip she calculates Barney’s clock is running at half the rate of hers.

## Barney sees the distance as 4 light years so the round trip will take 9.24 years. During the trip he calculates Betty’s clock is running at half the rate of his.

## Barney will be 4.62 years older than Betty when she gets back, but where did the extra clock ticks come from? (Think Doppler shift!)

## On the way out, Betty sees (with light) ticks from Barney’s clock every 3.231 seconds. On the way back, every 0.232 seconds.