A set of twins, Andrea and Courtney, are initially 10 years old. While Courtney remains on Earth, Andrea rides on a space ship which travels away from Earth at a speed of 0.6c for five years (as measured by Courtney), then turns around and comes back at 0.6c. When Andrea returns, Courtney is 20 years old. How old is Andrea upon her return?
A)10 years
B)12 years
C)15 years
D)18 years
E)20 years

Question

Quizplus · Accepted Answer

Explanation:Time dilation is a phenomenon in which time passes slower for an observer who is moving relative to another observer. In this case, Andrea is moving relative to Courtney, so time passes slower for Andrea. When Andrea returns to Earth, she has only aged 5 years (as measured by her own clock), while Courtney has aged 10 years (as measured by her clock). Therefore, Andrea is 15 years old upon her return.

A Set of Twins, Andrea and Courtney, Are Initially 10