A set of twins, Andrea and Courtney, are initially 10 years old. While Courtney

Question

A set of twins, Andrea and Courtney, are initially 10 years old. While Courtney remains on Earth, Andrea rides on a spaceship that travels away from Earth at a speed of 0.60c for 10 years (as measured by Courtney). At the end of the trip, Courtney is 20 years old. How old is Andrea?

20 years

10 years

18 years

12 years

Suppose one twin takes a ride in a spaceship traveling at a very high speed to a distant star and then back again, while the other twin remains on Earth. Compared to the twin who remained on Earth, the astronaut twin is

the same age as the Earth-twin.

The ages cannot be determined from the information provided.

younger than the Earth-twin.

older than the Earth-twin

20 years

10 years

18 years

12 years

Explanation / Answer

A moving clock runs slow.

So it a time t passes for Courtney, the proper time t0 that passes for Andrea is:

t0 = t / gamma = t sqrt (1 - (v/c)^2)

andrea's age = initial age + (courtney's age - initial age) sqrt (1 - (v/c)^2)

= 10 +(20-10)(1- (0.6c/c)^2)^0.5 = 10+8 = 18 years

correct option is (c)

younger than the Earth-twin.

Correct option is (c)

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