SR, what is the difference in ages between twins after space trip

1. Jun 17, 2013

amiras

1. The problem statement, all variables and given/known data
Consider two twins, Joe and Ed. Joe goes off in a straight line traveling at speed of 24/25 c for 7 years as measured on his clock, then reverses and returns at half the speed. Ed remains at home.

2. Relevant equations
What is the age difference between twins after return?

3. The attempt at a solution
Joe is moving relative to stationary Ed, so time passes slower for Joe
Ed measures time: 7/sqrt(1-(v/c)^2) = 25, where v = 24/25 c

For backward trip:
Since the speed is halfed, the time it takes for Joe to return is doubled (in his frame of reference) 7*2=14 years.

Now the speed is halfed, Ed is again stationary relative to Joe
Ed measures time: 14/sqrt(1-(v/2c)^2) = 16 years

So It takes for Joe total of 7+14=21 years
and for Ed: 25+16 = 41 years.

What is the problem here?

2. Jun 17, 2013

TSny

The problem is with the statement shown in blue. Think about why that isn't correct.

Last edited: Jun 17, 2013
3. Jun 18, 2013

amiras

Yesterday while walking home, I understood what I was doing wrong, thanks for the reply!