
#1
Jan2810, 07:51 PM

P: 146

If i travel at .9c in a spaceship from earth to a star 20 lightyears away, and come back (assuming uniform motion the entire time). When i arrive back to earth why does is my twin older than me?
My twin moved at .5c relative to me, so why does time slow down for me and not him? when reading about special relativity the book said i would arrive on earth and see my twin much older than i am, but isnt my twin and everyone on earth moving at .9c relative to me in the space ship? 



#2
Jan2810, 08:23 PM

P: 250





#3
Jan2810, 08:40 PM

P: 15,325





#4
Jan2910, 01:35 AM

Emeritus
Sci Advisor
PF Gold
P: 9,018

time slows down for who? 



#5
Jan2910, 10:31 AM

P: 250





#6
Jan2910, 10:38 AM

Emeritus
Sci Advisor
PF Gold
P: 9,018





#7
Jan2910, 11:33 AM

P: 250





#8
Jan2910, 12:10 PM

Emeritus
Sci Advisor
PF Gold
P: 9,018





#9
Jan2910, 07:40 PM

P: 446

As Kev pointed out one party travels in a straight line in spacetime and one party travels on a path with two separate legs. The straight line is always the shortest path.




#10
Jan3010, 10:07 PM

P: 42

Why all the emphasis on the turnaround?
Suppose the traveling twin arrives at his destination and stays there, wouldn't he still be younger than the earth twin? After one year at his destination suppose both he and the earth twin leave at the same time and travel at .9c and meet in the middle. They compare ages. Wouldn't the twin who stayed on earth be older? Thecla 



#11
Jan3010, 10:57 PM

P: 15,325





#12
Jan3010, 11:02 PM

Mentor
P: 16,485





#13
Jan3110, 12:55 AM

P: 42

Let me see if I understand this.
The twin travels at .9c a distance of 20 light years and stays at his new planet. No return. Doesn't the traveling twin have to be younger even without a change in direction for the return voyage? How will he know he is younger? Both twins can send each other pictures and compare. The question I am asking:Does the traveling twin have to make a return trip to earth to be younger? Thecla 



#14
Jan3110, 07:05 AM

Mentor
P: 11,255





#15
Jan3110, 10:40 AM

P: 3,967





#16
Jan3110, 12:41 PM

Sci Advisor
PF Gold
P: 1,809

To illustrate what kev just said, consider this "quadruplets paradox".
Alice and Bob set out from Earth at 0.9c. Carol and David stay on Earth. After 9 years 8 months have passed on Earth, David follows out on the same route and speed as Alice and Bob. Carol remains on Earth. Meanwhile, after 9 years 8 months have passed on Alice and Bob's ship, they reach their destination (20 light years from Earth). Alice jumps off ship, decelerates and stops (relative to Earth), but Bob continues at 0.9c away from Earth. David is at rest relative to Bob. When they compare their ages by exchanging messages and allowing for the delay, they find that, at the moment P that David left Earth he had aged 9 years 8 months since the experiment began but Bob had aged 22 years 3 months. Alice is at rest relative to Carol. When they compare their ages by exchanging messages and allowing for the delay, they find that, at the moment Q that Alice arrived at the planet she had aged 9 years 8 months since the experiment began but Carol had aged 22 years 3 months. To summarise, Alice and Carol agree that Carol is older, and David and Bob agree that Bob is older. But, until they separated, Alice and Bob travelled together and so were the same age; until they separated, Carol and David travelled together and so were the same age. According to David & Bob, immediately after David left Earth (P), Carol = David < Bob. According to Alice & Carol, immediately after Alice landed on the planet (Q), Bob = Alice < Carol. So everyone disagrees over whether Bob or Carol is the oldest. Complications like this are the reason why the twins paradox is formulated in terms of twins who meet each other again so that there can be no disagreement over who is older. 



#17
Jan3110, 08:32 PM

P: 42

In response to KEV I don't think there has to be a return trip to establish an age difference. Consider the following:
A third planet is situated exactly halfway between the Earth and target planet that the twin landed on. This planet is stationary with respect to the other two planets and the three planets form a straight line. The halfway observer has a very powerful telescope and points it East to see the Earth twin ten light years away. He swings the telesope West and sees the traveling twin just landing on the target planet 10 light years away. According to the halfway planet observer looking through his telescope, which twin has more gray hair and wrinkles? Thecla 


Register to reply 
Related Discussions  
Time slows at the speed of light  Special & General Relativity  1  
time slows down when you approach the speed of light?  Special & General Relativity  80  
since time slows down in a strong gravitational field, does time stop in a black hole  Special & General Relativity  12  
Time slows as you speed up right, so..... ???  General Discussion  5 