1. Aug 17, 2004

### Gfoxboy

Okay, everyone knows about the twins paradox, you know one twin flies around near the speed of light in a spaceship or whatever and when he comes back he's 50 years younger than his twin brother. Well... according to relativity you can reverse any motion and say that the universe is passing by his spaceship at the speed of light, and therefore everything else in the universe would be younger than him when he was finished with his little trek. It sounds retarded and I really want to find a way of making this wrong. I love Einstein and his theories so please help.

2. Aug 17, 2004

### DW

See problem 5.4.4c at
http://www.geocities.com/zcphysicsms/chap5.htm#BM65

3. Aug 17, 2004

### Staff: Mentor

You know that its the spaceship moving because to travel away from Earth it had to accelerate.

4. Aug 17, 2004

### Gfoxboy

well yes he did have to accelerate initially however... does time dilation still affect you if you aren't accelerating anymore? think about it this way, yes it makes sense when you are accelerating becasue if you said that the ship was staying still then the passenger would have to be accelerating backwards to account for the force on the seat, however when he reached a speed near the speed of light, time would slow down for him according to time dilation, however if he stoppped accelerating it would be easy to use the ship as the 0,0,0 coordinates and say that the universe was passing him by at near the speed of light. Oh and i read your reply page DW and i'm not that good at reading really high level math considering i'm 17... however i think i might understand what that was getting at but that only applies to acceleration not constant velocity

5. Aug 17, 2004

### Gfoxboy

And an additional thing DW, the link you sent me refers to circular motion, which would imply constant acceleration.

6. Aug 17, 2004

what russ meant was once the person in the feels the acceleration, he knows he is the one who is gonna have his time pass by slower. You can make this work for time too. Thats how time knows when to slow down, when it feels who is acceleration, then it knows who is in motion, its a farfetched way to look at it, but it works.

7. Aug 17, 2004

### blue_sky

If this is true, we could think that not all the co-ordinate systems are equivalent... the 1 with no acceleration it's special.....

8. Aug 17, 2004

### DW

Your welcome, but problem 5.4.4c isn't the one about circular motion. Thats 5.4.5

9. Aug 17, 2004

### pervect

Staff Emeritus
Time dilation is not directly related to acceleration. Consider the triangle inequality from geometry. The shortest distance between two points is a straight line. The observer who travels from point A to point B in a straight line will travel the shortest distance, the one who travels from A to C to B will travel a longer distance. The observer who doesn't travel in a straight line must at some point make a "bend" in his path. This "bend" in his path is like the acceleration.

The situation with time is very similar to that with the triangle inequality, the only difference is the sign. The observer who travels in a straight line will measure the longest amount of time on his clock - the observer who travels a crooked path will measure a shorter amount of time.