# Time dilation

1. Sep 9, 2007

### C_Ovidiu

It's my firt time reading special relativity and this question bug's me . It is said that times runs slower at greater velocities .
Let's say I leave home in a space craft and fly for some time at 99% the speed of light measured from earth. It is said that when i come home i will be older than my twin brother .
Now let's transfer ourselves in the space craft . The earth now will move with 99% the speed of light from me . So when my brother comes back to me (he remained back on earth ) he should be younger than me .

So which one is it ?

2. Sep 9, 2007

### yogi

When the journey is completed, the brother that traveled will be younger. This follows from the transforms of SR which relate space and time in relatively moving inertial frames

3. Sep 9, 2007

### C_Ovidiu

Can you be more explicit please .
How can you say which is moving and which isn't ? Acording to the space craft the earth is moving .

4. Sep 9, 2007

5. Sep 9, 2007

### C_Ovidiu

One more questions .
Let's say we have to space crafts moving towards each other . We know nothing of their movement before . We also have no still point to determine which has which velocity . All we know is their relative velocity .
Can we say know if the time runs slower in the second craft compared to the first , or is it the other way around ? Or it's a complete mistery ?

6. Sep 9, 2007

### yogi

With no initial conditions given - two spaceships that pass each other have no way to know which is moving - if they make measurments using clocks and rulers as they pass each other, each will see the clock on the other spaceship running slow, and they will measure the other spaceship to be contracted. In other words nothing can be concluded because the situation is symmetrical

On the other hand, if there is an initial synchronization of clocks on two separated spaceships at rest in the same frame, and one is accelerated so that it is now moving toward the other, Einstein predicted that when the two ships meet, the clock on the spaceship that was put in motion will show less time when the two clocks are brought together and compared in the same frame

Last edited: Sep 9, 2007
7. Sep 9, 2007

8. Sep 9, 2007

### JesseM

But Einstein would also say there's no "objective" sense in which the two clocks were "synchronized" before the acceleration, they were only showing the same time in their rest frame but not in other frames.

9. Sep 9, 2007

### yogi

Of course - there is no universal time in SR