# Time Sync Problems

1. May 28, 2011

### whereisnomar

http://galileoandeinstein.physics.virginia.edu/lectures/synchronizing.html" [Broken] the website talks about synchronizing clocks on a moving train. But I have a few questions about it:

1) Why are you allowed to add ct+vt on the left-hand side of the first equation below the picture of the train? I thought one of the main tenants of Special Relativity was that you can't add velocities (or I'd assume distances) without taking into account the dilation/ contraction of that quantity.

2) What would happen if you stopped the train, and then looked at what the clocks say? Would they be synchronized or not? I understand that the train would enter a noninertial reference frame, which would change how they behave, BUT wouldn't both clocks be affected equally... so who is right once the train is stopped- the person who says they're synchronized, or the person who says they're not?

THANK YOU SO MUCH!

Last edited by a moderator: May 5, 2017
2. May 28, 2011

### ZealScience

It didn't say that velocity is added, ct+vt is the distance wich had been through a length contraction.

Though I didn't learnt much of it, I don't think that the clock would be synchronized. Because according to the rest frame it is not synchronized even when the train is still moving, for different frames have different synchronization.

Last edited by a moderator: May 5, 2017
3. May 28, 2011

### schaefera

So at what point would the person on the train see them leave synchronization, then?

4. May 28, 2011

### ZealScience

I think just when the train changes speed. Look at the time of rear and front, they depend on c+v and c-v, so when v changes time would change. Synchronization means when the two times are equal, but if they are equal at one v they gotta be different when v changes.

5. May 29, 2011

### schaefera

Any other ideas?