# Synchronization of time impossible?

1. Dec 17, 2013

### chowyunfat321

Sorry, strange question, but let's do a thought experiment: if a train was moving at near light speed and hit a slow moving object on the tracks, would the two timelines synchronize? since the train is moving at near light speed, according to relativity, time on the train would slow down relative to stationary objects. However, once they hit, it would not be possible that they would be in the same timeline because such a synchronization would mean that in effect, time travel would be possible.

but if the timelines are not synchronized, wouldn't we have a strange state of affairs where two different timelines existed side by side??

2. Dec 17, 2013

### phinds

You seem to be positing infinite acceleration (instantaneous slowing of the faster train, etc.)

3. Dec 17, 2013

### chowyunfat321

No, the train is going close to the speed of light, fast enough for space-time distortion to be occurring. But once the collision happens, they would be in the same timeframe wouldn't they? but how would this be possible?

4. Dec 17, 2013

### phinds

Why would you expect this to work any differently than if you were riding a motorcycle at 50 MPH and a car going 70 MPH hit you from behind and you got stuck in his bumper and you both ended up moving forward at 65 MPH. The exactly numbers would be different but conceptually it's the same thing (overlooking, of course, the actual fact that the real result would be one really massive explosion).

5. Dec 17, 2013

### chowyunfat321

hehe yes it would be a fireball.
But at 50MPH there would be no time distortion. at close to the speed of light we would be dealing with a different time frame in terms of relativity. his clock would be slower ... the clock of the poor sucker on the tracks would be faster. they would be in different times. but once they collided, wouldn't the two times synchronize since they would be involved in one event? how does relativity resolve this?

6. Dec 17, 2013

### phinds

I just don't see what there is to resolve. I see no qualitative difference between the two scenarios.

7. Dec 17, 2013

### chowyunfat321

No? perhaps... but aren't there two different times both moving at different speeds? if there is no synchronization, how can they 'share' this event of crashing?

8. Dec 17, 2013

### phinds

How could they possibly NOT share it. You are getting confused by time dilation. Time dilation is NOT a characteristic of an event it is an artifact of how that event is perceived by an observer in a different reference frame. Each of the two objects sees itself as stationary with the other object approaching it and hitting it.

Last edited: Dec 17, 2013
9. Dec 18, 2013

### julcab12

Assuming most variables are neglected(mentioned by Phind).In an event where 2 object with different speed with the same mass moving away relative to each other(considering relativistic velocity-addition formula and adding 3rd law of conservation). Both will exert equal force during collision. After that depending on the impulse. Both will experience the same time(During collision)and gradually losing speed(due to resistance and friction) as they're moving away to each other. The faster the object move the slower the time relative to the other object. We can measure such small almost negligible difference in femtoseconds at lower speed.

Furthermore. We’re always traveling forwards through time. Imagine this scenario. Considering you and a friend. The moment you stand up. You’re now further from the Earth, and so gravity is a tiny bit weaker for you. Which means you're experiencing a femtosecond slower relative to your friend who’s sitting down. If your moving away from him/her. Your time is slower than hers/him. the faster you move, the slower time will pass for you relative to hers/him.^^

10. Dec 22, 2013

### Staff: Mentor

Are you thinking that two observers must be in the same inertial frame, and thus have their clocks ticking at the same rate, for there to be an interaction? If so, that is not true.