# Special relativity question

1. Apr 16, 2012

### chris2112

An intersting question was asked in my class today when learning about time dilation. We were shown the classic example of a light ray perpendicular to the floor of a moving train reflecting off the ceiling back to it's source. We were shown the lightray make an angle at the ceiling for an observer outside the train to derive the time dilation. A student asked the teacher to consider a photographic plate right above the ceiling and asked if the ray were to keep going, would the obsever outside the train see a different spot than theobserver inside the train considering the angle. The teacher said yes, they would see different spots, but it doesn't seem right to me. Does the light ray really make an angle to the observer outside the train in reality or is it just a convenient way to derive time dilation?

Last edited: Apr 16, 2012
2. Apr 17, 2012

### clamtrox

I think the key here is that the observer outside will see the entire train deformed accordingly. So while they do not agree on the position of the beam hitting ceiling, they do agree on which atoms it hits. So they are not "different spots", just the distance measure is different.

3. Apr 17, 2012

### S1lent Echoes

 Sorry, I deleted my comment and decided to start my own post for it. Although it was about the same subject, the question about it was different. Sorry for intruding your post.

Last edited: Apr 17, 2012