# Simple Special relativity question

1. Feb 22, 2009

### theneedtoknow

1. The problem statement, all variables and given/known data

The star alpha goes nova, 10 years later (in the ref frame of an astornomer observingthe stars), the star beta also explodes. The distance between them according to the same observer is 100LY. An alien spacecraft passing through the galaxy finds that the distance b/w the explosions is 120ly. According to the aliens, what is the time difference between the explotions?

3. The attempt at a solution
I don't really get the question...the 2 explosions are 100ly away in a reference frame in which they're at rest wrt each other. Shouldn't the distance be contracted for a spaceship passin through the galaxy? how can a spaceship moving measure the distance between them as greater than the astronomer's reference frame?

2. Feb 23, 2009

### tiny-tim

Hi theneedtoknow!

You're reading too much into this question …

the question doesn't give you any information about what the stars are doing, only the position and time coordinates of two specific events