If the epicenter distance is 5,500 km, what would the difference in arrival time be between the P and S-waves?

Answer were lower;

Some of the great contributes to advancement in the us were immigrants

1. why do astronomers think that there may be other advanced civilizations?
because  it was discovered that there are other planets like earth out there, able to  accommodate life forms.
2. what technologies are astronomers using to look for life on other planets? why?  
they  have used radio telescopes, but now they use laser telescopes   to track laser communications.