I’m sure this has been covered before, but there are a lot of threads to wade thru, so here goes:
Two spaceships are traveling in opposite directions (to an outside observer) at 0.6 c.
To the observer, the distance between the spaceships is increasing at 1.2 c (more or less), right?
Now, one of the spaceships shoots a laser at the other. The laser beam is still traveling at c, so, to the observer, it will be gaining on the other spaceship at a a rate of .4 c, right?
What would the crew aboard the other ship receive in the form of a laser? Would it be Doppler shifted into a radio wave or something when it reached the other ship?