1. The problem statement, all variables and given/known data Two spaceships having rest length 100 m pass each other traveling opposite directions with a relative speed of 0.901 c As the front of the spaceships just cross, each pilot sets off a small flare at the back of her own ship, synchronized to the same instant (t=0 in her own frame). To each pilot, how far in front does the flare of the back of the other ship occur? 2. Relevant equations x'=[itex]\gamma[/itex](x-vt) L=Lp/[itex]\gamma[/itex] 3. The attempt at a solution I got the correct answer (x'=291 meters) with the help of a TA, but I'm a bit confused by why this is correct. I thought lengths contracted - here the length is extending. My book says that the proper length is measured by an observer for whom the endpoints of the length remain fixed in space. Also it says that L=[itex]\gamma[/itex]Lp. If that is the case, then why is the length extending? To me, it seems like x' and L are the same thing in this problem because we are finding how far back the flare occurs. Any help?