Not every world needs to be perfectly physically accurate, or even follow the same physics as our universe. However, it is a natural part of world building to want to understand the science underlying concepts you want to play with - even if you don't want to stick to the restrictions. For example if I want to write a story that must involve faster than light travel, then I'm going to do it regardless of whether it is physically realistic. However, it would still be very useful to me to gain an understanding of what is already possible, so that I can limit the amount of rule bending and make sure those rules that I do bend do not detract from the story too much.
So I definitely see faster than light and physics questions in general as on topic. Sometimes they can't be asked on physics stack exchange because that concerns our universe and current rather than future science. Even when they would be a good fit for physics stack exchange, that needn't make them off topic here.