What is the ceiling on the perceived relative speeds of two bodies, from the point of view of a third observer's reference frame?
We all know the ceiling on perceived speed relative to oneself is $c$. But what about when I see two bodies heading towards each other in opposing directions? What's the ceiling on the speed I can perceive them to approach each other at, as perceived from my reference frame?
Do relativity and quantum theory agree on this?
Edit (This edit is not from the OP): This question is different from the other (two because one leads to another) two linked closed questions as the other two regard speed with respect to one of the moving objects. This question is from a perspective of a third observer. OP asks if from a third observer's reference frame can approaching two objects look as exceeding the vacuum speed of light. So clearly this resolves the original closed reason. This is a unique question that deserves an explanation. I am surprised that this is closed.
