This isn't homework. I just read another person's post and it jogged my head into having questions, so this is just for my curiosity. Question 1: Aside from the practical problem of finding a fuel source that travels the speed of light or faster... Why is this a fundamental rule? I really do not get this. i get that: speed = distance/time Ok here is scenario #1: 2 spaceships are travelling at 75% the speed of light toward each other in opposite directions, one from earth to Proxima Centauri and another from Proxima Centauri to earth. Each spaceship's speed is relative to earth. Are not technically BOTH spaceships travelling at 1.5x the speed of light relative to each other? Why would this be possible but one spaceship travelling from earth to Proxmia Centaruri at 1.5x the speed of light not be possible? (again excluding the problem of finding a propellant/fuel source to do so) I mean if gravity can affect light how do we know what the "Actual" speed of light is? How do we know that this is in face a constant, as opposed to everything else in the universe where if an object is shot/thrown/launched/etc off a moving object that it's speed is added to the speed of the object it was shot/thrown/etc from (newton's 2nd and 3rd laws)? Gravity is everywhere pulling in multiple directions with all different amounts of force. Question 2: I read https://www.physicsforums.com/showpost.php?p=66811&postcount=25" by chroot (the thread actually brought me to this site) "The term "wave" is meaningless unless you also specify what it is that is waving." and it got me to wondering. In ocean waves it is the water that is waving in sound waves it is the air (usually) that is waving So what is waving in light waves? It cant be matter because they propagate in a vacuum.