Here's a few smattered questions I've had a hard time finding good answers for: 1. Why is the speed of light a constant? Is it related to the fact that it is massless? Is it because it isn't subject to time dilation? 2. Why is the speed of sound, or EVERYTHING for that matter, not constant? For example, if you throw a ball at me, and I measure it at 5mph, and then you throw it at me again while I'm moving towards you, why do I not still measure it at 5mp due to time dilation and length contraction, just like I would light? 3. This one may be a blunder, but I have to ask: Is the speed of light really constant, or is it just that we always measure it at the same velocity due to relativistic effects? Is there any difference between these two things? Thank you!