Hello! I'm hoping someone or group of someones can help me with something I'm stuck on, having to do with the Michelson Morley Experiment (MME for short if that's okay). I want to say that I'm learning Relativity and I'm getting it, as long as I accept the Given that the speed of light will always be measured to be c, regardless of what reference frame the observer measuring it is in. I get all that and it's totally cool. What I'm stuck on is how exactly the MME proves that. I get the idea and understand it (not 100% obviously, as I wouldn't be asking this question I suppose if I did), but to explain what I'm stuck on, I have to talk about the Tennis Balls now. Say I'm in a space ship cargo hold. I'm hanging out between galaxies in empty space and the thrusters of my ship are off and everything is quiet and off to save energy (I've gone green!). The cargo hold has had all the air pumped out. It's a vacuum. It's exactly 10m by 10m, or 100 m2. There are no gravitational effects acting on it and the ship isn't accelerating in any way. In the middle of this cargo hold is a device bolted to the floor that shoots tennis balls. One "gun" shoots tennis balls in direction x and the other "gun" shoots them in direction y, perpendicular to the gun shooting in the x direction. I activate the machine and it shoots tennis ball X (tbX for short) in the x direction and tennis ball Y (tbY for short) in the y direction. It shoots them both at the exact same time and at exactly 10m/sec. After 2 seconds, both balls return to their firing point. That's 1 second to hit the cargo hold wall, and 1 second to come back after bouncing off the wall. So, there it is... I fired two tennis balls in different directions and they came back at the exact same time. How is this not the same as the MME and light? Why could I not conclude, whilst hanging out in my spaceship eating a sandwich, playing with tennis balls, that the speed of a tennis ball is the same for all observers regardless of their reference frame? This is what my brain says the MME is saying, but I'm sure it's saying something else. What is different about the MME that I'm missing? Any thoughts and/or help is appreciated! - Noyhcat P.S.: I do get that the MME was attempting to prove the existence of the aether, but when reading about Relativity, it always references this experiment and how because the returning light waves did NOT interfere destructively, it means there was no variation in speed, and therefore, the speed of light is the same in all reference frames.