I am just trying to understand the concept regarding space-time with regards to the observer. I will use the concept I heard of the ping pong ball bouncing straight up and down on a table in a moving train with regards to an observer on the train and one outside of it. I understand the concept in general that the distance traveled is different between observers but that the light speed is constant. What I do not understand is the point this concept is trying to make. To the observer on the train, it may not "seem" like the ball traveled the same path or distance as it did to the person outside of the train but surely they acknowledge that fact on some level. Assuming they acknowledge the fact that they are moving. Right? Even if it is only on some subconscious level? Also, regardless of observer perspective the ball did travel along the diagonal path through space-time, right? Thanks in advance for helping to explain the idea behind this concept to me.