It begins, there are two people. One (Bill), is standing on the platform of a railroad station waiting for the train to pull in. The other, Mary, is on a train coming towards the train station. The only thing abnormal about this scene is that the train on one side, the side facing the platform, is missing its side but only to anyone on the outside of the train. People on the outside of the train can see into one side of the train because that one side seems to be missing its wall, but if your inside the train it seems like the wall is there and you can’t see outside. I know it sounds weird but just follow me because we need it to be that way but it doesn’t have much to do with what we’ll be talking about. So again Bill on the platform can see into the train but Mary inside the train cannot see out. That’s easy enough. Now inside the train, Mary is sitting on a chair in the center of the train, against the wall on the side opposite of the train platform where Bill is standing and she is facing the platform side. As she sits, Mary is watching two people play ping pong on a table in the center of the train car. Looking at the train the player towards the back of the train is about to serve the ball towards his opponent on the side of the table towards the front of the train. He is serving in the direction the train is moving. At the same time he makes his first serve, the train is passing the train station platform. The train is moving at a speed of 90 miles an hour and traveling past the train station going to the next station.
Now as the train passes Bill on the platform, his eye catches the ping pong game so both he and Mary are watching the player make his first serve. Here comes the strange part of who, what, where and when. When the player hits the ball, the ball will be moving 10 miles an hour in the direction the train is moving. As Mary watches the player hit the ball, she sees the ball move towards his opponent at a speed of 10 miles an hour. Because Mary is traveling inside the train at the trains speed, she sees the ball hit and move at 10 miles an hour. As Bill watches standing on the platform, the train is wizzing by him at 90 miles an hour. When the ball is hit he sees the ball move at 100 miles an hour because he not only sees the ball move at 10 miles an hour, he also sees the train moving at 90 miles an hour so it appears to him that the ball is traveling 100 miles an hour. Which one is right, Mary who sees the ball move at 10 miles an hour or Bill who sees the ball move at 100 miles an hour. The answer is, they are both right. How can that be you ask? Good question.
The answer is relativity. What they see is relative to where they are. Mary sitting inside, is moving at the speed of the train but can’t see outside and doesn’t perceive the movement of the train so for her the only thing she sees moving is the ball that when hit moves at 10 miles an hour. For Bill, he has a different perspective as he sees the surroundings and sees the movement of the train with the movement of the ball. For Bill, the ball is moving much faster than it is for Mary.