What is truth? - continued.

Here's a little thought experiment- you and I are on a train sitting on opposite sides of the carriage. We've measured the distance between us as two metres. I take a ball and roll it over to you. How far has the ball travelled? Two metres, yes? Now suppose that the train has a glass roof and at the moment I roll the ball to you the train is passing under a bridge, and there is an observer on the bridge looking down at us. What will he (let's assume it's a he) see? He will see the ball leave me and travel across to you. But in the time it takes the ball to get from me to you the train will have moved on, so he will not see the ball travel in a straight line, as you and I do, he will see it move in a diagonal line, and simple geometry tells us that that diagonal line will be more than two metres long. Just how much longer will depend on how fast the train is going but for sure it will be more than two metres. So in our frame of reference on the train the ball has travelled two metres, but in the observer's frame of reference on the bridge it has travelled two metres plus! But if you think that's weird it is as nothing compared with what we find if we ask how fast the ball has travelled - more to come.