What is truth - continued.

Speed is not a measurement - speed is a calculation. It is distance (which is a measurement) divided by time (which is a measurement). We have seen (post dated 14/7) that the ball travels further for the observer on the bridge than it does for us on the train. So what about its speed? Either it must have travelled faster, or it must have travelled for longer. One of the most difficult aspects of the Theory of Relativity to get our heads round is the idea that time is relative - that what lasts a second, say, for me may last more or less than a second for somebody else in a different frame of reference. But substitute a beam of light for our ball and this becomes the only explanation, because we know that light travels at the same speed no matter where it is measured from. The beam of light will travel further for the observer on the bridge and therefore must have taken a longer period of time to get from me to you than it did for us on the train. All this has strayed somewhat from our original question - what is truth - and we'll try and get back more on track next time.