Q: The following paradox has puzzled me since I was a child. A fly is flying in the opposite direction to a moving train. The fly hits the train head-on. As the fly strikes the front of the train, its direction of movement changes through 180掳, because it hits the windscreen and continues as an amorphous blob of fly-goo on the front of the train.
At the instant it changes direction, the fly must be stationary and since, at that instant, it is also stuck onto the front of the train, the train must also be stationary. Thus a fly can stop a train. Where is the logical inconsistency in this (or does it explain something about British Rail)? (continued)
A: Readers鈥� explanations about the fly hitting the train cover many aspects from the width of the fly to the pliability of the windscreen. (What if the fly hits the boiler instead?)
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But they completely miss the implied point of the question, which is philosophical rather than physical. For 鈥渇ly鈥� substitute 鈥渙ne atom of the fly鈥�. This is just a rerun of the paradox posed by Zeno of Elea. Around BC 450 he said that a moving object is always in motion, and yet at any given time it is somewhere (that is, stationary). We humans cannot see, measure, or imagine an infinitely small time any more than we can truly imagine infinity. We never will.