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The Flying Bird Paradox
Suppose two people, Alice and Bob, are walking towards one another
and a bird is flying back and forth between them. When it reaches
Alice, it turns around and flies until it reaches Bob, then turns
around again until it reaches Alice, and so forth. (Assume the bird
takes no time to turn around.) Which direction is the bird facing
when Alice and Bob meet? This is just like Thomson's lamp again: it
seems it can't be facing either way.
Suppose that Alice and Bob begin walking apart again and the bird
begins flying between them in the same way as before. If the two of
them travel at 1 Km/h and the bird at 10 Km/h, where will the bird
be when they are 1 Km apart?
Answer: the bird can be anywhere along the path! Proof:
place the bird anywhere along the path and reverse the process. No
matter where the bird started from, it will end up at the same place,
in the same amount of time.
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