Pretty nice puzzle. I won't spoil it with the answer, but it was a toughie. There is a real answer, although I doubted it on first reading the puzzle.

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The ether of general relativity therefore differs from that of classical mechanics or the special theory of relativity respectively, in so far as it is not 'absolute', but is determined in its locally variable properties by ponderable matter.

Albert Einstein, "On the Ether", 1924