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Many years ago (in the '70s I think) I read an explanation of the meaninglessness of simultaneity at large distances. The example had to do with two people walking along a sidewalk in opposite directions, and an alien race on a planet millions of light-years away planning an invasion of the Solar System. The example showed that in one walker's reference frame the invasion fleet had departed, but in the other reference frame the fleet had not.

At the time, the explanation made perfect sense, but I have forgotten the details and have never run across this example again.

Does anybody know where this was, or have the text of the explanation?

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The regular space time diagram makes sense of this. Applying google fu... encyclopedia2.thefreedictionary.com/relativity look at Simultaneity in prerelativity physics and the following paragraph –  Captain Giraffe Sep 22 '11 at 18:05

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I think you are talking about the Rietdjik-Putnam argument.

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I'm very sorry, I could not read past "If special relativity is true". It's not allowed within my religion. –  Captain Giraffe Sep 22 '11 at 18:32
@Giraffe, every physical theory only holds true until proven otherwise. To assume special relativity is 100% true would stagnate physics. –  Michael Sep 22 '11 at 18:38
@Michael Yes of course. But to me SR is basic geometry and geometry is useful. SR has a kind of special place among theories. Not to mention that all the accelerators on the planet relies on SR being good. –  Captain Giraffe Sep 22 '11 at 19:05
@Michael I very much doubt we will ever find a paper "we have found faster than light communication in an inertial frame" The Michelson Morley still stand in their tower. –  Captain Giraffe Sep 22 '11 at 19:11
That's it, thanks! I read Penrose's "The Emperor's New Mind", which is where I must have seen this. I notice it was published in '89, so I couldn't have seen it in the '70s (time is relative, especially as you get older :-) –  Jim Garrison Sep 22 '11 at 20:29

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