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RichFelker's profile
Rich Felker
Rich Felker
Rich Felker
@RichFelker

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Rich Felker

@RichFelker

Yeah, I do @musllibc, FOSS & infosec stuff. But now is not the time for a mostly-/only-tech Twitter feed.

musl-libc.org
Joined March 2014

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    1. zack HOWL berg‏ @elwoz Feb 23

      I have a puzzle for the experimental physicists in the room, which occurred to me this morning in the shower. Suppose you are the sort of experimental physicist who works on relativity -- stuff like Gravity Probe B.

      2 replies 2 retweets 2 likes
      Show this thread
    2. zack HOWL berg‏ @elwoz Feb 23

      One morning you get to your office and there's a package on your desk containing two black boxes. Each has a switch labeled ON/OFF, a standard UHF coax jack labeled SIGNAL, and a pair of banana jacks labeled +12V and GND.

      1 reply 0 retweets 0 likes
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    3. zack HOWL berg‏ @elwoz Feb 23

      There is also a note. It just says "These are an ansible." There is no return address or further explanation.

      1 reply 0 retweets 0 likes
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    4. zack HOWL berg‏ @elwoz Feb 23

      Now. Technically, relativity only says that whenever a signal goes from A to B faster than light in one reference frame, there is another frame in which it goes backward in time. We assume this means FTL signaling is impossible, because we assume time travel is impossible.

      1 reply 0 retweets 0 likes
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    5. zack HOWL berg‏ @elwoz Feb 23

      But here is this pair of black boxes, and it is easy to confirm that they do what the note says they do. You put 300 km between the boxes, you synchronize two clocks, you observe a signal go into one box and come out the other in less than a millisecond.

      2 replies 0 retweets 0 likes
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      Rich Felker‏ @RichFelker Feb 23
      Replying to @elwoz

      The margin (just <1ms) probably contributes to difficulty with current tech. If it were a few ns you wouldn't need extreme accel or gravity, I think...

      10:49 AM - 23 Feb 2018
      • 1 Like
      • zack HOWL berg
      1 reply 0 retweets 1 like
        1. New conversation
        2. zack HOWL berg‏ @elwoz Feb 23
          Replying to @RichFelker

          I should have been more specific: a signal goes in Box A and comes out of Box B order of 10μs later, and, as best you can tell by putting one end and an atomic clock in a car and driving around, that's a _constant_, independent of distance.

          1 reply 0 retweets 0 likes
        3. Rich Felker‏ @RichFelker Feb 23
          Replying to @elwoz

          I understood that (except the improved 10µs timing you added). I meant you should need less-exotic reference frames to get a paradox the lower the ansible latency can be bounded.

          1 reply 0 retweets 0 likes
        4. zack HOWL berg‏ @elwoz Feb 23
          Replying to @RichFelker

          Right, I understood that. Part of what I'm wondering, by posing the puzzle in the first place, is how blatant an SR violation do you need to do the experiment with current tech. Like "here's the tabletop setup that would work, but only if the box had 1ns latency".

          0 replies 0 retweets 1 like
        5. End of conversation

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