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  • DeLacue@lemmy.world
    0·
    1 month ago

    Given that the speed of light is the upper limit for changes to propagate through the universe; if you had a four light-year long indestructible rod and you move one end of the rod how long until the other end moves?

        • DigitalDruid@lemmy.sdf.org
          0·
          1 month ago

          you’re right, I meant 4 years as aboundary even if that’s actually high.

          Since it’s a hypothetical material we don’t know what max speed information can travel through it and the actual answer would be related to that, so a lower boundary at 4 years anyway.

          If the material was truly somehow magically incompressible then I expect we would see some kind of time dilation so that the information could move at the right speed without deforming the rod. It would still take at minimum 4 years.

    • ripcord@lemmy.world
      0·
      1 month ago

      Somewhat related: let’s say you’re moving at 80% the speed of light away from earth. Then you turn on a flashlight and point it away from earth.

      • How fast is the light emitted moving relative to you?
      • How fast is it moving relative to earth?
        • ripcord@lemmy.world
          0·
          1 month ago

          OK, so after 1 hour:

          • How much further away are you from earth?
          • How much further away from you is the light?
          • How much further away from earth is the light?
          • Jojo, Lady of the West@lemmy.blahaj.zone
            0·
            1 month ago

            It depends. The light is definitely moving at c, no matter who checks and from everyone’s perspective, but as a consequence of that the answers to those other questions change depending on how you measure. The coordinate system (technically the “reference frame”) you choose changes the answer. Even more, time isn’t the same for each frame, so you even have to specify for whom it has been 1 hour.