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I Communication at a near-light speed

  1. May 14, 2016 #1
    So, I've been wondering: how would time dilation affect communications?
    For the sake of visualisation, imagine the Flash is running at 99% the speed of light in a circle around a fixed position. There's a building in this position, and inside this building are his friends. Due to time dilation, they should be experiencing time at different rates. Assuming the usual problems(wind muffling comms, drag etc.), are a non-issue, and that he is talking to his friends through conventional radio communications, would others perceive him to be talking much slower than normal, if, from his perspective, he was talking at a normal rate? If so, at what rate would he have to talk to be able to communicate with them?

    If you'd prefer, you can swap the Flash for a spaceship and the building for an asteroid; like I said, it's just to help with visualisation.
  2. jcsd
  3. May 14, 2016 #2


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    Well, to start off, his friends would have to tune their receiver differently as the radio signal would be affected by relativistic Doppler shift. Exactly what would be observed depends on the direction the Flash is running in.
  4. May 14, 2016 #3


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    Taking the spaceship case, the Lorentz factor for 99% of c is about 7. You take it from there.
  5. May 14, 2016 #4


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    Draw a spacetime diagram with a sequence of periodic transmissions.
    Note the receptions.
    As mentioned by @Orodruin , this involves the Doppler Effect.
  6. May 15, 2016 #5
    Seriously, do this.
  7. May 15, 2016 #6


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    Signalling between circular moving observer and observer at the center of the circle (presumed inertial) is a special case . The situation is stationary with a lot of symmetry (rotation, time translation). Using this, the exact answer can be written down with virtually no computation or diagramming. Nor do you need to worry about Doppler.

    After the fact, it is interesting to explain both observers' point of view with proper application of Doppler, helping understand the nuances of transverse Doppler.
    Last edited: May 15, 2016
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