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B Thought experiment: FTL Paradox?

  1. Dec 18, 2015 #1
    So here it goes:

    2 points in space A & B.
    1 superluminal object (fixed at 10c, no acceleration) that periodically transmits photons as soon as it starts moving.
    1 observer with a very high accuracy photon counter removed a sufficiently large distance from the 2 points for light to take a small while to arrive but the distance between A & B is 10 times as large.

    Now things are going to be set in motion: The superluminal object moves from A to B. In my view the observer detects the following depending on where he is:

    1) If the observer (indicated by X) is positioned as follows:


    he will detect photons from A, then a trail of photons between A and B and finally detect photons from B

    2) If the observer (indicated by X) is positioned as follows:


    he will detect photons from B, then a trail of photons from B to A and finally photons from A.

    Now in situation 2 one could state causality is broken to the observer, but in reality no causality has been broken, we just don't have the means to detect the proper order of events using fotons. I'd also like to state that any human being would just see 1 small flash of light since the timescale is so small that we can't distinguish anything more, nothing close to seeing my grandfather, unless perhaps he lived in a different galaxy but then he'd be dead already.

    Now what am I missing in this famous grandfather paradox. Thanks for your time :D
    Last edited: Dec 18, 2015
  2. jcsd
  3. Dec 18, 2015 #2


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    Staff: Mentor

    You aren't missing anything, but as you started with a false premise (that there is faster than light travel) the conclusions drawn from that premise need not be either correct or consistent. When you're reasoning about the behavior of light emitted by an object moving faster than light, you're basically saying: "let's apply the laws of physics to a situation in which they do not apply"

    For an example of a more striking logical inconsistency that results from assuming faster-than-light travel, google for "tachyonic antitelephone".

    This thread is closed, as the issue has been discussed in many previous threads.
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