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Light cones in faster than light travel

  1. Sep 2, 2008 #1
    Looking for clarification:
    FTL travel is said to imply signals moving backwards in time for some inertial frames. Is this because in certain cases, depending on how much greater than light speed one is able to achieve, someone can signal someone else's "past light cone" in the graphical sense? In other words, if you were to graph someones light cone, in certain cases, would a faster than light signal intercept the "past" light cone and therefore send a signal back in time?
    Let me know if i should explain myself better. or please give me a link about Faster than light Travel.
  2. jcsd
  3. Sep 2, 2008 #2


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    FTL signals from a given point in spacetime always go outside both the past and future light cone of that point. What's true is that if FTL signals work the same way in every frame, then if I send a signal to a friend moving away from me which travels FTL in my frame, and the friend then sends a reply which travels FTL in his frame, it is possible for his reply to enter the past light cone of the original event of my first sending the signal, so I get the reply before I sent the original message. There are some good spacetime diagrams of such a thing here:

  4. Sep 2, 2008 #3
    thanks alot thats what I thought.
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