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Faster than light and time travel

  1. Sep 10, 2011 #1
    Why does faster than light travel or propagation mean time travel backwards to violate causality?
  2. jcsd
  3. Sep 10, 2011 #2


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  4. Sep 10, 2011 #3
    I see,so if i were in a spaceship and were to faster than light,i would travel backwards in time to the past?
  5. Sep 10, 2011 #4


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    That is one possible interpretation of the Lorenz time contraction formula where, if it were possible for v to be larger than c, then t becomes an imaginary number. A more reasonable interpretation would be that this is yet another reason why one can't go faster than c.
  6. Sep 10, 2011 #5
    Because of the relativity of simultaneity. If you are traveling faster than light then there are frames of reference where you arrive before you start and that's a violation of causality.
  7. Sep 10, 2011 #6
    is there anything is physics that travels simultaneity?
  8. Sep 10, 2011 #7
    how come entanglement happens faster than light?
  9. Sep 10, 2011 #8
    Because nothing travels and no information is transmitted.
  10. Sep 10, 2011 #9


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    What do you mean by "travels simultaneity?"
  11. Sep 10, 2011 #10
    Quantum entanglement means, basically:

    If 1=A, 2=B.
    If 1=B, 2=A.

    But what happens is that, when 1 is made to equal A, then 2 equals B.

    Or does changing the value of A result in decoherence? I think so, I would just like a confirmation.
  12. Sep 11, 2011 #11
    what i meant is are there things or situation in physics that travel simultainisly?
  13. Sep 11, 2011 #12
    ok let me make something clear sorry i didnt,Faster than light travel has to cope with time travel because in one frame it will be seen that the traveler traveled backwards in time,does entanglement deal with faster than light time travel backwards,meanung will some one see entanglement happen backwards in time?
  14. Sep 11, 2011 #13
    I've read that if group velocity exceeds speed of light, t2- t1 to some reference frame would be reversed to t1- t2, which means to some reference frame there is violation of causality. According to the principle of relativity, no frame is preferred. Therefore this argument is valid. But I forgot how to derive that...
  15. Sep 11, 2011 #14
    what about lijun wang's expierament done in 2000,does that deal with time travel?
  16. Sep 11, 2011 #15
    If I was the one about to go faster than c, I would be less concerned about time and causality, much more concerned about length contraction...

    Length to "0", then
    Through "0" into what?
    Negative length?
    Imaginary length?
    Hyper-real length?

    Or length expansion beyond c...

    Even if within my frame I measured nothing unusual, how much of "me" continues to function after that transition through true zero length?

    Maybe this suggests why nothing should pass through c from either the slower or faster side?
  17. Sep 11, 2011 #16
    also i forgot to add,can something travel backwards in time and not violate causality?
  18. Sep 11, 2011 #17
    cause cannot preceed event so to answer your question the answer is no any backward movement in time can only violate causality
  19. Sep 12, 2011 #18
    Well, you should look at the definition of causality.

    "Causality is the relationship between an event (the cause) and a second event (the effect), where the second event is understood as a consequence of the first."

    based on this, if you travel backward, what would happen is that you have travelled back before you start travelling. So I think this is the violation
  20. Sep 12, 2011 #19
    I know that group velocity can exceed the speed of light,but does it travel backwards in time since it travels faster than light?
    Last edited: Sep 13, 2011
  21. Sep 13, 2011 #20
    ok,i was doing some research and it seems that the group velocity can exceed the speed of light and travel backwards in time it just can't send information,is this correct? can something travel faster than light without it traveling backwards in time in any frame?
    Last edited: Sep 13, 2011
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