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Question of speed of light

  1. Dec 19, 2007 #1
    Because of the speed of light, and the distance between us and certain supernovas, we're just seeing the light of it, even though it occured 170,000 years ago.

    If we were able to travel twice the speed of light, I know it's impossible, but what if it were, and someone traveled to a planet a light year away, and then returned. Would, in one light year, we be able to see that person returning to earth, and would those same images follow the life he led up to his death?
  2. jcsd
  3. Dec 19, 2007 #2


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    Staff: Mentor

    If it's impossible physically, how are we supposed to answer your question? What do we base our answer on? Fantasy?
  4. Dec 19, 2007 #3


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    The question itself is nonsensical since the laws we use to describe the events that occur around us prohibit speeds greater than c. Do you not see the paradox of asking physics to predict what will happen if you do something that physics says is impossible?

    Edit: You've got faster berkeman...
  5. Dec 19, 2007 #4
    fantasy, reality

    I realize the question is a paradox.... it's like asking if God were omnipotent enough to create a rock that he couldn't lift.
    I am confused by the question myself, but I thought if there were laws of physics, that they could be applied to such a question. After all, we are seeing things that happened in the past, (such as the supernova) and we're using that same laws and technology to look beyond that. By reversing the motion of light that we're now seeing from 170,000 years ago, why wouldn't we be able to see something man made, such as the space ship the man traveled.
    Yeah, it's fantasy. No getting around that. We can't travel that fast, but the bottom line is, the laws of physics don't change just because we created the space ship.

    I really do appreciate your replies. I'm not trying to be difficult, it's just that my mind races constantly with questions that nobody I know even understands the questions, never mind helping me with an answer.

    Thanks again.
  6. Dec 19, 2007 #5
    It depends on how long he would stay in the other planet? Say 1 year, then after returned home, he could see what happened to him in the other world 1/2 year back. Not to his death.
  7. Dec 19, 2007 #6


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    It means you're supposed to be working in the complex plane :biggrin:

    Things like this, if they were possible, would be dominated by doppler shift.
  8. Dec 19, 2007 #7


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  9. Dec 19, 2007 #8


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    [suspend reality]
    In a Galilean universe where c is finite but not the "universal speed limit", simultaneity is not relative, time dilation doesn't happen, etc.: If someone were to leave here at 2x the speed of light travel for 6 months to a point 1 light-year away, turn around and travel another 6 months to return then the following things would happen.
    T=0mo -> he leaves, we see him leave
    T=6mo -> he arrives, we see him outbound at about .3 light years distance
    T=1yr -> he returns, we simulataneously see him return and see him outbound at about .7 light years distance
    T=1yr-1.5yr -> we simultaneously see one image of his return trip running backwards and one image of his outbound trip running forwards
    T=1.5yr -> we see the first image of his return trip merge with the last image of his outbound trip and the images end
    [/suspend reality]
    Last edited: Dec 19, 2007
  10. Dec 19, 2007 #9

    Thank you for such great explanations.
    It's quite settling to know that there are answers, explanations, and yes, even sometimes just expectations as to what might be.
  11. Dec 19, 2007 #10


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    A more "realistic" version of this question would be assume that you traveled a few light years using a "warp engine", i.e. some type of artificial "wormhole". This MIGHT be possible, it is at least not ruled out by SR.
  12. Dec 19, 2007 #11
    Nice point, forces one to take the question more seriously.

    Regardless of what your own clock does, the different stages of your progress will *appear* condensed if you are approaching rapidly, and reversed if you exceed (what I will term) "ambient light speed". If you recede rapidly, your stages of progress will *appear* delayed.

    So, you will see the traveller depart, then as you are watching the traveller move away she will arrive back again. As you see her in front of you, you will simultaneously start seeing her return journey (in reverse) in addition to watching her outward journey. Triple vision.

    So she herself can now take a telescope and watch two of her self going to the distant star. In Albuquerque we can assume zero time dilation, so the smaller her *appears* to move slow whilst the closer her does double-time in reverse. Very soon (compared to how long her journey took) she'll see herself arrive at the destination, then greet the locals at normal-speed, get back into her ship, and annihilate with her other image. :biggrin:
    Last edited: Dec 19, 2007
  13. Dec 28, 2007 #12
    "Double Light Speed" at C x SQR (0.8) ?

    It could be mentioned, that "double light speed" in the sense of reaching a star in half the time of a light ray, judged from spaceship, according to SR gives speed = C x SQR(0.8) = appr. 0.9 C . I mean at that speed the space ship reaches the 1 lightyear
    (judged from Earth) distant star in just half a year, in spaceship local time.

    I.e. length contraction gives factor SQR(1 - (V/C)exp 2) and if spaceship velocity is V relative to Earth then the equation becomes V / SQR(1-(V/C)exp2) = (2xC)
    and V = C x SQR (0.8) (If not anything wrong)

    Of course that is not the presumptions in question, but it could be reminded that
    in some sense not even SR forbids unlimited speeds. May be the concept of "speed" is in some way philosophical ... :grumpy:
    Last edited: Dec 28, 2007
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