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Faster than light

  1. Jul 20, 2009 #1
    I know that relativity doesn't permits a motion at speed of light or fast, as the Lorentz factor becomes infinite and we get absurd results, but what about wormholes or something, say wrap drive and what about http://en.wikipedia.org/wiki/Alcubierre_drive" [Broken] and
    tachyons.
    Also I've heard that in quantum chromodynamics that electrons normally travel at speed greater than c, ans some less than c, so that average speed is c.
    So does this all means that we can go faster than light, if so than what will happen to casualty?
     
    Last edited by a moderator: May 4, 2017
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  3. Jul 21, 2009 #2
    Special relativity explains that an information cannot travel at speed greater than c, because the Lorentz factor would become a complex number ; but there are situations where "something" goes faster than c. For example, imagine a laser pointed forward a planet ; then, you turn the laser forward another planet ; this action during one second. If there is a distance of one light-year between the two planets, the spot will have a speed of one light-year per second.
     
    Last edited by a moderator: May 4, 2017
  4. Jul 21, 2009 #3

    HallsofIvy

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    "The spot" not being an actual physical entity, of course.
     
  5. Jul 21, 2009 #4

    Dale

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    The whole point about wormholes etc. is that you always have a timelike worldline (ie v<c always). Wormholes are about taking a shortcut, not about going fast.
    You can have any two of the three: relativity, causality, and FTL. Right now it looks like relativity and causality, not FTL.
     
  6. Jul 21, 2009 #5
    No, light will reach the spot only at c so the speed of the spot will prove to be c The light would be like a string rotated in the air.
     
  7. Jul 21, 2009 #6

    Doc Al

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    No, there's nothing preventing a "spot" from moving faster than c. As has been pointed out, the spot is not a physical entity, so there's no issue with relativity.
     
  8. Jul 21, 2009 #7

    diazona

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    The spot will appear to travel at 1 ly/s, since at one time the laser is shining on one planet, and 1 second later the laser is shining on the other planet. But all this will only be seen on the planets some time after the person holding the laser moves it, because of the finite speed of light.
     
  9. Jul 21, 2009 #8
    No, the spot does not move at>c, because you do not see it to be moving at>c as light takes longer to reach the spot and reflect back
     
  10. Jul 21, 2009 #9

    Doc Al

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    The travel time of the light to the surface is irrelevant.
     
  11. Jul 21, 2009 #10
    The point here is that he said the spot moves at >c and all agreed.That is what is clearly wrong to me you cannot say the spot has moved unless you see it
     
  12. Jul 21, 2009 #11

    Doc Al

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  13. Jul 21, 2009 #12

    sylas

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    Doc Al is correct. The time it takes for the light to get from the spot to you is irrelevant.

    For example, consider a laser beam swept across the face of the moon; assume that it is bright enough that you can see it; and that it takes 1 microsecond to sweep from one side to the other. The dot moves faster than c.

    From Earth, you see the dot sweeping across the face of the moon a bit over a second afterwards. But you still see the dot sweeping over the face of the moon in a microsecond.

    If you are closer, or further away, you'll change the time between when you see the spot moving and when it actually swept across the moon; but that makes no difference to how fast the spot is moving or how fast you see it moving.

    Cheers -- sylas

    PS. For pedants in the audience; yeah, I know there will be tiny differences due to different angles at one side of the sweep or the other. It's still seen to be moving at almost the same velocity it actually did sweep across the surface, and that speed is much greater than lightspeed.
     
  14. Jul 21, 2009 #13
    The scissor Gedanken exp is wrong The theory essentially says no mass can move at speeds greater than c Mass elements farther than approximately 3*10^7 away are not free to move in(neglect curvature)
    Also, there is shadow only if there is light, so the speed of shadow is restricted to be equal to the speed of light
     
    Last edited: Jul 21, 2009
  15. Jul 21, 2009 #14

    sylas

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    "scissor"?

    I'm guessing you mean a case where you close scissors sufficiently fast and the cutting edge where the two arms meet moves faster than light.

    This edge is not a "mass element". The sweeping dot is not a "mass element". There is no violation of relativity or the limit of light speed on transmission of information or particles.

    Added in edit: Found it. You are referring to The Superluminal Scissors. It is indeed precisely as I guessed, and it is not "wrong". In the specific example given, the contact point moves faster than the speed of light, but there is no matter moving faster than light speed.
     
  16. Jul 21, 2009 #15
    Yes?Then what is it?
    It is not, but if you think of my answer you will find it not to be greater than c
     
  17. Jul 21, 2009 #16
    No no no
    Look at doc's reply#11
     
    Last edited: Jul 21, 2009
  18. Jul 21, 2009 #17

    sylas

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    The point in space where the two blades meet of course. The particles involved in that point keep changing as the scissors close.

    Your answer is flatly wrong. This isn't hard. Abstract points like the meeting point of two scissor blades or a dot of light sweeping over the face of the moon easily go much faster than c. There's no particles moving with the dot, or the cutting point on scissors.

    I have no idea why you don't follow this. It isn't hard.
     
  19. Jul 21, 2009 #18
    OK I hadn't read it properly
    The explanation is right, but it takes much much longer to close the scissor as written there,it is written that it takes longer for the tips to come close, and note that I had initially written the light travels as a string rotated in the air, analogous to bending of blades here and from the numeric mention in the previous post the contact point moves at c towards the tip, not greater than c
     
    Last edited: Jul 22, 2009
  20. Jul 21, 2009 #19
    I'll explain. In the gedanken expt, the scissor is closed in 1/10 of a sec. If the edges were not to bend, the scissor had to be of length c/10 meters the farthest mass element has velocity c.If in 1/10 sec the edges overlap, the velocity of the farthest element and the contact point are both same=c.Even if they were bent, the velocity of the farthest element(at 1 ly meters,original lt)is c and the velocity of cp is c
    The blades are said to be bent because it takes apparently longer time for the farther elements to come close
    The scissors flex at c/10 meters
     
    Last edited: Jul 22, 2009
  21. Jul 21, 2009 #20
    There's no particles moving with the dot, but that does not mean you can see the universe rolling around you at superluminal speeds.You perceive it as moving slower than supposed, because it takes time for light from such large distances to reach you
     
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