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Ok can some one explain me what it mean that laser spot can move faster the light.

  1. Oct 4, 2011 #1
    I have read this millions of time on internet that laser spot can move faster than light. but i can't understand how.

    For example if we point our laser source towards moon then after 1.2 seconds our laser light reaches the surface of moon and creates a spot there, now if i move my laser source in order to move the spot on surface of moon then after moving my laser source the first photons again take 1.2 seconds to reach the moon so the laser spot on moon can only move after 1.2 seconds to the new location. so there will always be a lag between the previous and new position of the spot.
    Now can some one explain me how it is possible to move the laser spot faster than light ?

    I would be very thankful.
     
  2. jcsd
  3. Oct 4, 2011 #2
    Re: Ok can some one explain me what it mean that laser spot can move faster the light

    Laser spots do not move at all, two spots are not made of the same light.

    If I shoot with a gun and make a hole somewhere and then move the gun slightly to the left and shoot again does the hole move?
     
  4. Oct 4, 2011 #3

    ghwellsjr

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    Re: Ok can some one explain me what it mean that laser spot can move faster the light

    The photons that are hitting the moon at one moment in time are not the same ones that are hitting a different part of the moon a short time later so nothing is moving faster than light. What if you turned the laser beam off just before you aimed it to a new location and then back on again, would you say the first spot on the moon was the same as the second spot at a different location and that it had traveled faster than light?
     
  5. Oct 4, 2011 #4
    Re: Ok can some one explain me what it mean that laser spot can move faster the light

    Nice analysis.
     
  6. Oct 4, 2011 #5
    Re: Ok can some one explain me what it mean that laser spot can move faster the light


    But it does not matter if they are made of same light or not, because even then it's not possible to move the spot of these new photons on the surface of the moon faster than light.

    my question has nothing to do with the information i am just saying that it is absolutely not possible to make the spot of light appear at another location than the previous location faster than the speed of light.
     
  7. Oct 4, 2011 #6

    ghwellsjr

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    Re: Ok can some one explain me what it mean that laser spot can move faster the light

    If you are talking about how the spot on the moon appears to the person on earth who is aiming the laser, then you are correct, because he has to wait not only for the laser beam to travel to the moon, but also for the image of the spot to get back to earth, so including this delay the spot will never appear to travel faster than light.
     
    Last edited: Oct 4, 2011
  8. Oct 4, 2011 #7

    DaveC426913

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    Re: Ok can some one explain me what it mean that laser spot can move faster the light

    AH_AHSAN, would you agree that I can have a light bulb on Jupiter switch on then one second later have a light bulb on Mercury switch on? Has the light travelled faster than c?Obviously no. They're two separate lights.

    Same with the laser. It shoots out some photons. They hit the Moon's left edge. You swing it around in one zillionth of a second to the right edge and some other photons hit there.

    The two spots have nothing to do with each other. Nothing has travelled faster than c. The idea of "a spot of light" is entirely a construct of the person doing the observing, who supposes they are the same spot. They're not.
     
  9. Oct 4, 2011 #8
    Re: Ok can some one explain me what it mean that laser spot can move faster the light

    First of all to the title-question:

    Velocity is defined as the first time-derivative of position. The position of the laser spot is the point where the laser beam hits a surface. If the absolute value of the velocity of the spot exceeds c than it is faster than light.

    Imagine your laser hits the outer edge of the moon. Now you point to the laser within 0,0116 seconds to the center of the moon. 1.2 seconds later your laser will hit the edge and the center of the moon at almost the same time because the center of the moon is 0,0116 light seconds closer to your position. In this example the average speed of your laser spot is infinite and this is much faster than c. In this way you can "move" the spot with every speed you want.
     
  10. Oct 4, 2011 #9

    DaveC426913

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    Re: Ok can some one explain me what it mean that laser spot can move faster the light

    Stop right there.

    "The spot" has no velocity. "The spot" is not a discrete thing at all.

    "The spot" is a construct of the mind, erroneously linking a bunch of impinging photons here with a bunch of impinging photons there.

    The only velocities that exist are:
    1] the velocity of the photons traveling from Earth to Moon.
    2] the velocity of the laser's rotation.
    That's it.
     
  11. Oct 4, 2011 #10
    Re: Ok can some one explain me what it mean that laser spot can move faster the light

    Thanks a lot guyz now i understand it.
    I must be smoking some thing when i asked this.

    the point i was missing is that just moving the laser pointer direction the spot on moon would not disappear simultaneously at that time because photon emitted just before changing the direction of source are still in the space and moving toward the moon and hence spot should be there still after 1.2 seconds of moving my laser pointer.
     
    Last edited: Oct 4, 2011
  12. Oct 4, 2011 #11

    Borek

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    Re: Ok can some one explain me what it mean that laser spot can move faster the light

    I have a feeling you all answer not the question that was asked, but the question you know answer to.

    Laser sends photons continuously. When you are changing its direction it sends photons in every direction between the initial and final one. Yes, they will hit the Moon surface in 1.2 sec, but as they were sent all the time you will see all intermediate spots. Perhaps the image below will help you see what is happening.

    Edit: I see you got it in the meantime, I spent way too much time preparing the image :grumpy:
     

    Attached Files:

  13. Oct 4, 2011 #12

    DaveC426913

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    Re: Ok can some one explain me what it mean that laser spot can move faster the light

    Correct.

    (However, that doesn't change the fact that, the psot on the right side of the Moon could still appear instantaneously (faster than light) after the spot on the left side of the Moon winked out.)
     
  14. Oct 4, 2011 #13
    Re: Ok can some one explain me what it mean that laser spot can move faster the light

    thanks a lot buddy for your effort.
     
  15. Oct 4, 2011 #14
    Re: Ok can some one explain me what it mean that laser spot can move faster the light

    yes this is totally understandable to me now. thanks.
     
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