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Velovity of a light beam in an arc

  1. Aug 30, 2010 #1
    I was using a small penlight laser around the floor to have my cat chase the impossible dot. A though came that if I could move the dot fast enough from my porch , and with sufficient power, that at some distance along an arc the dot would move faster than the speed of light.

    Any comments would be appreciated.
  2. jcsd
  3. Aug 30, 2010 #2


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    That's entirely correct, and entirely unremarkable. The dot is not a physical object, so there's no reason to think it is limited to some maximum speed.
  4. Aug 31, 2010 #3


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    If you observe the movement of the spot on a distant object (say the surface of the Moon) it will appear to lag behind the 'beam' you are waving around. Imaging projecting a vertical slit of light from a revolving source (lighthouse style) onto a saucer shaped (shallow cone) ground. The shape of the line of light would have a curve to it because of the time taken for the beam to reach the further regions.
    Of course, this is a very subtle effect. It was used, in essence, in some of the first attempts to measure the speed of light, using rotating mirrors. You need to rotate the mirror very fast and use a very long 'throw' for the beam to observe the effect.
    The spot could, of course, be observed to move between two points on the Moon at faster than c because, as stated before, nothing with mass actually moves..
  5. Aug 31, 2010 #4
    On a similar note, an "object" on a digital screen (video game etc.) could be made to move faster than light.
  6. Aug 31, 2010 #5


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    Being a bit pedantic, I think the rate of writing separate objects on a screen and the separation of them (particularly with a conventional CRT) would not allow even virtual speeds approaching c. We're talking milliseconds and cms, rather than microseconds and hundreds of metres.

    edit - well, I guess a TV line interval is 64us, rather than 'milliseconds', but you're still pushing it.
  7. Aug 31, 2010 #6
    Ok now what you thought is really good and appreciable my friend, but what i think is that the dot is actually not matter it is just a point which you see and since that dot has no matter associated with it, it is a doubtless thought that the point can move with a speed much larger than light.
    You can understand it this way, first take a paper and pen and do what i say
    Step1: Draw x and y axis
    Step2: Now imagine a rod parallel to x axis and one rod parallel to y axis, now draw them at any positions.
    Step3: look at the intersection point of the two, and write down its coordinates(that is some (x,y))
    Step4: Now imagine the rods to be moving with the speed of light, The x paralell rod in direction of y and vice versa
    Step 5: Now look at the point of intersection, it is moving away from the origin with a speed larger than speed of light.
    So it is possible for the dot to move with a speed larger than that of light as you saw in this case. But i appreciate your thought, keep Innovating and share with me
  8. Aug 31, 2010 #7
    The best oscilliscopes certainly have a writing speed in excess of c but of course this is only phase velocity.
  9. Aug 31, 2010 #8


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    It's frequently expressed that no information can be transmitted faster than light. There is no way to encode a message into the intangible dot, so there is also no reason for it to be constrained as to speed.
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