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On beating the speed of light

  1. Apr 22, 2008 #1

    consider the following scenario. say i have torch, the light from which i shine on a screen. i was wondering if an arrangement were possible wherein i have the screen sufficiently far away from me so that when i trace an arc with the torch, the bright spot on the screen moves faster than the speed of light. something like a huge circular screen of radius 300000km. then when i flick the torch in my hand at more than 1 radian/sec the spot should move in the screen at a speed greater than c.

    is that allowed? i mean, the spot on the wall is not a 'physical' thing, but it does go faster than c..
  2. jcsd
  3. Apr 22, 2008 #2

    Doc Al

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    Staff: Mentor

    Sure it's allowed. There are plenty of examples of such non-things that move faster than c.

    Note that this moving "spot" cannot be used to transmit a message from one part of the wall to another.
  4. Apr 22, 2008 #3


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    No, search, this has come up many times.

    Yes, the "spot" moves faster than c. There is no spot however, just a "spot". It still takes photons the minimum time to get there so no information can be transmitted (apart from "Whohoo, I beat the speed of light with just this crappy laser pen!").
  5. Apr 22, 2008 #4
    The photons that are 'part of the spot of light' are not moving sideways when you move the torch. Instead, new photons are transmitted from your torch with a different angle in relation to the screen, so they hit the screen at a different point (effectively moving the spot of light). So while the spot of light might be moving faster than c, the photons themself are not.
  6. Apr 22, 2008 #5
    What exactly is meant by transmitting a message? If you have the whole wall covered in photodiodes then as the spot sweeps over from point a to point b you can transmit a message between those two points, cant you?

    Could you give any example of other such things moving faster than light?
  7. Apr 22, 2008 #6

    Doc Al

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    Staff: Mentor

    Nope. Imagine you are at point a and want to send a message to point b. How will someone else sweeping a light across from point a to point b help you send a message? You have no control over the moving "spot" of light so you cannot "modulate" it to create a message.

    Of course the person sweeping the light can send you a message by flicking his light on and off. But there's nothing "faster than light" about that.

    Check out: http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html#3"

    I almost forgot. To anyone interested in these kinds of things, Moses Fayngold wrote a nice little book a few years ago titled "Special Relativity & Motion Faster Than Light". I recommend it.
    Last edited by a moderator: Apr 23, 2017
  8. Apr 22, 2008 #7
    How exactly can you send a message by using these photodiodes? They might fire off very quickly one after the other but I don't see how that is faster than light...? Surely you can also fire two photodiodes simultaneously? Does that mean you are transmitting a message at infinite speed?

    For other examples see this:

    Woops, too slow :p
  9. Apr 22, 2008 #8
    hmmm.. good point about the photodiodes being fired simultaneously.. never thought abt it that way.. nice references Doc Al.. thanks guys..
  10. Apr 22, 2008 #9
    i've just had a look at this using a source of light and two points A and B to make a triange. [on paper!!]
    i got an expression for the time difference between point A recieving the light and poing B recieving the light.
    then diving the distance between point A and B, by the time difference above, you end up with the speed a dot of light seem to travel from A to B if you flicked the laser pen infinatly fast and you get

    V.apparant= c*sqrt((L2+L1)/(L2-L1))
    where L2 and L1 are the distances from the source to points A and B respectivly.

    now when L2>L1>0

    and then i see!
    if i imagine a photon leaving my laser pen for point A and then flicking infinatly fast to point B then, the next photon destined for point B has been given a head start, as you.re only going to start you timer once photon1 reaches point A. So by the time photon1 reaches point A, photon 2 is well on its way to pointB. [you jumped the gun, false start!]

    in fact you could make the velocity infinate by puttin point A and B directly opposite eachother with the source in the middle, then, although A and B are seperated by a distance, once light arrive at A it take NO TIME AT ALL to reach B!

    i know this dosnt add much to the discussion but i rarely ever manage to "formulate" a formula. so i just had to post :D
  11. Apr 23, 2008 #10
    that doesnt look right. take and equilateral triangle. then you have L1=L2. irrespective of how fast or slow you flick the source you get V.apparent to be infinite.

    i am not too good with the math, but the picture is a lot clearer in my head now than it was earlier.
  12. Apr 23, 2008 #11
    if its an equilateral triangle then L1=L2
    light leaves one vertex
    it arrives similtaneously at the other two [call them ptA and ptB]
    thus it seems it has travelled infinatly fast from A to B [thats is: there is no time interval between light arriving at A and then at B] [i even mentioned this in my above post!]

    also as i said above the above formula is for when you flick the laser pen so fast "infinatly fast" that you might as well have two laser pens, one pointing at A and one at B,
    obviously if you slew the pen very very slowly it wont work!
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