First statement of this question: In superluminal phase velocities, what is it that is traveling faster than light? says that

information cannot be transmitted at a velocity greater than speed of light

Also in the comment to my recent question: Is parallel programming a need of future and worth investing time and money? it is said that:

FTL communication between the two ends of our processors is impossible.

Now I have conceptual model of communication system as shown below: enter image description here

In the upper model, a laser is being moved up and down, and is being detected by the photo-detectors A & B. If the laser spot is at A , A's output is 1 else 0, and similarly for B.

In the lower model, a simple optical fiber based communication is being shown.

In both the cases we are transmitting data (1001 & 0010) from two sources Green and Red.

Now if we assume that the distance between laser and photo sensors A & B is very large, it can be proved that the spot (shown in red) can move at speed greater than light.

Can somebody explain this fallacy? In my first case information is being conveyed at a speed which is more than the speed of light, and is thus faster way of transmitting information as compared to fiber shown in the lower diagram. How is this possible?

Why information is being conveyed at much faster rate in upper case? Start from the case when the laser beam is in between the two detectors. Now let us transfer a 1 to A. So I need to move my hand up - let us say I do this at a speed of 1cm/s. Let us say with this velocity, in 1 second the spot reaches the detector A. So I have conveyed the information '1' in 1 second. And this comm took at a speed of spot which s greater that that of light. Now consider the case of usual optical comm. Assume that the distance is sufficiently large, so that light takes more than 1 second to reach the receiver.

What is wrong in my calculations?

Also Even If I do not compare upper and lower case, am I not proving that information is being conveyed at speed more than that of light. this is because the information is carried in spot that is capable of moving at a speed greater than C.

  • 5
    $\begingroup$ In my first case information is being conveyed at a speed which is more than the speed of light, You haven't shown that at all. $\endgroup$ Nov 11, 2013 at 15:09
  • $\begingroup$ @AlfredCentauri I updated my question $\endgroup$
    – gpuguy
    Nov 12, 2013 at 11:03
  • $\begingroup$ Related physics.stackexchange.com/q/6912/2451 $\endgroup$
    – Qmechanic
    Nov 12, 2013 at 11:05
  • 1
    $\begingroup$ The information speed is equal to 1/(time that you hand takes to move). It may be true that the spot travels at a higher speed in the screen. But the "information" needs to wait for your hand. After your hand moves, you need to wait for the photon to reach the detector. Therefore, you don't have information being transmitted at a speed faster than light. $\endgroup$
    – cinico
    Dec 5, 2013 at 17:42
  • $\begingroup$ I think this is a great question as this is a fallacy concerning physical phenomena, especially special relativity that is obvious after the explanation but can be quite misleading otherwise. The answer comes down to a good explanation of a whip where the tip breaks the barrier of sound. $\endgroup$
    – Alexander
    Dec 6, 2013 at 0:20

5 Answers 5


In your first case there is no information moving faster than light. The flow of information is from the actuator that controls the laser pointing to the detectors at A and B. While there is indeed "something" that moves faster than light in between A and B, there is no way for B to use that moving light spot to communicate information to A, or vice versa.

  • 2
    $\begingroup$ Information is being tranferred from laser to either the point A or B. No communication is required between A and B. That is A is not supposed to know any thing which B is getting V.V. they are simply independent. $\endgroup$
    – gpuguy
    Nov 12, 2013 at 12:59
  • $\begingroup$ That is essentially correct, yes. $\endgroup$ Nov 12, 2013 at 13:14
  • 3
    $\begingroup$ Also, the movement of the spot itself is delayed from the movement of the actuator by precisely the time it takes light to travel that way. $\endgroup$
    – Lagerbaer
    Dec 5, 2013 at 17:15

Yes, the spot on the receiver will "move" faster than the speed of light, and this does not break any physical principles. Why? Because no real thing is moving.

Consider a black box on a white background on your computer screen. Now suppose you animate it such that the box moves at 1cm/s. However, the box is just an image created by turning on and off pixels, while the pixels themselves are stationary. In short, only the image moves.

Similarly in your example, just the illuminated spot changes faster than the speed of light.

When it comes to information transfer, suppose the destination can make sense of the position of the light beam. Initially, it is pointed at an area which indicates zero. Now, it is pointed at an area which indicates one. However, the light emitted from your torch pointing at 1 will only reach the destination travelling at the speed of light.

When you start transmission at A(source), there is no data being received at B(destination).

|timestep |  A      | B       |
|    0    |  hello  |    -    |
|    1    |  1      |  hello  | 
|    2    |  2      |  1      |
|    3    |  test   |  2      |
|    4    |  -      |  test   |

As you can see, there will always be a difference of 1 year between data being sent and received, i.e. info sent at the speed of light.

Hope this makes sense to you.


Any time you read a report of superluminal whatever, it's another illustration of Phase vs Group velocity. The pattern indeed moves, but by pre-arrangement not by communication at that speed. A wonderful illustration is from Greg Egan: http://gregegan.customer.netspace.net.au/APPLETS/20/20.html

So playing with that applet is the way to explain it.


If you want to transmit information faster than the speed of light then you need to find a way to detect which type of measurement was made on particle a (alice) of an entangled pair without sending information back to particle b (bob)

To possibly prove this statement I quote;-)

If the measurement is unread it carries information.


  • $\begingroup$ -1. This doesn't answer the question at all. $\endgroup$
    – pho
    Dec 5, 2013 at 22:24
  • $\begingroup$ Which one? If people can't use a garden hose to figure out one of them they really need some schoolen;-) $\endgroup$
    – Jitter
    Dec 6, 2013 at 15:28

First I don't think that light moves faster than light. If your experiment isn't flawed it might be something to do with the frequency difference of the red light being lower.I am speaking here only from unvarified personal experience.I was once doing a radio moon bounce which had a delay on the information being sent.When i decreased the frequency to its lowest harmonic the information imposed on the radio waves appeared to be instantanious.So from my experiences information might be able to be sent faster than light.But this is not garanteed.As for parallel processing,there can be no other way.


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