# On beating the speed of light

jablonsky27
hi,

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..

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.

dst
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!").

Nick89
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.

jablonsky27
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.
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, can't you?

Could you give any example of other such things moving faster than light?

Mentor
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, can't you?
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.

Could you give any example of other such things moving faster than light?
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.

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Nick89
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:
math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html

Woops, too slow :p

jablonsky27
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.

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.

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?

hmmm.. good point about the photodiodes being fired simultaneously.. never thought abt it that way.. nice references Doc Al.. thanks guys..

phlegmy
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 receiving the light and poing B receiving 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
V.apparant>c

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 each other with the source in the middle, then, although A and B are separated by a distance, once light arrive at A it take NO TIME AT ALL to reach B!

i know this doesn't add much to the discussion but i rarely ever manage to "formulate" a formula. so i just had to post :D

jablonsky27
V.apparant= c*sqrt((L2+L1)/(L2-L1))
where L2 and L1 are the distances from the source to points A and B respectivly.

that doesn't 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.

phlegmy
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 traveled 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 won't work!