# How to send a faster-than-light signal (spot Paradox)?

Mueiz

Suppose you have a source that send a very strong straight beam of light (say laser )
Then you direct the light toward a distant wall with angle 45 degree
as show in the drawing below
Then if you rotate the source 90 degree in one second, what is the velocity of the visual effect (spot)on the wall?
According to the distances given in the drawing it must be greater than the spead of light !

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Mentor
How does someone standing at the top of the screen use that spot to send a signal to someone standing at the bottom of the screen?

Mueiz
How does someone standing at the top of the screen use that spot to send a signal to someone standing at the bottom of the screen?

He can use another spot signal to the person who hold the torch.
and the total velocity of the signal from the top to the bottom will be still greater than C

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darkhorror
Why not just shine the light at both places at once?

The spots that the light shows hold no information about that point. It's just light sent from the laser to that specific point. If you want some information from that point it has to send a signal back.

Sure that point might look solid to you but it's just light being sent from the light source. Or think of you light source as a machine gun and the light as a bullet. Sure it might look like you drew a line on the wall but it's just a bunch of bullets.

Mueiz
Why not just shine the light at both places at once?

The spots that the light shows hold no information about that point. It's just light sent from the laser to that specific point. If you want some information from that point it has to send a signal back.

Sure that point might look solid to you but it's just light being sent from the light source. Or think of you light source as a machine gun and the light as a bullet. Sure it might look like you drew a line on the wall but it's just a bunch of bullets.

light spot can be made a signal easly if the three person agree to use certain code . say for example the number of spots per second or the time one spot stay in the bottom.
If the distance between those person is so great (hundereds of light-years) then the spot method will be very fast compared with
other ways

darkhorror

The light can carry information from the light source to the spot. But the spot can't carry information from one spot to the other spot unless it sends information back to the light source.

The only information is coming from the light source so you aren't sending any information from first spot and the last spot.

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Q-reeus

Suppose you have a source that send a very strong straight beam of light (say laser )
Then you direct the light toward a distant wall with angle 45 degree
as show in the drawing below
Then if you rotate the source 90 degree in one second, what is the velocity of the visual effect (spot)on the wall?
According to the distances given in the drawing it must be greater than the spead of light !
View attachment 31272
I like to let links do the talking where possible: http://www.weburbia.com/physics/FTL.html" see 3. Shadows and Light Spots. One point not mentioned there is that the 'spot' won't be a spot for very fast motion of pointer - becoming an increasingly elongated ellipse along the apparent direction of motion. Ellipticity here being ~ (2*1011)/(3*108) = 666 - basically a faint streak!

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Mueiz

The light can carry information from the light source to the spot. But the spot can't carry information from one spot to the other spot unless it sends information back to the light source.

The only information is coming from the light source so you aren't sending any information from first spot and the last spot.

I read what you said well..but you did not read my second post
I said that if the person in the top of the screen want to send a signal to the person on the bottom He can send first a spot signal to the source the source will send a similar signal to the bottom ..the final result is a signal from the top to the bottom which is faster than light also.

Mueiz
I like to let links do the talking where possible: http://www.weburbia.com/physics/FTL.html" see 3. Shadows and Light Spots. One point not mentioned there is that the 'spot' won't be a spot for very fast motion of pointer - becoming an increasingly elongated ellipse along the apparent direction of motion. Ellipticity here being ~ 2*1011/3*108 = 666 - basically a faint streak!

The shape of the spot is not important we can look at the center of the spot only and let it then be ellipse or even donkey-shaped

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Q-reeus
The shape of the spot is not important we can look at the center of the spot only and let it then be ellipse or even donkey-shaped
Well OK but don't rush to patent just yet - peer review can save real money!:tongue:

NoDoubt
I read what you said well..but you did not read my second post
I said that if the person in the top of the screen want to send a signal to the person on the bottom He can send first a spot signal to the source the source will send a similar signal to the bottom ..the final result is a signal from the top to the bottom which is faster than light also.

Are you serious? Please calculate the your distance from top of the wall to the light source and then add the distance from source to the bottom of the wall, and you will find out that it will actually take longer than sending a signal straight from the top of the wall to the bottom. Please do the math. Thank you.

Mueiz
Well OK but don't rush to patent just yet - peer review can save real money!:tongue:

This wicked wikipeadia leaves nothing for us to discover:grumpy:
But there is a mistake in what is said in the wikipeadia.
The effect cannot be regarded as contradiction to special relativity not because it can not transmit a signal (it can as i proved in my last post) but because of the fact that the moving spot is not a single object ..the photons of the spot seen at the top are not those seen at the bottom.

Mueiz
Are you serious? Please calculate the your distance from top of the wall to the light source and then add the distance from source to the bottom of the wall, and you will find out that it will actually take longer than sending a signal straight from the top of the wall to the bottom. Please do the math. Thank you.

No need for calculation it is simply the same velocity divided by 2..because we need only additional one second to send the signal from the top to the source..the final result of the velocity(of the signal and not the spot) from the top to the bottom is 10^11m/s.

Q-reeus
This wicked wikipeadia leaves nothing for us to discover:grumpy:
But there is a mistake in what is said in the wikipeadia.
The effect cannot be regarded as contradiction to special relativity not because it can not transmit a signal (it can as i proved in my last post) but because of the fact that the moving spot is not a single object ..the photons of the spot seen at the top are not those seen at the bottom.
Right. So great for letting a whole bunch of beings on some distant planet each know you're here more or less at the same time. But won't speed things when it comes to getting back a reply message!

Mueiz
Right. So great for letting a whole bunch of beings on some distant planet each know you're here more or less at the same time. But won't speed things when it comes to getting back a reply message!

when we become able to live in such distant planets i think we will find a way to convert signal into things and cause Einstein to become angry for breaking his laws.:rofl:

darkhorror
I read what you said well..but you did not read my second post
I said that if the person in the top of the screen want to send a signal to the person on the bottom He can send first a spot signal to the source the source will send a similar signal to the bottom ..the final result is a signal from the top to the bottom which is faster than light also.

lol why not just send it directly to the bottom? Your going to have to send the signal at the speed of light to the source. Then the source will have to send it at the speed of light to the bottom. Why have the extra step of sending it to the source why not just send it from the top directly to the bottom thus removing the extra step/distance it would have to travel to get to the source.

darkhorror
No need for calculation it is simply the same velocity divided by 2..because we need only additional one second to send the signal from the top to the source..the final result of the velocity(of the signal and not the spot) from the top to the bottom is 10^11m/s.

It might only take 1 second to turn your source from pointing at the top to pointing at the bottom. But once you point the source at the bottom the signal still needs to move from the source to the bottom. Just because you point the source at the bottom doesn't mean it instantly get's the message.

Mentor
I read what you said well..but you did not read my second post
I said that if the person in the top of the screen want to send a signal to the person on the bottom He can send first a spot signal to the source the source will send a similar signal to the bottom ..the final result is a signal from the top to the bottom which is faster than light also.
Nah, that won't work.

If person (A) at the top of the screen wants to use that torch to signal to a person (B) at the bottom, he must:
(1) Send a light signal to the torch holder (at T)
(2) Have the torch holder shine the light towards the second person (at B)

So the signal must travel a distance of A to T, then T to B. He'd be much better off shining his own light directly at B! (If the torch holder wastes time sweeping the light from A to B, it will take even longer for A to signal B.)

If the torch is simply swept from A to B, the apparent speed of the spot may well be greater than c but it is useless for signalling from A to B.

(Edit: I see that darkhorror made the same point. :uhh:)

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lightarrow
This kind of "superluminal" idea comes quite frequently. I suggest adding a new FAQ: "Why it's impossible to send faster than light signals moving a torch" (or something similar).

Mueiz
It might only take 1 second to turn your source from pointing at the top to pointing at the bottom. But once you point the source at the bottom the signal still needs to move from the source to the bottom. Just because you point the source at the bottom doesn't mean it instantly get's the message.

yes you are right ... my mistake is that i thought ( absent-minded) that the ray of light will conduct like a solid stick
but by the way what if we use a solid stick instead of the light ray ?

Mentor
2021 Award
but by the way what if we use a solid stick instead of the light ray ?
Then the stick will bend.

Mueiz
Then the stick will bend.

firstly;this is a thought experiment to test the postolute that it is impossible in principle (not only practically) to send a faster-than-light signal.it is not acceptable in physics to use practical difficulties to refuse a thought experiment .
secondly this experiment is a pure kinematical analysis or simply assume very small mass for the stick and almost zero gravitational field .

espen180
firstly;this is a thought experiment to test the postolute that it is impossible in principle (not only practically) to send a faster-than-light signal.it is not acceptable in physics to use practical difficulties to refuse a thought experiment .
secondly this experiment is a pure kinematical analysis or simply assume very small mass for the stick and almost zero gravitational field .

It will still bend. Relativity imposes theoretical upper bounds on the rigidity of materials. If you start pushing a material, the push cannot propogate through the material faster than c.

Mentor
2021 Award
firstly;this is a thought experiment to test the postolute that it is impossible in principle (not only practically) to send a faster-than-light signal.it is not acceptable in physics to use practical difficulties to refuse a thought experiment
I am well aware of that. Even in non relativistic classical mechanics it will bend in principle. This is because in principle mechanical disturbances in a material propagate at the speed of sound in the material.

The only difference with relativity is that the speed of sound is limited in principle to be less than the speed of light. This makes sense because the particles within the material interact electromagnetically so their interactions cannot possibly propagate faster than light.

Gold Member
Mueiz, the idea of a long rigid stick to send information >c has been proposed many, many times. It does not work.

Just like you cannot make a spaceship go faster than c, you cannot get the end of your stick to go faster than c.

Suppose you are strafing a machine gun at a brick wall, creating a row of bullet holes in the wall. Certainly if the wall is far away and you change the angle of your gun fast enough, the advancing row of bullet holes can "move" across the wall faster than the bullets travel from the gun to the wall. But does that mean if you are firing at point A on the wall, then some event E happens at your position and you want to send a signal to a person next to point B on the wall, you can get the signal to them faster than any individual bullet travels from the gun to point B? Of course not, until event E happened you weren't pointing the gun at point B, so even if you swing the gun around immediately to fire at point B, point B isn't going to get the message until there's been time for a bullet to travel from the gun to point B.

Nothing is changed here if the machine gun is changed to a laser and the bullets to photons--if the laser is aimed somewhere other than point B, and then an event E happens near the laser so someone changes the angle of the laser so it's aimed at point B, point B still won't get the message about event E until there's been time for a photon to travel from the laser to B at the speed of light.

I read what you said well..but you did not read my second post
I said that if the person in the top of the screen want to send a signal to the person on the bottom He can send first a spot signal to the source the source will send a similar signal to the bottom ..the final result is a signal from the top to the bottom which is faster than light also.
And he would send the signal to the source how? Anything that the sender does with his spot-light would be seen at the original spotlight with speed-of-light delay.

You are really not thinking this one through. No information can be carried by a moving light spot.

Mueiz
I am well aware of that. Even in non relativistic classical mechanics it will bend in principle. This is because in principle mechanical disturbances in a material propagate at the speed of sound in the material.

The only difference with relativity is that the speed of sound is limited in principle to be less than the speed of light. This makes sense because the particles within the material interact electromagnetically so their interactions cannot possibly propagate faster than light.

Relativity forbids faster-than-ligt signal without referring to any other theory concerning the structure of matter
If you use any theory other than relativity to show that the thought experiment is impossible that is your right but that implies a dangerous idea which is that :
Special Relativity is not enough to forbid faster-than-light signals and we need to use other theories.
so if you want to resolve a paradox concerning a foundamental theory like SR you must not use other theories.

Mueiz
And he would send the signal to the source how? Anything that the sender does with his spot-light would be seen at the original spotlight with speed-of-light delay.

You are really not thinking this one through. No information can be carried by a moving light spot.

Mueiz
Mueiz, the idea of a long rigid stick to send information >c has been proposed many, many times. It does not work.

Just like you cannot make a spaceship go faster than c, you cannot get the end of your stick to go faster than c.

I want to discuss it another time because I am not satisfied with old analysis
see my post #28
(I don't believe that such method can enable us to send a faster-than-light signal but I have a new analysis and i will use it in the suitable time.)

Mentor
Relativity forbids faster-than-ligt signal without referring to any other theory concerning the structure of matter
If you use any theory other than relativity to show that the thought experiment is impossible that is your right but that implies a dangerous idea which is that :
Special Relativity is not enough to forbid faster-than-light signals and we need to use other theories.
so if you want to resolve a paradox concerning a foundamental theory like SR you must not use other theories.
Your logic is flawed: All a materials science analysis tells us is that the motion of the rod is so slow that SR is inapplicable to the situation.

However, as Dale said, the ultimate limit of signal propagation in a material is determined by the speed of the interactions of the atoms. Atoms interact with each other electromagnetically and electromagnetic signals propagate at C. In a moving stick, though, the motion is transmitted via a combination of electromagnetic interaction and mechanical motion. Sending a purely electrical signal down a rod would be faster...

Mentor
2021 Award
Special Relativity is not enough to forbid faster-than-light signals and we need to use other theories.
This is correct. SR by itself is not enough to forbid FTL. According to SR you may have either FTL or causality, not both.

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Mueiz
Your logic is flawed: All a materials science analysis tells us is that the motion of the rod is so slow that SR is inapplicable to the situation.

However, as Dale said, the ultimate limit of signal propagation in a material is determined by the speed of the interactions of the atoms. Atoms interact with each other electromagnetically and electromagnetic signals propagate at C. In a moving stick, though, the motion is transmitted via a combination of electromagnetic interaction and mechanical motion. Sending a purely electrical signal down a rod would be faster...

But here you use the theory that matter is composed of atoms which is of course correct but it is not one of the postulate of SR
Do you believe that If we have a solid stick that made of continouos matter (however this is impossible) then we can send a faster-than-light signal ... if so we have to make a revision for the logical system of SR and add the postulate that matter is composed of atoms to make the statement that the speed of is the greatest correct.
I do not think that this is the solution .. I think that there is a pure relativistic analysis to show that a faster-than-light signal is impossible
I will introduse this analysis when i feel that there is agreement in the fact that referring to facts which are not relativistic is not the correct solution.

espen180
But here you use the theory that matter is composed of atoms which is of course correct but it is not one of the postulate of SR
Do you believe that If we have a solid stick that made of continouos matter (however this is impossible) then we can send a faster-than-light signal ... if so we have to make a revision for the logical system of SR and add the postulate that matter is composed of atoms to make the statement that the speed of is the greatest correct.
I do not think that this is the solution .. I think that there is a pure relativistic analysis to show that a faster-than-light signal is impossible
I will introduse this analysis when i feel that there is agreement in the fact that referring to facts which are not relativistic is not the correct solution.

It doesn't have to be a postulate of SR. According to this logic, you cannot use light to transmit a signal because SR does not tell you about atomic emission spectra.

Mueiz
It doesn't have to be a postulate of SR. According to this logic, you cannot use light to transmit a signal because SR does not tell you about atomic emission spectra.

No! i do not say that SR must tell us all facts about a phinomina before applying it .this would be too stupid thinking.
What I said is that the postulates of SR (as a fundamental theory) like that of the speed of light must stand on its own feet.
If we use other theories to defend the postulates of SR from paradoxes this is epistemologically incorrect
SR is a theory that independant from the theories of the structure of matter