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

#### JesseM

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.

#### K^2

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 refering 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 dont belive 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.)

#### russ_watters

Mentor
Relativity forbids faster-than-ligt signal without refering 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....

#### Dale

Mentor
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 refering 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 refering 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

#### Doc Al

Mentor
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
What's your point? Where is anyone claiming that SR's claim that perfectly rigid objects (like your 'solid stick') cannot exist depends on the details of the structure of matter?

#### Mueiz

What's your point? Where is anyone claiming that SR's claim that perfectly rigid objects (like your 'solid stick') cannot exist depends on the details of the structure of matter?
The important question here is not whether such perfectly object exist or not
The important question is that ;Does SR postulate of the speed of light holds regardless of the nature of the structure of matter ?...does it hold in the imaginary case of continual matter form (which is possible intellectually even not found experimentaly)?
Is it correct to defend a postoluate of a theory using an independant other theory.

#### espen180

The important question here is not whether such perfectly object exist or not
The important question is that ;Does SR postulate of the speed of light holds regardless of the nature of the structure of matter ?...does it hold in the imaginary case of continual matter form (which is possible intellectually even not found experimentaly)?
Is it correct to defend a postoluate of a theory using an independant other theory.
When you are sending a signal through a material, the signal propogates through stresses, shears and so on. I think you have to use GR to show that the signal speed is limited to c.

#### russ_watters

Mentor
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 refering to facts which are not relativistic is not the correct solution.
Why would we need SR to deal with a fictional scenario? Why not also assume superluminal unicorns?

#### russ_watters

Mentor
Should it also be necessary to use SR to prove why my car can't exceed C?

#### Mueiz

Should it also be necessary to use SR to prove why my car can't exceed C?
if someone claims that his car exceeded C you most be able to use SR only to deny his claims ...(there might be other ways to do so but ''SR only" most be one of them)

#### Mueiz

Why would we need SR to deal with a fictional scenario? Why not also assume superluminal unicorns?
No the two cases are different because SR does not forbid perfectly rigid stick but it forbid the superluminal unicorn !
If i use a superluminal unicorn as a paradox it will be poor logic (that would be like saying ;a theory is not correct because we can assume existance of something that it forbid)
but if i used the perfectly rigid stick in a paradox of SR that means i can not defend the theory unless i used another theory that forbid the existance of such stick

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#### Mueiz

When you are sending a signal through a material, the signal propogates through stresses, shears and so on. I think you have to use GR to show that the signal speed is limited to c.
Stresses and sheas are microscopically types of motion of atoms and melecules related to the theory of the structure of matter
so you again use the theory of the structure of matter to save SR from paradoxes!

#### Dale

Mentor
SR does not need to be saved from paradoxes. It is entirely self-consistent.

That has little to do with the current question, which is a question about the structure of matter (is it possible for a material to have a speed of sound > c) and therefore requires an answer from a theory of the structure of matter.

However, without regard to the current question and without regard to a theory of matter, it is my position that SR itself does not forbid FTL. It only forbids FTL and causality together.

#### Mueiz

SR does not need to be saved from paradoxes. It is entirely self-consistent..
That is right but we want SR to defend herself without help from other theories.
So if SR faces a paradox that does not contain any orders that contradict Her own rules ,she must then resolve it without involving other theories.
That has little to do with the current question, which is a question about the structure of matter (is it possible for a material to have a speed of sound > c) and therefore requires an answer from a theory of the structure of matter..
The current question is that can a pefectly rigid body be able to send FTL signal
using SR only to answer this question ?

However, without regard to the current question and without regard to a theory of matter, it is my position that SR itself does not forbid FTL. It only forbids FTL and causality together.
This is the fist clear answer to the question.

I have a different position and i will prove that SR alone is able to forbid any FTL even if we used a perfectly rigid stick .. I will show that after finishing the problem of using other theories as a resolution to the paradox.

#### lightarrow

The current question is that can a pefectly rigid body be able to send FTL signal
using SR only to answer this question ?
A perfectly rigid body cannot exist, simply because of the fact that forces cannot propagate faster than c. There is no need to invoke matter properties, SR is enough.

#### DaveC426913

Gold Member
I have a different position and i will prove that SR alone is able to forbid any FTL even if we used a perfectly rigid stick .. I will show that after finishing the problem of using other theories as a resolution to the paradox.
Before you post another word, I would remind you of https://www.physicsforums.com/showthread.php?t=414380" on overly-speculative posts.
I stronlgy recommend you re-read it.

Overly Speculative Posts:One of the main goals of PF is to help students learn the current status of physics as practiced by the scientific community; accordingly, Physicsforums.com strives to maintain high standards of academic integrity. There are many open questions in physics, and we welcome discussion on those subjects provided the discussion remains intellectually sound. It is against our Posting Guidelines to discuss, in most of the PF forums or in blogs, new or non-mainstream theories or ideas that have not been published in professional peer-reviewed journals or are not part of current professional mainstream scientific discussion. Personal theories/Independent Research may be submitted to our Independent Research Forum, provided they meet our Independent Research Guidelines; Personal theories posted elsewhere will be deleted. Poorly formulated personal theories, unfounded challenges of mainstream science, and overt crackpottery will not be tolerated anywhere on the site. Linking to obviously "crank" or "crackpot" sites is prohibited.

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#### Q-reeus

Mueiz - in #7 I posted a link to http://www.weburbia.com/physics/FTL.html Please check 4. Rigid Bodies, and then the rest. Interesting that when you substitute from E = mc2 in to the inequality Y< pc2 (p = m/L, L being length), one gets Y< E/L (ie. rest energy per unit length). How profound that is I'm not sure.

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#### Mueiz

A perfectly rigid body cannot exist, simply because of the fact that forces cannot propagate faster than c. There is no need to invoke matter properties, SR is enough.
This is a logical trick
Perfectly rigid body cannot exist because of the fact that forces cannot propagate faster than c = Perfectly rigid body cannot exist because it will prevent us from resolving the paradox
this cannot be a true logic to resolve any paradox
give me any paradox and i will resolve it using your method

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#### DrGreg

Gold Member
The current question is that can a perfectly rigid body be able to send FTL signal using SR only to answer this question ?
The very notion of a "perfectly rigid body" (in the sense of this thread) is an observer-dependent concept. Let's consider the logical consequences of a "rigid rod" to which you could apply a turning force at one end and the other end would instantly accelerate, in your frame of reference. The problem is that in another frame of reference, there would be a delay before the other end started to move, and so in that frame, the rod would become bent. So, rigid in one frame, bendy in another frame. All inertial frames are equally valid, so is the rod rigid or not?

But it's even worse than that. In a third frame of reference, the far end would start to move before the force was applied. Does that make much sense? If such "rigid rods" existed, you would be able to send a signal backwards in time, and give your former self a poke with a stick to push yourself into somewhere that you weren't, a logical contradiction.

We have to conclude that truly rigid rods don't exist.

(This is the causality problem that DaleSpam referred to. It's all to do with relative simultaneity; if you don't know about it, you need to find out. See also tachyonic antitelephone.)

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