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

[Doc Al]

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]

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.

Why would we need SR to deal with a fictional scenario? Why not also assume superluminal unicorns?

 

[russ_watters]

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 existence 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 existence of such stick

 

[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]

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]

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.

 

[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 into 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.

 

[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

 

[DrGreg]

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.)

 

[Mueiz]

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.

Yes I know this but I am not going to ceate a new theory here ..this is only a problem to be solved by SR like all those milions of problems written in textbooks and homework ..If PF rules prevent me from saying that certain solution to a problem according to certain theory is wrong and giving my solution i prefer to stop using it now!

 

[DaveC426913]

Yes I know this but I am not going to ceate a new theory here ..this is only a problem to be solved by SR like all those milions of problems written in textbooks and homework ..If PF rules prevent me from saying that certain solution to a problem according to certain theory is wrong and giving my solution i prefer to stop using it now!

No problem. But you'll want to step gingerly around postulating things like infinitely rigid rods to prove your case.

 

[lightarrow]

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

Don't know what kind of reasoning you have done, but the fact rigid bodies cannot exist can be easily proven, or you thought I was talking of something else?
How can be proven? Simply: apply a force to an end of a 1 light year long metal bar, ortogonally to it. Let's say the force is so high that you move the end of the bar of 1 m in 1 second. After how much time will the other end of the bar move, considered that no signal can propagate faster than c?
Which is the bar's shape during that time (= after the force is applied and before the signal arrives to the other end)?

 

[Dale]

The current question is that can a pefectly rigid body be able to send FTL signal
using SR only to answer this question ?

How do you define "perfectly rigid body" using only SR? I would define it as a body with an infinite speed of sound, which already goes beyond SR and incorporates a theory of matter.

 

[DaveC426913]

I would define it as a body with an infinite speed of sound...

Well, that wouldn't be a definition of a rigid object; that would be a property of a rigid object.

 

[Dale]

OK, so what would be the definition?

 

[DaveC426913]

OK, so what would be the definition?

Well, the definition would be something along the lines of an object that does not deform from its orginal shape under the application of any force.

An infinite speed of sound would be a property derived directly from that definition.

 

[Dale]

Well, the definition would be something along the lines of an object that does not deform from its orginal shape under the application of any force.

OK, with that definition you could prove that there is no such thing as a perfectly rigid body in SR.

An infinite speed of sound would be a property derived directly from that definition.

I would think that even a body with an infinite speed of sound would not be perfectly rigid by the above definition. It seems to me that an object with an infinite speed of sound would not deform in its rest frame, but it would deform in other frames.

 

[DaveC426913]

OK, with that definition you could prove that there is no such thing as a perfectly rigid body in SR.

Well, yes.

I think the OP was trying to prove it without SR though, or something.

I would think that even a body with an infinite speed of sound would not be perfectly rigid by the above definition.

Agreed. That's why they're not reciprocal. One is the definition, the other is derived.

Though a body with a infinite speed of sound does not have to be perfectly rigid, a perfectly rigid body does have to have an infinite speed of sound.

"If apple then round" is true.
"If round then apple" is not true.

Apple is the definition, round is the property.


"If rigid then iSoS" is true.
"If iSoS then rigid" is not true.

rigid is the definition, iSoS is the property.

 

[Dale]

Though a body with a infinite speed of sound does not have to be perfectly rigid, a perfectly rigid body does have to have an infinite speed of sound.

Sorry, I guess I didn't make my point well. I was not trying to say that infinite speed of sound does or does not imply perfect rigidity. I was trying to point out that the above definition of perfectly rigid is simply not compatible with SR at all due to the relativity of simultaneity. Even the most rigid material you can possibly imagine would not qualify, and any less rigid material would not qualify either. Any object which accelerates necessarily has a distorted shape in some frame.