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

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!

Last edited by a moderator:

DaveC426913

Gold Member
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

Mentor
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

Gold Member
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

Mentor
OK, so what would be the definition?

DaveC426913

Gold Member
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

Mentor
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

Gold Member
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.

Last edited:

Dale

Mentor
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.

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving