# Regarding the 1light year pole question

the question was " if there was a pole that is 1 light year long...person A at one side and person B at another" if person A pushes the pole, does person B see it instanteously...

most people said that it would not be instanteous because impact would need to travel 1 light year in order to get to the other side...
HOWEVER i have one thing to point out...
matter only gets "pushed" when the force provided travels the whole length of the object...not individual atoms, in the molecular level.

i.e. it would take person A a long time to push the pole because he has to let the force to travel the length of the pole...however, once he pushes it...person B sees the pole move instantaneously...

tell me if im wrong...i got this from a very trustworthy source

cristo
Staff Emeritus
There's probably a reason that thread was closed, and that your posts were deleted-- I'm sure you shouldn't start another thread on the exact same topic!
tell me if im wrong...i got this from a very trustworthy source

my source is from a teacher at james ruse, therefore i cannot link you to him...

Sounds like the teacher is struggling to explain the issue in a manner you understand. How about you provide your teacher with a link to this thread?

russ_watters
Mentor
Either your teacher is oversimplifying it for you or doesn't understand it himself.

A push is the same as an impact.

mini

I thought of the very thing you are talking about a few years ago. I wanted to get around faster than light communication by constructing a rigid rod 1 l.y. long. Then an actuator on one end would strike the rod in a pattern like morse code, upon which the receiver could decipher the code and voila, faster than light communication.

My error, was that every time you strike the rod, the 'strike' is really just a tiny deformation made in the rod's end. This deformation travels along the rod at the speed of sound of the given material, in this case a metal rod. This speed is very slow compared to C and therefore it makes faster than light speed communication impossible for this scenario.

Even if you could strike the rod's end with an actuator that strikes it at the speed of light (which again, is impossible) the deformation of the rod would still be a bottleneck and no communication at the speed of light would be attained.

So, your idea is very clever, but its just impossible.

AlephZero
Homework Helper
My error, was that every time you strike the rod, the 'strike' is really just a tiny deformation made in the rod's end. This deformation travels along the rod at the speed of sound of the given material, in this case a metal rod. This speed is very slow compared to C and therefore it makes faster than light speed communication impossible for this scenario.

True. "Rigid rods" (where both ends would move at the same instant in time) are fairytale creatures that only live in school mechanics textbooks, not in the observable universe.

True. "Rigid rods" (where both ends would move at the same instant in time) are fairytale creatures that only live in school mechanics textbooks, not in the observable universe.

ZapperZ
Staff Emeritus
the question was " if there was a pole that is 1 light year long...person A at one side and person B at another" if person A pushes the pole, does person B see it instanteously...

most people said that it would not be instanteous because impact would need to travel 1 light year in order to get to the other side...
HOWEVER i have one thing to point out...
matter only gets "pushed" when the force provided travels the whole length of the object...not individual atoms, in the molecular level.

i.e. it would take person A a long time to push the pole because he has to let the force to travel the length of the pole...however, once he pushes it...person B sees the pole move instantaneously...

tell me if im wrong...i got this from a very trustworthy source

If you ask someone who studies solid state physics (which I did), you'll get the answer that ALL of solids are held up together via electromagnetic forces. Now this may not mean anything to you and your "trustworthy source", but if both of you have studied any significant amount of classical electromagnetic theory, you will notice that light is also governed by such electromagnetic theory.

This means that the bonding that forms between atoms and molecules that causes a solid to hold its shape is nothing more than electromagnetic forces. Since light is electromagnetic in origin, the speed in which something can be transfered from one location to another in a solid is, at most, the speed of light. In fact, there's considerable evidence that it is significantly smaller than the speed of light because what is being transfered isn't purely an electromagnetic signal, but rather, a vibration signal. This is slower because the vibraton is trying to move heavy ions that make up the material, and there will be a significant delay in time when one tries to move something heavy. That is why the speed of vibration transfer (the speed of sound) in the material is significantly less than the speed of light!

The last thread on this that was locked, I believe, because there were simply way too many speculative and outright wrong "explanations". People obviously either forgot, or did not pay attention to the PF Guidelines.

Zz.

russ_watters
Mentor
If you ask someone who studies solid state physics (which I did), you'll get the answer that ALL of solids are held up together via electromagnetic forces. Now this may not mean anything to you and your "trustworthy source", but if both of you have studied any significant amount of classical electromagnetic theory, you will notice that light is also governed by such electromagnetic theory.
Heck, you don't need even need to be a physicist for this - as an engineer, I see all beams, rods, etc. as essentially just very stiff springs.